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A control system for an automatic transmission having friction coupling elements, at least one of which is locked in 3rd and 4th gears and unlocked in 1st and 2nd gears, includes a first shift valve shiftable between 3rd and 4th gears, and second shift valve shiftable between in 2nd gear and 1st gear in which engine brake available, and a solenoid valve providing control pressure which is selectively directed to these shift valves and cause them to shift so as to lock or unlock the friction coupling elements. What is claimed:1. A hydraulic pressure control system for an automatic transmission of a type having a plurality of friction coupling elements which are selectively operated with operating pressure for placing the automatic transmission having first and second gears for a low speed gear range and at least third and fourth gears for a high speed gear range into desired gears, a specific one of said friction coupling elements being differed in operated condition between said high speed gear range and said low speed gear range, said hydraulic pressure control system comprising: operating pressure supply means for supplying operating pressure to said friction coupling elements; a first switching valve capable of switching over between two operative conditions of transmission of said operating pressure to one of said friction coupling element in said high speed gear range; a second switching valve capable of switching over between two operative conditions of transmission of said operating pressure to another one of said friction coupling elements in said low speed gear range; solenoid valve means for providing control pressure with which each of said first switching valve and said second switching valve is switched over from one of said operative conditions to another of said operative conditions; and pressure connecting means for selectively connecting and disconnecting alternatively transmission of said control pressure to said first switching valve and said second switching valve; respectively-according to and during a rise of said operating pressure to said specific friction coupling element. 2. A hydraulic pressure control system as defined in claim 1, wherein said specific friction coupling element comprises a3-4 clutch, at least which is locked in a gear of said high speed gear range and is unlocked in a gear of said low speed gear range, said one friction coupling element comprises a 2-4 brake of a type having a brake apply pressure chamber and a brake release pressure chamber, at least which is locked in said second gear and said fourth gear when said operating pressure is supplied to said brake apply pressure chamber only, and unlocked in said first gear and said third gear when said operating pressure is released from both said brake apply pressure chamber and said brake release pressure chamber, when said operating pressure is supplied to both said brake apply pressure chamber and said brake release pressure chamber, and when said operating pressure is supplied to said brake release pressure chamber only, and said another friction coupling element comprises a low-reverse brake, which is locked in at least said first gear in which engine brake is available. 3. A hydraulic pressure control system as defined in claim 2, wherein said first switching valve connects transmission of said operating pressure to said brake release pressure chamber in one of said operative conditions of said first switching valve and disconnects transmission of said operating pressure to said brake release pressure chamber in another of said operative conditions of said first switching valve, and said second switching valve connects transmission of said operating pressure to said low-reverse brake in one of said operative conditions of said second switching valve and disconnects transmission of said operating pressure to said low-reverse brake in another of said operative conditions of said second switching valve. 4. A control system as defined in claim 1, wherein said specific friction coupling element comprises a 3-4 clutch, at least which is locked in a gear of said high speed gear range and is unlocked in a gear of said low speed gear range, said one friction coupling element comprises a lock-up clutch for mechanically coupling a torque converter incorporated in said automatic transmission together, and said another friction coupling element comprises a low-reverse brake locked in at least said first gear in which engine brake is available. 5. A hydraulic pressure control system as defined in claim 4, wherein said first switching valve connects transmission of said operating pressure to said lock-up clutch in one of said operative conditions of said first switching valve and disconnects transmission of said operating pressure to said lock-up clutch in another of said operative conditions of said first switching valve, and said second switching valve connects transmission of said operating pressure to said low-reverse brake in one of said operative conditions of said second switching valve and disconnects transmission of said operating pressure to said low-reverse brake in another of said operative conditions of said second switching valve. 6. A hydraulic pressure control system for an automatic transmission of a type having a plurality of friction coupling elements which are selectively locked and unlocked with hydraulic operating pressure for changing transmission paths through which driving power from a power source is transmitted in said automatic transmission, said friction coupling elements including at least a first friction coupling element, a second friction coupling element which is controlled to lock while said first friction coupling element is locked, and a third friction coupling element which is controlled to lock while said first friction coupling element is unlocked, said hydraulic pressure control system comprising: first operating pressure supply means for supplying said operating pressure to said first friction coupling element; second operating pressure supply means for supplying said operating pressure to said second friction coupling element; third operating pressure supply means for supplying said operating pressure to said third friction coupling element; first switching means for switching over between two operative conditions of transmission of said operating pressure to said second friction coupling element; second switching means for switching over between two operative conditions of transmission of said operating pressure to said third friction coupling element; control pressure providing means for providing control pressure with which each of said first switching means and said second switching means is switched over to an operative conditions where each said switching means connects transmission of said operating pressure to said respective friction coupling elements to another operative condition where each said switching means disconnects transmission of said operating pressure to said respective friction coupling elements; and pressure connecting means for selectively connecting transmission of said control pressure to said first switching means and said second switching means according to and during a rise of said operating pressure to said first friction coupling element. 7. A hydraulic pressure control system as defined in claim 6, wherein said control pressure providing means comprises pressure developing means for developing a specified level of source pressure as said control pressure and a solenoid valve for connecting and disconnecting transmission of said control pressure to said pressure connecting means. 8. A hydraulic pressure control system as defined in claim 6, wherein said pressure connecting means comprises a shift valve having a spool shiftable between two operative positions according to which said pressure connecting means selectively connects and disconnects transmission of said control pressure to said first switching means and said second switching means, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said operating pressure supplied to said first friction coupling means so as thereby to shift to another of said operative positions against said return spring. 9. A hydraulic pressure control system as defined in claim 6, wherein each of said first switching means and said second switching means comprises a shift valve having a spool shiftable between two operative positions according to which said operative conditions are provided, respectively, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure so as thereby to shift to another of said operative positions against said return spring. 10. A hydraulic pressure control system as defined in claim 6, wherein said second switching means comprises sub-control pressure providing means for providing sub-control pressure, sub-switching means for transmission of said sub-control pressure while connection of transmission of said control pressure is made by said pressure connecting means to said second switching means, and switching means for switching over transmission of said operating pressure between connection to and disconnection from said third friction coupling element. 11. A hydraulic pressure control system as defined in claim 10, wherein said sub-control pressure developing means develops a specified level of source pressure as said sub-control pressure and a solenoid valve for connecting and disconnecting transmission of said sub-control pressure to said switching means. 12. A hydraulic pressure control system as defined in claim 11, wherein said sub-control pressure switching means comprises a solenoid valve shiftable between two operative positions according to said control pressure, said sub-control pressure being delivered in one of said operative positions and being withdrawn in another of said operative positions. 13. A hydraulic pressure control system as defined in claim 6, wherein said second friction coupling element is operated in at least a high speed gear range, and said third friction coupling element is operated in at least a low speed gear range. 14. A hydraulic pressure control system as defined in claim 13, wherein said first friction coupling element comprises a3-4 clutch, at least which is locked in third and fourth gears for said high speed gear range and unlocked in first and second gears for said low speed gear range, said second friction coupling element comprises a 2-4 brake, at least which is locked in said second gear and said fourth gear and unlocked in said first gear and said third gear, and said third friction coupling element comprises a low-reverse brake, which is locked in at least said first gear in which engine brake is available. 15. A hydraulic pressure control system as defined in claim 14, wherein each of said first switching means and said second switching means comprises a shift valve having a spool shiftable between two operative positions according to which said operative conditions are provided, respectively, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure so as thereby to shift to another of said operative positions against said return spring. 16. A hydraulic pressure control system as defined in claim 13, wherein said first friction coupling element comprises a3-4 clutch, at least which is locked in third and fourth gears for said high speed gear range and is unlocked in first and second gears for said low speed gear range, said second friction coupling element comprises a lock-up clutch for mechanically locking a torque converter incorporated in said automatic transmission, and said third friction coupling element comprises a low-reverse brake locking in at least said first gear in which engine brake is available. 17. A hydraulic pressure control system as defined in claim 16, wherein each of said first switching means and said second switching means comprises a shift valve having a spool shiftable between two operative positions according to which said operative conditions are provided, respectively, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure so as thereby to shift to another of said operative positions against said return spring. 18. A hydraulic pressure control system as defined in claim 6, further comprising sub-control pressure providing means for providing sub-control pressure, sub-switching means comprising a shift valve having a spool shiftable between two operative positions according to which transmission of said sub-control pressure is connected and disconnected, respectively, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure supplied to said first friction coupling element so as thereby to shift to another of said operative positions against said return spring, wherein said pressure connecting means comprises a valve having a spool on which said sub-control pressure from said sub-switching means is exerted in one direction and a return spring for forcing said spool to shift to in another direction against said sub-control pressure. 19. A hydraulic pressure control system as defined in claim 18, wherein said first friction coupling element comprises a3-4 clutch, at least which is locked in third and fourth gears for said high speed gear range and is unlocked in first and second gears for said low speed gear range, said second friction coupling element comprises a lock-up clutch for mechanically locking a torque converter incorporated in said automatic transmission, said third friction coupling element comprises a low-reverse brake locking in at least said first gear in which engine brake is available, wherein each of said first switching means and said second switching means comprises a shift valve having a spool shiftable between two operative positions according to which said operative conditions are provided, respectively, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure so as thereby to shift to another of said operative positions against said return spring, said sub-control pressure switching means switches said operative position to one where transmission of said sub-control pressure is connected while transmission of said operating pressure is disconnected from said 3-4 clutch and to another where transmission of said sub-control pressure is disconnected while transmission of said operating pressure is connected to said3-4 clutch, and further said pressure connecting means connects transmission of said control pressure to said second switching means so as to cause said second switching means to switch over to said one of said operative positions where said second switching means connects transmission of said operating pressure supplied by said third operating pressure supply means to said low-reverse brake while said sub-control pressure switching means takes said one of said operative positions and connects transmission of said control pressure to said first switching means so as to cause said first switching means to switch over to said one of said operative conditions where said second switching means connects transmission of said operating pressure supplied by said second operating pressure supply means to said lock-up clutch while said sub-control pressure switching means takes said another of said operative positions. 20. A hydraulic pressure control system as defined in claim 19, wherein said sub-control pressure providing means is interrupted to provide said sub-control pressure in said second gear where said sub-control pressure switching means switches said sub-control pressure over to delivery while transmission of said operating pressure is disconnected from said 3-4 clutch. 21. A hydraulic pressure control system as defined in claim 6, wherein each of said first to third operating pressure supply means comprises a duty solenoid valve. 22. A hydraulic pressure control system for an automatic transmission of a type having a plurality of friction coupling elements which are selectively locked and unlocked with hydraulic operating pressure for changing power transmission paths of the automatic transmission through which source power is transmitted in said automatic transmission, said friction coupling elements including at least a first friction coupling element, a second friction coupling element which is controlled to lock while said first friction coupling element is locked, a third friction coupling element which is controlled to lock while said first friction coupling element is unlocked, and a fourth friction coupling means which is controlled to lock while said first friction coupling element is locked and said third friction coupling element is unlocked, said hydraulic pressure control system comprising: first operating pressure supply means for supplying said operating pressure to said first friction coupling element; second operating pressure supply means for supplying said operating pressure to said second friction coupling element; third operating pressure supply means for supplying said operating pressure to said third friction coupling element; fourth operating pressure supply means for supplying said operating pressure to said third friction coupling element; first control pressure providing means for providing first control pressure to allow one of said second friction coupling element, said third friction coupling element and said fourth friction coupling element to operate; second control pressure providing means for providing second control pressure to control one of said third friction coupling element and said fourth friction coupling element to operate; first switching means for switching over between two operative conditions of transmission of said operating pressure to said second friction coupling element according to said first control pressure; second switching means for switching over between two operative conditions of transmission of said operating pressure to said third friction coupling element according to said second control pressure; third switching means for switching over between two operative conditions of transmission of said operating pressure to said fourth friction coupling element according to said second control pressure; first control pressure connecting means for selectively connecting transmission of said second control pressure to said second and said third switching means; and second control pressure connecting means for selectively connecting transmission of said first control pressure to said first control pressure connecting means and said first switching means according to said operating pressure supplied to said first friction coupling element; said first control pressure connecting means selectively connecting transmission of said second control pressure to said second and said third switching means when said second control pressure connecting means connects transmission of said first control pressure to said first control pressure connecting means. 23. A hydraulic pressure control system as defined in claim 22, wherein said first friction coupling element comprises a3-4 clutch, at least which is locked in a gear of said high speed gear range and is unlocked in a gear of said low speed gear range, said second friction coupling element comprises a 2-4 brake, which is at least which is locked in said second gear and said fourth gear and unlocked in said first gear and said third gear, said third friction coupling element comprises a low-reverse brake, which is locked in at least said first gear in which engine brake is available, said fourth friction coupling element comprises a lock-up clutch for mechanically locking a torque converter incorporated in said automatic transmission, and each of said first to third switching means comprises a shift valve having a spool shiftable between two operative positions according to which said operative conditions of operating pressure transmission are selectively provided, and a return spring for forcing said spool to one of said operative positions, said spool being applied with said control pressure so as thereby to shift to another of said operative positions against said return spring. 24. A hydraulic pressure control system as defined in claim 23, wherein said second control pressure connecting means connects transmission of said first control pressure to said first control pressure connecting means while said first operating pressure supply means does not supply said operating pressure to said3-4 clutch and to said first switching means while said first operating pressure supply means supplies said operating pressure to said3-4 clutch, and said first control pressure connecting means connects transmission of said second control pressure to said second switching means so as to cause said second switching means to provides one of said operative conditions where transmission of said operating pressure is connected to said low-reverse brake when said second control pressure connecting means connects transmission of said first control pressure to said first control pressure connecting means and to said third switching means so as to cause said third switching means to provides one of said operative conditions where transmission of said operating pressure is connected to said lock-up clutch when said second control pressure connecting means disconnects transmission of said first control pressure to said first control pressure connecting means. 25. A hydraulic pressure control system as defined in claim 23, wherein said first control pressure providing means is interrupted to provide said first control pressure in said second gear where said second control pressure connecting means connects transmission of said first control pressure while transmission of said operating pressure is disconnected from said 3-4 clutch. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic transmission control system, and, more particularly, to a hydraulic control system, for an automatic transmission, having a switching means which provides selective pressure transfer to and from friction coupling elements according to control pressure applied thereto. 2. Description of Related Art Typically, an automatic transmission for an automobile has a torque converter and a transmission gear mechanism operationally coupled to and driven by the torque converter. Such a transmission gear mechanism includes a plurality of friction coupling elements, such as clutches and brakes, which in turn are selectively coupled or locked and released or unlocked so as thereby to place automatically the automatic transmission into desired gears according to driving conditions. Selectively locking and unlocking those friction coupling elements is performed by means of a hydraulic pressure control system. A number of torque converters are equipped with a locking feature. A locking unit, i.e. a lock-up clutch, mechanically locks its input and output shafts, such as, for instance, an engine output shaft and a turbine shaft when activated to prevent slippage allowed even at cruising speeds due to that the only connection between pump and turbine is transmission fluid. A lock-up condition is allowed in driving conditions in which there is not so strong demand for torque multiplication or torque reduction. Locking and unlocking the lock-up clutch are governed by means of the hydraulic pressure control system as well as locking and unlocking the friction coupling elements. As is well known in the art, the hydraulic pressure control system is provided with various valves, such as, for instance, a pressure regulator valve for regulating pressure discharged from an oil pump to a fixed level, a manually operated shift valve for manually placing the automatic transmission into desired ranges, and a plurality of shift valves, which are selectively operated to control pressure transfer, including pressure supply to and pressure discharge from the frictional coupling elements so as to lock and unlock the frictional coupling elements, thereby shifting the automatic transmission into selected gears. In order for those shift valves to timely operate or timely shift their spools between two operative positions, the hydraulic pressure control system is typically provided with a timing means, by means of which control pressure is timely and independently supplied to the shift valves. In many instances where the control pressure is differed in order for the shift valves to shift the spool positions, a control pressure transfer means must be installed one for each shift valve, which is always undesirable for the hydraulic pressure control system in the light of space and cost efficiency. In an automatic transmission control system equipped with a down-shift timing valve for controlling a down-shift timing and an engine brake control valve for causing a specific friction coupling element which makes engine brake available in a specific gear, an approach to eliminate those problems is that described in Japanese Unexamined Patent Publication No. 62(1987)-61838. Means used in the Japanese Unexamined Patent Publication No. 62-61838 is a shuttle valve installed between the control pressure transfer means and those down-shift timing valve and engine brake control valve. The shuttle valve is able to change its spool position according to engine load proportional pressure and operates as a control pressure transfer means commonly to engine brake control and down-shift timing control. Another approach is to utilize a single control pressure transfer means which bears the function of supplying control pressure commonly to a plurality of shift valves, such as a 3-2 down-shift timing control valve and a low-clutch timing control valve, which are differed in operation timing. The control pressure transfer means operates to control selectively and directly the shift valves at desired timings. Such a hydraulic pressure control system is known from, for instance, Japanese Unexamined Patent Publication No. 2(1989)-138562. Further, Japanese Unexamined Patent Publication No. 2(1989)-278076 has proposed a hydraulic pressure control system making utilization of a single control pressure transfer means which provides control pressure which effects selectively control of an engine brake clutch for making engine brake available in some specific gears and a lock-up clutch according to manually selected ranges. Some automatic transmissions controlled by means of hydraulic pressure control system of these kinds include a friction coupling element, as one of elements necessary for providing forward gears, which is differed in operation between for high speed gears and for low speed gears. In such an automatic transmission, there is installed in the hydraulic pressure control system valves which are selectively operated to provide high speed gears and low speed gears, respectively. For example, what is called a 3-4 clutch is installed in a four forward gear automatic transmission, which is locked in a high speed gear such as a third gear and a fourth gear and unlocked in a low speed gear such as a first gear and a second gear. This type of automatic transmission typically has a switch valve operative between the third gear and fourth gear and a switch valve operative between the second gear and first gear in which engine brake is available. While the automatic transmissions controlled by these types of hydraulic pressure control systems have various advantages, nevertheless, various constraints must be imposed on mechanical structures, which are always undesirable for simplified and inexpensive hydraulic pressure control systems. SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydraulic pressure control system for an automatic transmission of the type having a friction coupling element differed in operation between different speed ranges, such as a range covering high speed gears and a range covering low speed gears, in which only one control pressure transmission means is made of common utilization for connecting and disconnecting transmission of control pressure to switching valves used in the different speed ranges, respectively. The foregoing object of the present invention is accomplished by providing a hydraulic pressure control system for an automatic transmission of a type having a plurality of friction coupling elements, such as clutches and brakes which in turn are selectively locked and unlocked so as to provide for the automatic transmission desired gears, including, for instance, first and second gears for a low speed gear range and at least third and fourth gears for a high speed gear range, a specific one of the friction coupling elements being operated only in the high speed gear range. The hydraulic pressure control system includes first and second switching means, each of these switching means being able to operate in two operative conditions in which transmission of operating pressure is connected to and disconnected from the related friction coupling element, respectively, and a control pressure developing means, such as a solenoid valve, which develops or provides control pressure with which each switching means is caused to switch over from one operative condition to another operative condition. The hydraulic pressure control system further includes a pressure connecting means which selectively connects transmission of the control pressure to the first and second switching means according to and during supply of the operating pressure to the specific friction coupling element. In a four forward speed automatic transmission, the specific friction coupling elements may comprise a 3-4 clutch, at least which is locked in third and fourth gears of the high speed gear range and is unlocked in first and second gears of the low speed gear range. In this instance, the first switching means connects and disconnects transmission of the operating pressure to one of the friction coupling elements, which is locked in the second and fourth gears and is unlocked in the first and third gears, such as a 2-4 brake of a type having a brake apply pressure chamber and a brake release pressure chamber. The 2-4 brake is locked in the second gear and fourth gear when the operating pressure is supplied to the brake apply pressure chamber only, and is unlocked in the first gear and third gear when the operating pressure is released from both the brake apply pressure chamber and brake release pressure chamber, when the operating pressure is supplied to both brake apply pressure chamber and brake release pressure chamber, and when the operating pressure is supplied to the brake release pressure chamber only. Alternatively, the one of the friction coupling elements may comprise a lock-up clutch for mechanically coupling input and output shafts of a torque converter together. Further, the second switching means connects and disconnects transmission of the operating pressure to another one of the friction coupling elements which is locked in the second and fourth gears and is unlocked in the first and third gears, such as a low-reverse brake locked in at least the first gear in which engine brake is available. According to another aspect of the present invention, in the hydraulic pressure control system for an automatic transmission having a plurality of friction coupling elements, of which a second friction coupling element is controlled to lock while a first friction coupling element is locked and a third friction coupling element is controlled to lock while the first friction coupling element is unlocked, in addition to first to third operating pressure supply means for supplying the operating pressure to the first to third friction coupling elements, respectively, there are installed first and second switching means, a control pressure providing means and a pressure connecting means. The first switching means switches over between two operative conditions, in one of which transmission of the operating pressure is connected to the second friction coupling element, and in another of which transmission of the operating pressure is disconnected from the second friction coupling element. Similarly, the second switching means switches over between two operative conditions, in one of which transmission of the operating pressure is connected to the third friction coupling element, and in another of which transmission of the operating pressure is disconnected from the third friction coupling element. The pressure connecting means selectively connects transmission of the control pressure to the first switching means and second switching means according to and during supply of the operating pressure to the first friction coupling element. With the control pressure, each of the second friction coupling element and third friction coupling element is forced to switch over from one operative condition to another. The control pressure providing means may comprises a solenoid valve for transmits the control pressure developed or provided as a specified level of source pressure. The pressure connecting means may comprises a shift valve having a spool shiftable between two operative positions according to which the pressure connecting means selectively connects and disconnects transmission of the control pressure to the first switching means and second switching means, and a return spring for forcing the spool to one operative positions. Exerted on the spool is the operating pressure supplied to the first friction coupling means which serves against the return spring to force the spool to another operative position. The second switching means may comprise a means for providing connection and disconnection of sub-control pressure according to the control pressure and a means for causing the second switching means to selectively connect and disconnect transmission of the operating pressure to the third friction coupling element according to the sub-control pressure. These second and third friction coupling elements are operated in at least a high speed gear range and at least a low speed gear range, respectively. More specifically, when the first friction coupling element is a 3-4 clutch, at least which is locked in the third and fourth gears for the high speed gear range and unlocked in the first and second gears for the low speed gear range, the second friction coupling element may comprise a 2-4 brake, at least which is locked in the second gear and fourth gear and unlocked in the first gear and third gear, and the third friction coupling element may comprise a low-reverse brake, which is locked in at least the first gear in which engine brake is available. Alternatively, when the first friction coupling element employs the 3-4 clutch, the second friction coupling element may comprise a lock-up clutch for mechanically coupling a torque converter incorporated in the automatic transmission, and the third friction coupling element may comprise a low-reverse brake which is locked in at least the first gear in which engine brake is available. The hydraulic pressure control system may further comprise a means for providing sub-control pressure and a means such as comprising a shift valve having a spool shiftable between two operative positions where providing connection and disconnection of transmission of the sub-control pressure, respectively and a return spring for forcing the spool to one operative position. The spool is applied with the operating pressure to the first friction coupling means so as thereby to shift to another operative position against said return spring. Specifically, in the case where each of the first and second switching means, which may comprise a solenoid valve having a spool shiftable between two operative positions and a return spring, the sub-control pressure switching means provides connection of transmission of the sub-control pressure on one operative position while transmission of the operating pressure to the 3-4 clutch is disconnected and withdraws transmission of the sub-control pressure in another operative position while said operating pressure is supplied to said 3-4 clutch while transmission of said operating pressure is connected. Further, the pressure connecting means connects transmission of the control pressure to the second switching means so as to cause it to switch over to the one operative position where the second switching means connects transmission of the operating pressure to the low-reverse brake while the sub-control pressure switching means is in the one operative position and connects transmission of the control pressure to the first switching means so as to cause it to switch over to the one operative condition where the second switching means connects transmission of the operating pressure to the lock-up clutch while the sub-control pressure switching means is in the other operative position. According to still another aspect of the present invention, the hydraulic pressure control system for an automatic transmission of the type having a plurality of friction coupling elements selectively locked and unlocked with operating pressure for changing transmission paths for transmitting driving power from a power source, which include at least a first friction coupling element, a second friction coupling element which is controlled to lock while the first friction coupling element is locked, a third friction coupling element which is controlled to lock while the first friction coupling element is unlocked, and a fourth friction coupling means which is controlled to lock while the first friction coupling element is locked and the third friction coupling element is unlocked. The control system includes first to fourth operating pressure supply means, each supplying operating pressure to a related first friction coupling element, a first control pressure providing means for providing first control pressure with which one of the second to fourth friction coupling element is controlled to lock, a second control pressure providing means for providing second control pressure with which one of the third and fourth friction coupling elements are controlled to lock, first to third switching means for switching over between their two operative conditions of operating pressure transmission to the second to fourth friction coupling elements, respectively, according to the first and second control pressure, a first control pressure connecting means for selectively connecting transmission of the second control pressure to the second and third switching means, and a second control pressure connecting means for connecting transmission of the first control pressure selectively to the first control pressure connecting means and first switching means according to the operating pressure to the first friction coupling element. The first control pressure connecting means connects transmission of the second control pressure selectively to the second and third switching means when the second control pressure connecting means connects transmission of the first control pressure to the first control pressure connecting means. In the cases where the first to fourth friction coupling elements are, respectively a 3-4 clutch which is locked in the high speed gear range and unlocked in the low speed gear range, a 2-4 brake which is locked in the second and fourth gear and unlocked in the first gear and third gear, a low-reverse brake which is locked in the first gear in which engine brake is available, and a lock-up clutch for mechanically coupling a torque converter, each of the first to third switching means may comprise a shift valve having a spool shiftable between two operative positions where transmission of the operating pressure is connected and disconnected, selectively, and a return spring for forcing the spool to one operative position. The spool is applied with the control pressure so as thereby to shift to another operative position against the return spring. Further, the second control pressure connecting means connects transmission of the first control pressure to the first control pressure connecting means while the first operating pressure supply means does not supply the operating pressure to the 3-4 clutch and connects the first control pressure to the first switching means while supplying the operating pressure to the 3-4 clutch. On the other hand, the first control pressure connecting means connects transmission of the second control pressure to the second switching means so as to cause it to provide one operative condition where transmission of the operating pressure is connected to the low-reverse brake while the second control pressure connecting means connects transmission of the first control pressure to the first control pressure connecting means, and connects the second control pressure to the third switching means so as to cause it to provides one operative condition where the operating pressure transmission is connected to the lock-up clutch while disconnecting the first control pressure transmission to the first control pressure connecting means. With the hydraulic pressure control system of the present invention, the means for providing the control pressure, such as a solenoid valve, cooperates selectively with the first and second switching means through connection and disconnection of transmission of the control pressure by the pressure connecting means according to whether a gear shift is intended to a high speed gear or a low speed gear. That is, the control pressure is common to the first and second switching means and timely supplied to each switching means in response to operative conditions of the first or specific friction coupling element. Specifically, the control pressure developing or providing means is used to cause, on one hand, the 2-4 brake to operate in the high speed gear range and, on the other hand, to cause the low-reverse brake to operate in the low speed gear range. Otherwise, the control pressure developing or providing means is used to cause, on one hand, the lock-up clutch to operate in the high speed gear range and, on the other hand, to cause the low-reverse brake to operate in the low speed gear range. The utilization is made of a single control pressure developing or providing means common to two switching means, or two friction coupling elements, which do in no way experience coincidental activation, resulting in simplified and inexpensive hydraulic pressure control systems. Further the utilization is made of two control pressure developing or providing means common to three switching means, or three friction coupling elements, also resulting in simplified and inexpensive hydraulic pressure control systems. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will be clearly understood from the following description with respect to a preferred embodiment thereof when considered in conjunction with the accompanying drawings, wherein same reference numerals has been used throughout the drawings to denote the same or similar elements, and in which: FIG. 1 is a schematic illustration of an automatic transmission incorporating a hydraulic control system of the present invention; FIG. 2 is a cross-sectional view of the automatic transmission shown in FIG. 1; FIG. 3 is a diagrammatic view of a hydraulic control circuit according to an embodiment of the present invention; FIG. 4 is a block diagram illustrating a control system for various solenoid valves of the hydraulic control circuit of FIG. 3; FIG. 5 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a first gear; FIG. 6 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a second gear; FIG. 7 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a third gear; FIG. 8 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a fourth gear; FIG. 9 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a first gear in a low speed (L) range; FIG. 10 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides a reverse gear; FIG. 11 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides lock-up control in the third gear; FIG. 12 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 3 which provides lock-up control in the fourth gear; FIG. 13 is a diagrammatic view of a hydraulic control circuit according to another embodiment of the present invention; FIG. 14 is a circuit diagram of a regulator valve of the hydraulic control circuit of FIG. 13; FIG. 15 is a block diagram illustrating a control system for various solenoid valves of the hydraulic control circuit of FIG. 13; FIG. 16 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a first gear; FIG. 17 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a second gear; FIG. 18 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a third gear; FIG. 19 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a fourth gear; FIG. 20 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a first gear in a low speed (L) range; FIG. 21 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides a reverse gear; FIG. 22 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides fail-safe control for the reverse gear; FIG. 23 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides lock-up control in the second gear; FIG. 24 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides lock-up control in the third gear; FIG. 25 is an enlarged view of an essential part of the hydraulic control circuit of FIG. 13 which provides lock-up control in the fourth gear; FIG. 26 is a flowchart illustrating a control sequence for a first duty solenoid valve (DSV) during a 3-4 shift; FIG. 27 is a flowchart illustrating a control sequence for a third duty solenoid valve (DSV) during the 3-4 shift; FIG. 28 is a time chart showing changes in various factors during the 3-4 shift; FIG. 29 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during a 1-4 shift; FIG. 30 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during the 1-3 shift; FIG. 31 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 1-3 shift; FIG. 32 is a time chart showing changes in various factors during the 1-3 shift; FIG. 33 is a flowchart illustrating a switching control sequence for a first solenoid valve (SV) during the 1-4 shift; FIG. 34 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during a 2-4 shift; FIG. 35 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 2-4 shift; FIG. 36 is a time chart showing changes in various factors during the 2-4 shift; FIG. 37 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during a 4-3 shift; FIG. 38 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 4-3 shift; FIG. 39 is a time chart showing changes in various factors during the 4-3 shift; FIG. 40 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during a 4-2 shift; FIG. 41 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 4-2 shift; FIG. 42 is a time chart showing changes in various factors during the 4-2 shift; FIG. 43 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during a 4-1 shift; FIG. 44 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during the 4-1 shift; FIG. 45 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 4-1 shifting; FIG. 46 is a time chart showing changes in various factors during the 4-1 shifting; FIG. 47 is a flowchart illustrating a scheduled down-shift control sequence for the third duty solenoid valve (DSV) during a 4-3 shift; FIG. 48 is a time chart showing changes in various factors during the scheduled 4-3 shift; FIG. 49 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during a 2-L1 shift; FIG. 50 is a time chart showing changes in various factors during the 2-L1 shift; FIG. 51 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during a 3-L1 shift; FIG. 52 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during the 3-L1 shift; FIG. 53 is a flowchart illustrating a relay valve position determination sequence for a relay valve during the 3-L1 shift; FIG. 54 is a time chart showing changes in various factors during the 3-L1 shift; FIG. 55 is a flowchart illustrating a control sequence for the first duty solenoid valve (DSV) during a 4-L1 shift; FIG. 56 is a flowchart illustrating a control sequence for the second duty solenoid valve (DSV) during the 4-L1 shift; FIG. 57 is a flowchart illustrating a control sequence for the third duty solenoid valve (DSV) during the 4-L1 shift; FIG. 58 is a time chart showing changes in various factors during the 4-L1 shift; and FIG. 59 is a hydraulic control circuit incorporated in the automatic transmission according to a further preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is to be noted that the term "left end" position used in the following description shall means and refer to one of operative positions of a valve as viewed throughout in figures which a valve spool occupies and the term "right end" position used in the following description shall means and refer to another one of the operative positions of the valve as viewed throughout in figures which the valve spool occupies. Referring to the drawings in detail, in particular to FIGS. 1 and 2 showing an overall structure of an automatic transmission equipped with a torque converter which incorporates a hydraulic control system in accordance with a preferred embodiment of the present invention, an automatic transmission 10 has a mechanical configuration including, as main components, a torque converter 20, a transmission gear mechanism 30 driven by means of the torque converter 20, and a plurality of friction coupling elements 41-45, such as clutches and brakes, and a one-way clutch 46 which are selectively coupled and uncoupled by the hydraulic control system to change power transmission paths of the transmission gear mechanism 30 so as to place the automatic transmission into desired gears and ranges, namely first (1) to fourth (4) forward gears in a drive (D) range, a second speed (S) range and a low speed (L) range, and a reverse gear in a reverse (R) range. The torque converter 20 has a pump 22, a turbine 23 and a stator 25. The pump 22 is placed within and secured to an transmission housing 11 secured to an engine output shaft 1. The turbine 23 is placed facing the pump 22 within the transmission housing 11 and driven by the pump 22 by means of a special lightweight oil. The stator 25 is inserted between the pump 22 and turbine 23 and mounted on the transmission housing 11 through a one-way clutch 24 so as to multiply engine torque. The torque converter 20 further has a lock-up clutch 26 placed between a converter housing 21 and turbine 23 for mechanically coupling the engine shaft 1 and the turbine 23 together when it is activated. The torque converter 20 thus structured connects transmission of turbine rotation to the transmission gear mechanism 30 through a turbine shaft 27. Behind the torque converter 20, i.e. on one side of the torque converter 20 opposite to the engine 1, there is provided an oil pump 12 driven by the engine output shaft 1 through the housing 21 of the torque converter 20. The transmission gear mechanism 30 comprises a first planetary gearset 31 and a second planetary gearset 32. The first planetary gearset 31 is comprised of a sun gear 31a, a plurality of pinion gears 31b in mesh with the sun gear 31a, a pinion carrier 31c for carrying the pinion gears 31b, and a ring gear 31d in mesh with the pinion gears 31b. Similarly, the second planetary gearset 32 is comprised of a sun gear 32a, a plurality of pinion gears 32b in mesh with the sun gear 32a, a pinion carrier 32c for carrying the pinion gears 32b, and a ring gear 32d in mesh with the pinion gears 32b. The automatic transmission includes a forward clutch (FWD) 41 disposed between the turbine shaft 27 and the sun gear 31a of the first planetary gearset 31, a reverse clutch (REV) 42 disposed between the turbine shaft 27 and the sun gear 32a of the second planetary gearset 32, a 3-4 clutch (3-4C) 43 disposed between the turbine shaft 27 and the pinion carrier 32c of the second planetary gearset 30, and a 2-4 brake (2-4B) 44 for locking the sun gear 32a of the second planetary gearset 32. Further, a low-reverse brake (L/R) 45 and a one-way clutch (OWC) 46 are disposed in parallel with each other between the transmission housing 11 and the ring gear 31d of the first planetary gearset 31 connected to the pinion carrier 32c of the second planetary gearset 32. A transmission output gear 13 is connected to the pinion carrier 31c of the first planetary gearset first 31 connected to the ring gear 32d of the second planetary gearset 32. Rotation of the transmission output gear 13 is transmitted to right and left axles 6 and 7 through a differential 5 via gears 3 and 4. Those frictional coupling elements 41-45 and the one-way clutch 46 are selectively activated so as to place the automatic transmission 10 into available gears as shown in the following Table I. In Table I, a circle indicates that a frictional coupling element is locked; a circle in parentheses indicates that a frictional coupling element is locked in the low speed (L) range only; and a dark spot indicates that a friction coupling element (the 2-4 brake 44) is not locked as a result of both servo apply pressure chamber (S/A) and servo release pressure chamber (S/R) being supplied with hydraulic pressure as will be described later. TABLE I __________________________________________________________________________ 2-4B(44) Gear FWD(41) S/A S/R 3-4C(43) L/R(45) REV(42) OWC(46) __________________________________________________________________________ 1ST .largecircle. (.largecircle.) .largecircle. 2ND .largecircle. .largecircle. 3RD .largecircle. .circle-solid. .largecircle. .largecircle. 4TH .largecircle. .largecircle. REVERSE .largecircle. .largecircle. __________________________________________________________________________ Specifically, the forward clutch (FWD) 41 locks in the first, second and third gears; the reverse gear (REV) 42 locks in the reverse gear; the 2-4 brake (2-4B) 44 locks in the second and fourth gears; and the low reverse brake (L/R) 45 locks in the reverse gear and in the first gear in the low speed (L) range where engine brake is available. In gears where simultaneously locking of two of the friction coupling elements 41-45, such as the 3-4 clutch 43 and low reverse brake 45, the low reverse brake 45 and 2-4 brake 44, and the 2-4 brake 44 and reverse clutch 42, will occur, what is called interlocking causes in the automatic transmission 10. In order for the automatic transmission 10 to be prevented from interlocking, the hydraulic control system includes a shift valve which will be described in detail later. The following description will be directed to a hydraulic control circuit shown in FIG. 3 for controlling operating hydraulic pressure supply to and operating hydraulic pressure discharge from a pressure chamber of each friction coupling element 41-45. Valves utilized in the hydraulic control system have two different spool positions which are shown by upper and lower halves in FIG. 3. One of the friction coupling elements 41-45, for instance the 2-4 brake (2-4B) 44 including a band brake, has a servo apply pressure chamber (S/A) 44a and a servo release pressure chamber (S/R) 44b to which operating pressure is supplied. Specifically, the 2-4 brake (2-4B) 44 locks when the operating pressure is supplied to the servo apply pressure chamber (S/A) 44a only, and unlocks when the operating pressure is not supplied to both pressure chambers (S/A and S/R) 44a and 44b, when the operating pressure is supplied to both pressure chambers (S/A and S/R) 44a and 44b, or when the operating pressure is supplied to the servo release pressure chamber (S/R) 44b only. The remaining friction coupling elements 41-43 and 45 have single hydraulic pressure chambers and lock when operating pressure is supplied to the related hydraulic pressure chambers. As shown in FIG. 3, a hydraulic pressure control circuit 50 includes various valves as essential elements, such as a regulator valve 51 for generating a line pressure (control source pressure), a manual shift valve 52 for shifting ranges of the automatic transmission 10 according to manual range selections through a shift lever, first to third shift valves 56-58 for changing pressure lines according to gears, first and second ON-OFF solenoid valves 61 and 62 (which are hereafter referred to as 1st and 2nd SVs, respectively for simplicity) utilized as an actuating pressure generator means for supplying actuating pressure to these shift valves 56-58 and a lock,-up control valve 59, first to third duty solenoid valves 66-68 (which are hereafter referred to as 1st, 2nd and 3rd DSVs, respectively, for simplicity) utilized as operating pressure supply means for generating, regulating and discharging operating pressure supplied to pressure chambers of the friction coupling elements 41-45. These duty valves 66-68 as the operating pressure supply means are utilized with the effect of accurate regulation of operating pressure for the friction coupling elements 41-45 and, in particular, precise timing control of supply and discharge of operating pressure during gear shifts, which yield a reduction in shift shock caused in the automatic transmission and, consequently, improve feelings of gear shifting. In this instance, those solenoid valves 61 and 62 and duty valves 66-68 are of a three-way type having operating modes, namely a communication mode where hydraulic pressure is allowed to flow in a pressure line from the upstream side to the downstream side and vice versa and a drain mode where hydraulic pressure is drawn from the downstream pressure line. In the drain mode, since the valve shuts of its related upstream pressure line during draining hydraulic pressure from the downstream pressure lines, there is no aimless drainage of hydraulic pressure from the upstream pressure line, decreasing driving loss of the oil pump 12. As is well known, each of the solenoid valve valves 61 and 62 opens the downstream pressure line when it is ON or is applied with electric current and brings the upstream and downstream pressure lines into communication with each other when it is OFF or is not applied with electric current. On the other hand, each of the duty valves 66-68 develops a specifically regulated level of hydraulic pressure in the downstream pressure line according to a given level of line pressure in the upstream pressure line by means of duty control. The regulator valve 51 regulates operating pressure discharged by the oil pump 12 to a fixed level of line pressure which in turn is delivered to the manual shift valve 52 through a main pressure line 100 and also to a reducing valve 71 and the second duty valve 67 as control source pressure. Further, as described later, according to operated conditions of the first shift valve 56, the line pressure is supplied to an accumulator 72 used to lock the forward clutch 41 through the shift valve 56 as back pressure. The line pressure is reduced by the reducing valve 71 to a fixed pressure level and then, delivered to the first and second solenoid valve valves 61 and 62 through a pressure line 101. When the first and second solenoid valve valves 61 and 62 are OFF, the fixed level of line pressure is further delivered downstream as solenoid pressure. Specifically, when the first solenoid valve valve 61 is OFF, the solenoid pressure is directed as pilot pressure to the control port 56a of first shift valve 56 through a pressure line 103 via the third shift valve 58 from a pressure line 103 or to the control port 57a of second shift valve 57 through a pressure line 104 via the third shift valve 58 from the pressure line 103. As a result, these shift valves 56 and 57 place their spool in left end positions as viewed in FIG. 3. On the other hand, when the first solenoid valve 62 is OFF, the solenoid pressure is directed to the control port 59a of lock-up control valve 59 through a pressure line 105 and places its spool in the right end position as viewed in FIG. 3. Further, the fixed level of operating pressure from the reducing valve 71 is also delivered to the regulation port 51a of regulator valve 51 through a pressure line 106. In this instance, the operating pressure is regulated according to, for instance, engine load by means of a linear solenoid valve 69 installed in the pressure line 106. In this way, the line pressure is regulated according to engine load by means of the regulator valve 51. The line pressure delivered to the manual shift valve 52 through the main pressure line 100 is directed to a forward pressure line 107 in the forward ranges, i.e. the drive (D) range, slow speed (S) range and low speed (L) range, or to a reverse pressure line 108 in the reverse (R) range. The forward pressure line 107 branches off into three pressure lines 111-113, the first pressure line 111 leading to the first duty valve 66 so as to supply the line pressure as control source pressure; the second pressure line 112 leading to the first shift valve 56 so as to supply the line pressure to it; and the third pressure line 113 being brought into communication with a pressure line 114 so as to supply the line pressure as control source pressure to the third duty valve 68 when the second shift valve 57 has placed its spool in the left end position. A pressure line 121 downstream from the first duty valve 66 which is supplied with control source pressure at the upstream side thereof leads to the lock-up control valve 59 and is brought into communication with the pressure chamber of forward clutch 41 through a forward clutch pressure line 122 equipped with an orifice 73 when the lock-up control valve 59 has placed its spool in the right end position. In this connection, a pressure line 123 branching off from the forward clutch pressure line 122 is connected to the first shift valve 56. A pressure line 124 downstream from the second duty valve 67 is equipped with an orifice 74 and leads to the second shift valve 57 and to the first shift valve 56 through a pressure line 125 branching off from the pressure line 124. A pressure line 126 downstream from the third duty valve 68, which is called a servo-apply pressure line, leads to the servo-apply pressure chamber 44a. The reverse pressure line 108, into which the line pressure is introduced through the manual shift valve 52 in the reverse (R) range, leads to the second shift valve 57 so as to deliver the line pressure to the second shift valve 57. In this instance, the reverse pressure line 108 also delivers the line pressure to the intensifying port 51 of regulator valve 51 so as to regulate the line pressure to a level generally higher in the reverse (R) range than in the forward ranges. A pressure line 128 branching off from the pressure line 127 leads to the control port 57b of second shift valve 57 so as to deliver the line pressure to the second shift valve 57 as pilot pressure for forcing it to place its spool in the right end position. The lock-up control valve 59 is supplied with operating pressure through a torque converter pressure line 131 via a relief valve 75 from the regulator valve 51 and is connected to the pressure line 121 downstream from the first duty valve 66. Further, the lock-up control valve 59 is supplied at its control port 59a with pilot line pressure through the pressure line 105 via the solenoid valve 62. While the solenoid valve 62 has been OFF, the pilot pressure forces the lock-up control valve 59 to place its spool in the right end position, bringing the torque converter pressure line 131 into communication with a pressure line 132 leading to a front pressure chamber (T/CF) 26a of the lock-up clutch 26 so as to supply operating pressure to the front pressure chamber (T/CF) 26a, thereby unlocking the lock-up clutch 26. Simultaneously, the pressure line 121 downstream from the duty valve 66 is brought into communication with the forward clutch pressure line 122 through the lock-up control valve 59 with a result of supplying operating pressure regulated by the first duty valve 66 to the pressure chamber of forward clutch 41 through the forward clutch pressure line 122. As apparently shown in FIG. 3, outlets of both front pressure chamber (T/CF) 26a and rear pressure chamber (T/CR) 26b of lock-up clutch 26 are connected with each other by means of a pressure relief line 133 equipped with an oil cooler 76. When the second ON-OFF valve 62 is ON, operating pressure is drained out from the control port 59a of lock-up control valve 59 through the pressure line 105. Resultingly, the lock-up control valve 59 shifts its spool to the left end position, so as to bring the torque converter pressure line 131 into communication with a pressure line 134 leading to the rear pressure chamber (T/CR) 26b of lock-up clutch 26, thereby forcing the operating pressure into the rear pressure chamber (T/CR) 26b of lock-up clutch 26 and locking the lock-up clutch 26. Simultaneously, the pressure line 132 leading to the rear pressure chamber 26b (T/CR) of lock-up clutch 26 is brought into communication with the pressure line 121 downstream from the first duty valve 66 through the lock-up control valve 59, enabling the first duty valve 66 to regulate the operating pressure in the rear pressure chamber 26b of lock-up clutch 26. As a result, the lock-up clutch 26 is controlled precisely in locked condition, which is always desirable to effectively increase power transmission performance of the torque converter 20. A source pressure supply line 135, which is installed between the lock-up control valve 59 and first shift valve 56, is brought into communication with the forward clutch pressure line 122 through the lock-up control valve 59 while the lock-up control valve 59 has placed its spool in the left end position, i.e. the first duty valve 66 has been in communication with the pressure line 132. Further, the source pressure supply line 135 is brought into communication with the pressure line 112 branching off from the forward pressure line 107 through the first shift valve 56 while the first shift valve 56 has placed its spool in the left end position, and with the drain port 56b of first shift valve 56 while the first shift valve 56 has placed its spool in the right end position. In the hydraulic pressure control circuit 50 of this embodiment, the first shift valve 56 is connected to the accumulator 72 through a pressure line 136 as well as being connected to the pressure line 123 branching off from the forward clutch pressure line 122. While the first shift valve 56 places its spool in its left end position, the pressure line 136 is in communication with the branch pressure line 123 so as to force the accumulator 72 to discharge the pressure into the pressure chamber of forward clutch 41. That is, accumulation of pressure in the accumulator 72 and the discharge of accumulated pressure to the forward clutch 41 are managed by shifting the spool of first shift valve 56. The first shift valve 56 is connected to the servo release pressure chamber 44b of 2-4 brake 44 through a servo release pressure line 141. The servo release pressure chamber 44b of 2-4 brake 44 and the pressure chamber of forward clutch 41 are brought into communication through the forward clutch pressure line 122, pressure lines 123 and 141 and simultaneously supplied with operating pressure when the first shift valve 56 shift its spool to the right end position. Further, the first shift valve 56 and second shift valve 57 are interconnected by means of the pressure lines 124 and 125. The servo release pressure chamber 44b of 2-4 brake 44, leading to the first shift valve 56, and the pressure chamber of 3-4 clutch 43 leading to the second shift valve 57 through a 3-4 clutch pressure line 142 are brought into communication with each other through the pressure lines 141, 142, 124 and 125 when both first shift valve 56 and second shift valve 57 shift their spool to their left end positions, respectively. Causing the first shift valve to shift the spool in position changes communication of the servo release pressure chamber 44b of 2-4 brake 44 between with the forward clutch 44 and with the 3-4 clutch 43. The second shift valve 57 is connected with the reverse clutch 42 and the low reverse brake 45 through a reverse clutch pressure line 143 and a low reverse brake pressure line 144, respectively, as well as being connected to the servo apply pressure chamber 44a of 2-4 brake 44 through the pressure lines 114 and 126 via the third duty valve 68. Accordingly, the 2-4 brake 44 receives at the servo apply pressure chamber 44a servo apply pressure directly from the third duty valve 68. While the second shift valve 57 has placed its spool in the left end position, it keeps the 3-4 clutch 43 in communication with second duty valve 67 through the pressure lines 124 and 142 and provides communication between the pressure lines 113 and 114, supplying operating pressure to the third duty valve 68. During this, both reverse clutch 42 and low reverse brake 45 are connected to the drain ports 57c and 57d of second shift valve 57, respectively. On the other hand, while the second shift valve 57 has placed its spool in the right end position, it keeps the pressure chamber of reverse clutch 42 in communication with the reverse pressure line 108 as well as connecting the low reverse brake 45 to the second duty valve 67 through the pressure lines 124 and 144. In this instance, the 3-4 clutch 43 and the servo apply pressure chamber of 2-4 brake 44 are connected to the drain ports 57c and 57d of second shift valve 57, respectively. As described above, the second shift valve 57 has the drain port 57a common to pressure chambers of 3-4 clutch 43 and reverse clutch 42 for pressure discharge and the drain port 57b common to the pressure chamber of low reverse brake 45 and the servo apply pressure chamber 44a of 2-4 brake 44 for pressure discharge. This structure of second shift valve 57 makes it possible to install a shortened axial length of second shift valve 57 as compared with cases where the shift valve 57 is provided with drain ports peculiar to the respective friction coupling elements 41-45. As was previously described, while, in gears where simultaneously locking of two of the friction coupling elements 41-45, namely the 3-4 clutch 43 and low reverse brake 45, the low reverse brake 45 and 2-4 brake 44, and the 2-4 brake 44 and reverse clutch 42, will occur, interlocking causes in an automatic transmission, nevertheless, the hydraulic pressure control circuit 50 of this invention prevents the automatic transmission 10 from interlocking, which results from changing pressure lines with the use of a single shift valve, i.e. the second shift valve in this embodiment. Specifically stating, when the 3-4 clutch 43 or 2-4 brake 44 is locked, which occurs while the second shift valve 57 has placed its spool in the left end position, both low reverse brake 45 and reverse clutch 42 are simultaneously unlocked. When the low reverse brake 45 or reverse clutch 42 is locked, which occurs while the second shift valve 57 has placed its spool in the right end position, the 3-4 clutch 43 and 2-4 brake 44 are simultaneously unlocked. In such a way, the interlocking of the automatic transmission 10 is certainly prevented by changing pressure lines with the use of the second shift valve 57 which is realized by a relatively simple structure. Supply of operating pressure to two friction coupling elements which are not simultaneously locked, namely the 3-4 clutch 43 and low reverse brake 45, is performed through the selective connection of second duty valve 67 to the two friction coupling elements caused by shifting the spool of second shift valve 57, enabling to provide a simple structure of the hydraulic pressure control circuit 50. While the second shift valve 57 is supplied at the control port 57b with the control source pressure through the pressure lines 127 and 128 from via the manual shift valve 52, it is also supplied at another control port 57a with the solenoid pressure from the first solenoid valve 61 through the pressure lines 102 and 104 via the third shift valve 58. The second shift valve 57 is forced to shift its spool with pilot pressure at either one of the control ports 57a and 57b, selectively providing operative pressure lines. The third shift valve 58 is connected at the upstream side with pressure lines 145 and 146 branching off from the pressure line 101 leading to the reducing valve 71 and a line pressure 147 branching off from the pressure line 124 downstream from the second duty valve 67, as well as the pressure line 102. Together, the third shift valve 58 is connected at the downstream side with the pressure line 148 leading to the pressure line 125 upstream from the first shift valve 56 and a line pressure 149 branching off from the 3-4 clutch pressure line 142 extending between the second shift valve 57 and 3-4 clutch 43, as well as the pressure lines 103 and 104. While the third shift valve 58 has placed its spool in the right end position, it keeps the pressure line 146 in communication with the pressure line 103, directing the fixed level of pilot pressure to the first shift valve 56 from the reducing valve 71 so as to cause the first shift valve 56 to shift its spool to the left end position. At this time, the second shift valve 57 is brought into communication with the first solenoid valve 61 through the pressure lines 102 and 104, allowing the solenoid pressure from the first solenoid valve 61 to be directed to the second shift valve 57 as pilot pressure. When the second shift valve 57 receives the pilot pressure at the control port 57a, it shifts its spool to the left end position and, as a result, brings the 3-4 clutch 43 into communication with the second duty valve 67 through the pressure lines 124 a nd 142. Resultingly, while operating pressure is supplied to the pressure chamber of 3-4 clutch 43 from the second duty valve 67, the operating pressure is also supplied to the control port 58a of third shift valve 58 through the pressure line 149. When the operating pressure exceeds a fixed level, the third shift valve 58 shift its spool to the left end position and, as a result, brings the pressure line 145 into communication with the pressure line 104, directing the fixed level of pilot pressure to the control port 57a of second shift valve 57 from the reducing valve 71. At this time, the first solenoid valve 61 is brought into communication with the control port 56a of first shift valve 56 through the pressure lines 102 and 103 and, as a result, allows the solenoid pressure from the first solenoid valve 61 to be directed to the first shift valve 56 as pilot pressure. As stated above, it can be summarized that the 3-4 clutch 43 is a friction coupling element which is locked in forward gears higher than the third gear and unlocked in gears slower than the second gear and that the first shift valve 56 is one which shifts between gears slower than the third gear where the forward clutch 41 is locked and the forth gear in which forward clutch 41 is unlocked, in other words, in a range of high speed gears. The second shift valve 57 shifts with a result of locking and unlocking the low reverse brake 45. That is, it is a shift valve which shifts between the second gear and the first gear in which engine brake is available, in other words, in a range of slow speed gears. Consequently, these first shift valve 56 and second shift valve 57 do not operate in any way simultaneously. By means of the hydraulic pressure control circuit 50, the first solenoid valve 61 is selectively connected to the control ports 56a and 56b of first shift valve 56 according to operated conditions of 3-4 clutch 43 which is differently operated between the high speed gears higher than the third gear and the low speed range lower than the second gear. This enables to supply solenoid pressure the shift valves 56 and 57 so as to operate them at a most appropriate timing and to use a single element, i.e. the solenoid valve 61, for shift operations of the shift valves 56 and 57. In other words, the first solenoid valve 61 is used commonly to the first shift valve 56 and second shift valve 57 which are in no way simultaneously operative, enabling to provide a simple structure of the hydraulic pressure control circuit 50. As was stated previously, the pressure line 125, which is brought into communication with the pressure line 141 through which the first shift valve 56 is connected to the servo release pressure chamber 44b of 2-4 brake 44 when the first shift valve 56 shifts its spool to the left end position, branches off from the pressure line 124 which extends from the second duty valve 67 to the second shift valve 57 and is equipped with the orifice 74. Further, the pressure line 125 is connected to the pressure line 148 leading to the third shift valve 58. When a first to second gear shift occurs, in other words, when operating pressure is supplied to the servo apply pressure chamber 44a of 2-4 brake 44 and is simultaneously discharged from the servo release pressure chamber 44b of 2-4 brake 44, the 3-4 clutch 43 is in no way supplied with operating pressure and the third shift valve 58 is not supplied at the control port 58a with pilot pressure, so that the third shift valve 58 holds the spool in the right end position, thereby keeping the pressure lines 147 and 148 in communication with each other. As a result, the operating pressure discharged from the servo release pressure chamber 44b through the pressure lines 141 and 125 runs not through the orifice 74 but through the pressure lines 148 and 147 via the third shift valve 58. It can accordingly be said that the pressure lines 148 and 147 form a bypass pressure line with regard to the orifice 74. Discharge of operating pressure from the servo release pressure chamber 44b is made quickly through the bypass pressure line 147 and 148, preventing a delay of operation of 2-4 brake in response to a gear shift which occurs due to residual operating pressure in the servo release pressure chamber 44b. During a second to third gear shift, i.e. when operating pressure is supplied to the servo release pressure chamber 44b of 2-4 brake 44, the operating pressure is introduced through the orifice 74. In this case, operating pressure is supplied to both servo release pressure chamber 44b of 2-4 brake 44 and pressure chamber of 3-4 clutch 43 at approximately the same level with the effect of providing comfortable feelings of gear shift. Shifting the third shift valve 58 necessary to directing operating pressure to the bypass pressure line 147 and 148 is caused by the operating pressure in the pressure chamber of 3-4 clutch 43 which is free from the first to second gear shift. Specifically, the third shift valve 58 is configured so that it shifts the spool in position to shut off the bypass pressure line 147 and 148 at a time when the operating pressure reaches a level slightly higher than a piston thrust force level for the 3-4 clutch 43. With the configuration, during the second to third gear shift, after the 3-4 clutch 43 and 2-4 brake 44 start their locking and unlocking operations resulting from the completion of a piston stroke of the 3-4 clutch 43, the orifice 74 is involved in the supply of operating pressure to the servo release pressure chamber 44b of 2-4 brake 44, causing smooth shift operations of the 2-4 brake 44 and 3-4 clutch 43 with the effect of providing comfortable feelings of gear shift. During the first to second gear shift, the operating pressure is discharged quickly from the servo release pressure chamber 44b of 2-4 brake 44. In this instance, while there is possibly a part of operating pressure impermanently left over in the servo release pressure chamber 44b of 2-4 brake 44 which in turn potentially reaches the control port 58a of third shift valve 58 through the pressure lines 125, 124 and 149 via the second shift valve 57, however, since the operating pressure necessary for the third shift valve 58 to make the bypass pressure line 147 and 148 function is set higher in level than the piston thrust force level, the residual pressure does not give any effect to the third shift valve 58. This is because that, during the shift operations of the3-4 clutch 43 and 2-4 brake 44, the operating pressure is developed so as to be held in both servo release pressure chamber 44b of 2-4 brake 44 and pressure chamber of 3-4 clutch 43 at approximately the same level and that, during the first to second gear shift, the third shift valve 58 is prevented from shutting off the bypass pressure line 147 and 148 which is caused by accidental shift operation due to the residual operating pressure in the servo release pressure chamber 44b of 2-4 brake 44. As shown in FIG. 4, the hydraulic control circuit 50 cooperates with a controller 150 for controlling operations of the valves 61, 62 and 66-69. The controller 150 receives various control signals, such as a signal representative of a vehicle speed from a speed sensor 151, a signal representative of engine throttle position or opening as an engine load from a throttle opening sensor 152, and a signal representative of a shift position or range selected by a shift lever from a position sensor 153, on the basis of which the controller 150 controls the various valves 61, 62 and 66-69 according to engine operating conditions. The following description will be directed to operation of the valves 61, 62 and 66-68 in connection with operating pressure supply to the friction coupling elements 41-45 in each gear shift. The valves 61, 62 and 66-68 operate in various patterns for the respective gears as shown in the following Tables II-VI. The Tables II, III and IV indicate patterns for up-shift, down-shifts and a down-shift to the first gear in the low speed (L) range, respectively. Further, the Tables V and VI indicates patterns for range selection through the manual shift valve 52 and lock-up control in the third gear and fourth gear, respectively. In each Table II-VI, a circle indicates the ON or activated state of each valve 61, 62, 66-68 in which a pressure line upstream is shut off to drain operating pressure out of a downstream pressure line of each related valve; a cross or X indicates the OFF or deactivated state of each valve 61, 62, 66-68 in which an upstream pressure line and downstream pressure line from each related valve are in communication with each other so as to allow operating pressure to be directed to the downstream pressure line; and a dark spot indicates that each duty valve 66-68 regulates source control pressure to a fixed level and directs it downstream. PL, Pacc, Ptc denote line pressure or control source pressure, accumulator pressure and converter pressure, respectively, and P1, P2 and P3 denote operating pressure regulated by the first, second and third duty valves 66, 67 and 68, respectively. Further, an arrow indicates a direction of gear shift or a direction of range shift. For instance, 3.fwdarw.4 indicates a gear shift from the third gear to the fourth gear; and N.rarw.D indicates a range shift from the drive (D) range to the neutral (N) range. In Table VI, a label L/U indicates a lock-up control condition. TABLE II ______________________________________ Gear 1 1.fwdarw.2 2 2.fwdarw.3 3 3.fwdarw.4 4 ______________________________________ FWD PL PL PL PL PL -- -- S/A -- P3 PL P3 PL P3 PL S/R -- -- -- P2 PL -- -- 3-4 -- -- -- P2 PL PL PL L/R -- -- -- -- -- -- -- REV -- -- -- -- -- -- -- T/CF Ptc Ptc Ptc Ptc Ptc Ptc Ptc T/CR Ptc Ptc Ptc Ptc Ptc Ptc Ptc 1st SV X X X X X .largecircle. .largecircle. 2nd SV X X X X X X X lst DSV X X X X X .largecircle. .largecircle. 2nd DSV .largecircle. .largecircle. .largecircle. .circle-solid. X X X 3rd DSV .largecircle. .circle-solid. X .circle-solid. X .circle-solid. X ______________________________________ TABLE III ______________________________________ Gear 1 1.rarw.2 2 2.rarw.3 3 *3.rarw.4 **3.rarw.4 4 ______________________________________ FWD PL PL PL PL PL Pacc PL -- S/A -- P3 PL P3 PL P3 P3 PL S/R -- -- -- P2 PL PL PL -- 3-4 -- -- -- P2 PL PL PL PL L/R -- -- -- -- -- -- -- -- REV -- -- -- -- -- -- T/CF Ptc Ptc Ptc Ptc Ptc Ptc Ptc Ptc T/CR Ptc Ptc Ptc Ptc Ptc Ptc Ptc Ptc 1st SV X X X X X X .largecircle. .largecircle. 2nd SV X X X X X X X X 1st DSV X X X X X X .largecircle. .largecircle. 2nd DSV .largecircle. .largecircle. .largecircle. .circle-solid. X X X X 3rd DSV .largecircle. .circle-solid. X .circle-solid. X .circle-solid. .circle-solid. X ______________________________________ *Power Off 43 shift; **Power On 43 shift TABLE IV ______________________________________ Gear L1 L1.rarw.2 2 ______________________________________ FWD PL PL PL S/A -- -- PL S/R PL P2 -- 3-4 -- -- -- L/R PL P2 -- REV -- -- -- T/CF Ptc Ptc Ptc T/CR Ptc Ptc Ptc 1st SV .largecircle. .largecircle. X 2nd SV X X X 1st DSV X X X 2nd DSV X .circle-solid. .largecircle. 3rd DSV .largecircle. .largecircle. X ______________________________________ TABLE V ______________________________________ Gear N N.fwdarw.D(3) D(1) ND NR R ______________________________________ FWD -- Pacc Pacc PL Pacc -- -- S/A -- -- -- -- -- -- -- S/R -- P2 -- -- -- -- -- 3-4 -- P2 -- -- -- -- -- L/R -- -- -- -- -- P2 PL REV -- -- -- -- -- PL PL T/CF Ptc Ptc Ptc Ptc Ptc Ptc Ptc T/CR Ptc Ptc Ptc Ptc Ptc Ptc Ptc 1st SV X X X X X X X 2nd SV X X X X X X X 1st DSV X .circle-solid. .circle-solid. X X X X 2nd DSV .largecircle. .circle-solid. .largecircle. .largecircle. .largecircle. .circle-solid. X 3rd DSV .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. ______________________________________ TABLE VI ______________________________________ Gear 3 3.about.3L/U 3L/U 4 4.about.4L/U 4L/U ______________________________________ FWD PL PL PL -- -- -- S/A PL PL PL PL PL PL S/R PL PL PL -- -- -- 3-4 PL PL PL PL PL PL L/R -- -- -- -- -- -- REV -- -- -- -- -- -- T/CF Ptc P1 -- Ptc P1 -- T/CR Ptc Ptc Ptc Ptc Ptc Ptc 1st SV X X X .largecircle. .largecircle. .largecircle. 2nd SV X .largecircle. .largecircle. X .largecircle. .largecircle. 1st DSV X .circle-solid. .largecircle. .largecircle. .circle-solid. .largecircle. 2nd DSV X X X X X X 3rd DSV X X X X X X ______________________________________ As shown in Tables II and III and in FIG. 5, in the first (1) gear in forward ranges excepting the low-speed (L) range, the second solenoid valve 62 is OFF, directing the pilot pressure to the control port 59a of lock-up control valve 96 through the pressure line 105. This causes the lock-up control valve 59 to shift its spool to the right end position as viewed in the FIG. 5, bringing the pressure line downstream from the first duty solenoid valve 66 into communication with the forward clutch pressure line 122. On the other hand, the second duty solenoid valve 66 is OFF, directing the line pressure directly to the pressure chamber of forward clutch (FWD) 41, so as to lock the forward clutch (FWD) 41 into coupling. At the beginning, the line pressure is supplied gently to the forward clutch (FWD) 41 through the orifice 73. Together, the converter pressure is supplied to the front chamber (T/CF) 26a of lock-up clutch 26 through the pressure line 131, unlocking the lock-up clutch 26. Further, in the first (1) gear, the second duty solenoid valve 67 is turned ON, draining the control port 58a of third shift valve 58 through the pressure lines 124, 142 and 149. As a result, the third shift valve 58 shifts its spool to the right end position, bringing the pressure line 146 into communication with the pressure line 103 so as to supply a fixed level of pressure to the control port 56a of first shift valve 56 through the pressure lines 101, 146 and 103 from the reducing valve 71 (see FIG. 3). Resultingly, the first shift valve 56 shifts its spool to the right end position, bringing the pressure line 123 branching off from the forward clutch pressure line 122 into communication with the pressure line 136 leading to the accumulator 72. The locking of the forward clutch 41 during, for instance, arrange shift from the neutral (L) to the drive (D) progresses gently due to the operation of the accumulator 72. The combined effects of the accumulator 72 and orifice 73 reduces shift shock occurring in the automatic transmission. In the first (1) gear where the first solenoid valve 61 is OFF, the pilot pressure is supplied to the control port 57a of second shift valve 57 through the pressure lines 102 and 104 via the third shift valve 58, forcing the second shift valve 57 to shift the spool to the left end position. As a result, the pressure line 114 leading to the third duty solenoid valve 68 is brought into communication with the forward pressure line 107 leading to the, so that, although the line pressure is supplied to the third duty solenoid valve 68 having been ON and drained at the downstream side, it prevents the line pressure from being delivered to the servo apply pressure chamber (S/A) 44a of 2-4 brake 44. As shown in Tables II and III and in FIG. 6, in the second (2) gear, the third duty solenoid valve 68 turns OFF, directing the line pressure directly to the servo apply pressure chamber (S/A) 44a of 2-4 brake 44 through the pressure lines 107, 114 and 126 via the second shift valve 57. The remaining valves 61, 62 66 and 67 hold the same states as in the first (1) gear. Resultingly, the 2-4 brake (2-4B) 44, in addition to the forward clutch (FWD) 41, is locked. As shown in Table II, in a transitional pattern (1.fwdarw.2) between the first (1) gear and second (2) gear, since the third duty solenoid valve 68 duty-controls the servo apply pressure and supplies it to the 2-4 brake (2-4B) 44, the locking of 2-4 brake (2-4B) 44 is smooth. After the achievement of shift to the second (2) gear, the third duty solenoid valve 68 terminates the control of line pressure and supplies the line pressure directly to the servo apply pressure chamber (S/A) 44a of 2-4 brake 44. In this instance, the third shift valve 58 has been placed its spool in the right end position and, consequently, holds the bypass pressure lines 147 and 148 in communication so as to form the pressure bypass line, the operating pressure is released not through the orifice 74 but through the bypass pressure line 147 and 148 during the first (1) to second (2) gear shift. This prevents a shift delay which is possibly caused by residual pressure left in the servo release pressure chamber (S/R) 44b of 2-4 brake 44. As shown in Table III, during a down-shift from the second (2) gear to first (1) gear, the same transitional pattern as during the up-shift from the first (1) gear to second (2) gear takes place. As shown in Tables II and III and in FIG. 7, in the third (3) gear, the second duty solenoid valve 67 takes the OFF state, delivering the line pressure directly to the first shift valve 56 and second shift valve 57 through the pressure line 125 and 124, respectively, from the main pressure line 100, and forces these shift valve to shift their spool to the left end positions, respectively. As a result, the line pressure is supplied to both pressure chamber of 3-4 clutch 43 and the servo release pressure chamber (S/R) 44b of 2-4 brake 44 through the pressure line 142 and 141, respectively, locking, on one hand, the 3-4 clutch 43 and unlocking, on the other hand, the 2-4 brake 44. The remaining valves 61, 62 66 and 68 hold the same states as in the second (2) gear. In a transitional pattern (2.fwdarw.3) between the second (2) gear and third (3) gear, since the second duty solenoid valve 67 duty-controls operating pressure on the basis of the line pressure from the main pressure line 100 and directs it to the first shift valve 56 and second shift valve 57 through the pressure lines 125 and 124, respectively. Because these pressure lines 124 and 125 leads to the 3-4 clutch pressure line 142 and servo release pressure line 141, respectively, the operating pressure regulated by the second duty solenoid valve 67 is supplied to both pressure chamber of3-4 clutch 43 and the servo release pressure chamber (S/R) 44b of 2-4 brake 44 as 3-4 clutch pressure and servo release pressure, respectively. The locking of 3-4 clutch 43 and the unlocking of 2-4 brake 44 are caused favorably. During the second (2) to third (3) gear shift, while the second duty solenoid valve 67 duty-controls the 3-4 clutch pressure and servo release pressure, the third duty solenoid valve 68, through which the line pressure was supplied to the servo apply pressure chamber (S/A) 44a of 2-4 brake 44 in the second (2) gear, duty-controls the servo apply pressure. Consequently, the locking of 3-4 clutch 43 and the unlocking of 2-4 brake 44 are timely caused with well reduced shift shock. At this time, since communication has been provided between the pressure chamber of3-4 clutch 43 and servo release pressure chamber (S/R) 44b of 2-4 brake 44, the operating pressure regulated by the second duty solenoid valve 67, after adjustment in supply timing by the orifice 74, is delivered to both pressure chamber of 3-4 clutch 43 and servo release pressure chamber (S/R) 44b of 2-4 brake 44 and held at approximately the same level in these pressure chambers. Although the third shift valve 58 holds its spool in the right end position so as to provide the bypass pressure line 147 and 148 in the second (2) gear immediately before the second (2) to third (3) gear shift, the third shift valve 58, as was previously described, forces the spool to the left end position when the3-4 clutch pressure raises and reaches slightly above the piston thrust force level, so as to disconnect the pressure lines 147 and 148 and shut off them as the bypass pressure line. Accompanying this, the pressure line 102 is changed in communication from with the pressure line 104 to with the pressure line 103. Resultingly, while the pilot pressure is directed to the control port 56a of first shift valve 56 from the first solenoid valve 61, the fixed level of pressure is directed to the control port 57a of second shift valve 57 through the pressure lines 145 and 104 from the reducing valve 71. As shown in Table III, during a down-shift from the third (3) gear to second (2) gear, the same transitional pattern as during the up-shift from the second (2) gear to third (3) gear. As shown in Tables II and III and in FIG. 8, while the first duty solenoid valve 66 is ON, through which the downstream forward clutch pressure line 122 is drained, the first solenoid valve 61 is ON, through which the control port 56a of first shift valve 51 is drained. Resultingly, the first shift valve 51 shift the spool to the right end position, so as to bring the forward clutch pressure line 122 into communication with the servo release pressure line 141 through the pressure line 123, thereby discharging simultaneously the forward clutch pressure and servo release pressure through the first duty solenoid valve 66 and pressure line 121 or through the first shift valve 56 and source pressure supply line 135 according to the operated condition of lock-up control valve 59. That is, the forward clutch 41 is unlocked, and the 2-4 brake 44 is locked again. As shown in Table II, in a transitional pattern (3.fwdarw.4) between the third (3) gear and forth (4) gear, since the third duty solenoid valve 68 duty-controls the operating pressure so as to provide appropriately regulated servo supply pressure, the third (3) to fourth (4) gear shift is caused smoothly without accompanying significant shift shock. Further, as shown in Table III, during a down-shift from the forth (4) gear to third (3) gear in a power-on mode, the first duty solenoid valve 66 turns OFF, directing the operating pressure from the pressure line 121 to the pressure chamber of forward clutch 41 through the forward clutch pressure line 122 via the lock-up control valve 59 and the servo release pressure chamber (S/R) 44b of 2-4 brake 44 through the pressure lines 123 and 141 via the first shift valve 56. Resultingly, the locking of forward clutch 41 and the unlocking of 2-4 brake 44 are simultaneously caused. During this, the third duty solenoid valve 68 duty-controls the servo apply pressure with the effect of performing unlocking smoothly the 2-4 brake. As shown in FIG. III, during the down-shift from the forth (4) gear to third (3) gear in a power-off mode in which the forward clutch 41 locks, the first solenoid valve 61 turns OFF, forcing the first shift valve to shift the spool to the left end position. The operating pressure having been accumulated in the accumulator in the fourth (4) gear is delivered to the pressure chamber of forward clutch 41. This results in shortening a time necessary to fill the pressure chamber of forward clutch 41 and the forward clutch pressure line 122 with the operating pressure and, as a result, enabling the forward clutch to lock quickly. As shown in Table IV and in FIG. 9, in the same manner as in the first gear in the drive (D) range, the first solenoid valve 62 delivers the solenoid pressure to the control port 59 a of lockup control valve 59, forcing the lock-up shift valve to shift the spool to the right end position. The duty solenoid valve 66 delivers the line pressure supplied through the pressure lines 107 and 111 to the forward clutch (FWD) 41 through the forward clutch pressure line 122 as the forward clutch pressure, locking the forward clutch 41. Simultaneously, the first solenoid valve 61 turns ON with a result of draining the pressure in the downstream pressure line, forcing the second shift valve 57 to shift the spool to the right end position. The second duty solenoid valve 67 delivers the line pressure to the low reverse brake pressure line 144 and servo release pressure line 141. Resultingly, in the first (1) gear in the low speed (L) range, both forward clutch 41 and low reverse brake 45 are simultaneously locked, creating the first (1) gear having the effect of engine brake. As shown in Table V and in FIG. 10, in the same manner as in the first (1) gear in the low speed (L) range, the second duty solenoid valve 67 delivers the line pressure to the low reverse brake pressure line 144 and servo release pressure line 141, by means of which the low reverse brake 45 is locked. Simultaneously, the line pressure is introduced into the reverse pressure line 108 leading from the manual shift valve 52 and then delivered to the reverse clutch pressure line 143 passing through the second shift valve 57. Resultingly, the reverse clutch 42 is supplied with the operating pressure. In this manner, locking is caused on both low reverse brake 45 and reverse clutch 42. In this instance, since the line pressure is not introduced into the forward pressure line 107 in the reverse (R) range, the forward clutch 41 is in no way supplied with operating pressure in spite of the operated condition of the first duty solenoid valve 66. As shown in Table VI and in FIGS. 11 and 12, when the lock-up control takes place in the third (3) gear, the second solenoid valve 62 turns ON with a result of draining the lock-up control valve 59 at the control port 59a, forcing the lock-up control valve 59 to shift the spool to the left end position. Communication is provided between the pressure line 121 downstream from the first duty solenoid valve 66 and pressure line 132 leading to the front pressure chamber (T/CF) 26a of lock-up clutch 26, and between the converter pressure line 131 pressure line 134 leading to the rear pressure chamber (T/CR) 26b of lock-up clutch 26. Simultaneously, the first duty solenoid valve 66 is drained at the downstream side. Resultingly, while the lock-up clutch 26 is supplied with the converter pressure to the rear pressure chamber (T/CR) 26b, it is drained from the front pressure chamber (T/CF) 26a, becoming locked. During the lock-up control, the line pressure is introduced into the forward clutch pressure line 122 from the forward pressure line 107 through pressure lines 122 and 135 via the first shift valve 56, holding the forward clutch 41 locked. With the hydraulic control circuit 50 equipped with the source pressure supply line 135, even while the first duty solenoid valve 66 executes the lock-up control, the forward clutch 41 is supplied with locking pressure through the forward clutch pressure line 122 from the source pressure supply line 135. During the control of transitional operation of the lock-up clutch 26 between locked and unlocked conditions (3.about.3L/U) and during the control of slippage of the lock-up clutch 26, the first duty solenoid valve 66 duty-controls of the locking pressure to the front pressure chamber 26a of lock-up clutch 26, performing the lock-up control and slippage control. When the lock-up control is made in the fourth (4) gear, in the same manner as in the third (3) gear, the second solenoid valve 62 turns ON, draining the lock-up control valve 59 at the control port 59a. Resultingly, as shown in FIG. 12, the lock-up shift valve 59 shifts the spool to the left end position, providing communication between the forward clutch pressure line 122 and source pressure supply line 135, between the pressure line 121 downstream from the first duty solenoid valve 66 and pressure line 132 leading to the front pressure chamber 26a of lock-up clutch 26, and between the converter pressure line 131 and pressure line 134 leading to the rear pressure chamber 26b of lock-up clutch 26. Simultaneously, the first duty solenoid valve 66 drains the downstream side. Since the first shift valve 56 holds the spool in the right end position, the source pressure line 135 also holds the communication with the drain port 56b of first shift valve 56. Accordingly, the converter pressure is supplied to the rear pressure chamber 26b of lock-up clutch 26 and locks the lock-up clutch 26. In this instance, the forward clutch pressure 122 is brought into communication with the drain port 56b of first shift valve 56 through the source pressure supply line 135, unlocking the forward clutch 41. With the hydraulic control circuit 50 thus structured, even while the first duty solenoid valve 66 performs the lock-up control, the forward clutch 41 is unlocked without any difficulty by means of shifting the spool of first shift valve 56. In this instance, during the control of transitional operation of the lock-up clutch 26 between locked and unlocked conditions (4.about.4L/U) and during the control of slippage of the lock-up clutch 26, the first duty solenoid valve 66 duty-controls of the locking pressure to the front pressure chamber 26a of lock-up clutch 26, performing the lock-up control and slippage control. In the hydraulic control circuit 50, if the lock-up clutch 26 is in the locked condition before the third (3) gear to fourth (4) gear shift, in order for the first duty solenoid valve 66 to hold the communication with the front pressure chamber 26a of lock-up clutch 26, the drain port 56a of first shift valve 56 is brought into communication with the source pressure supply line 135 so as to drain the pressure chamber of forward clutch (FWD) 41. On the other hand, if the lock-up clutch 26 is in the unlocked condition before the third (3) gear to fourth (4) gear shift, in order to reduce shift shock, the first duty solenoid valve 66 adjusts a timing of draining the pressure chamber of forward clutch (FWD) 41. For the purpose of certainly reducing shift shock during the fourth (4) to third (3) gear shift, while the lock-up control is prohibited during the fourth (4) to third (3) gear shift, the first duty solenoid valve 66 controls the operating pressure with a high accuracy. As described above, the hydraulic control circuit 50 is able to provide selectively the communication of first solenoid valve 61 with the first shift valve 56 and the second shift valve 57 according to the operating pressure of 3-4 clutch 43, in other words, according to whether a gear shift is made to high gears or low gears. The first solenoid valve 61 supplies operating pressure to the first shift valve 56 or the second shift valve 57 at a timing that it is brought into communication with. The first solenoid valve 61 is available commonly to the first shift valve 56 and second shift valve 57 which does not simultaneously function, enabling the hydraulic control circuit 50 to be simple in structure, to be spaceless and to be manufacturable at low costs. FIG. 13 shows a basic structure of a hydraulic pressure control circuit 1000 in accordance with another embodiment of the present invention, in which friction coupling elements and various valves per se are the same in structure and operation. The hydraulic pressure control circuit 1000 includes various valves as essential elements, namely a regulator valve 1001 for generating a line pressure (control source pressure), a manual shift valve 1002 for shifting ranges of the automatic transmission 10 according to manual range selections through a shift lever, first to third shift valves 1026-1028 (which are hereafter referred to as first to third duty solenoid valves (DSVs)) for changing pressure lines according to gears, first and second solenoid valves 1011 and 1012 (which are hereafter referred to as first and second solenoid valves (SVs), respectively, for simplicity) utilized to supply actuating or operating pressure to various shift valves, including a low reverse shift valve 1003, a bypass control valve 1004, a 3-4 shift valve 1005 and a lock-up control valve 1006. The first to third duty solenoid valves 1021-1023 are utilized to generate, regulate and discharge operating pressure to pressure chambers of the friction coupling elements 41-45. The hydraulic pressure control circuit 1000 further includes a solenoid relay valve (which is hereafter referred to simply as a relay valve) 1007 for selectively supplying operating pressure introduced from the first solenoid valve 1011. Those solenoid valves 1011, 1012 and 1021-1023 are of a three-way type having operating modes, namely a communication mode where hydraulic pressure is allowed to flow in a pressure line from the upstream side to the downstream side and vice versa and a drain mode where hydraulic pressure is drawn from the downstream pressure line. In the drain mode, since the valve shuts off its related upstream pressure line during draining hydraulic pressure from the downstream pressure lines, there is no aimless drainage of hydraulic pressure from the upstream pressure line, decreasing driving loss of the oil pump 12. Each of the solenoid valves 1011 and 1012 opens the upstream and downstream pressure lines when it turns ON. Each of the duty solenoid valves 1021-1023 provides its full opening so as to allow the whole part of pressure to run therethrough when it turns OFF or operates at a duty rate of 0%,. On the other hand, when each duty solenoid valve 1021-1023 turns ON, it shuts off its elated upstream pressure line so as to drain the downstream pressure line during operating at a duty rate of 100% and regulates operating pressure from the upstream pressure line according to duty rates at which it operates and delivers it to the downstream pressure line. The regulator valve 1001 regulates operating pressure discharged by the oil pump 12 to a fixed level of line pressure which in turn is delivered to the manual shift valve 1002 through a main pressure line 100 and also to a solenoid reducing valve (which is hereafter referred to simply as a reducing valve) 1008 and the 3-4 shift valve 1005. The line pressure is reduced by the reducing valve 1008 to a fixed pressure level and then, delivered to the first and second solenoid valves 1011 and 1012 through pressure lines 1101 and 1102, respectively. When the first solenoid valve 1011 is ON, the fixed level of line pressure is delivered to the relay valve 1007 through a pressure line 1103 and, when the relay valve holds its spool in the right end position as viewed in FIG. 13, further directed to the control port 1004a of bypass control valve 1004 as pilot pressure through a pressure line 1104. As a result, the bypass control valve 1004 shifts its spool to the left end position as viewed in FIG. 13. On the other hand, when the relay valve 1007 holds the spool in the left end position, the fixed level of line pressure is delivered to the control port 1005a of 3-4 shift valve 1005 as a pilot pressure through a pressure line 1105 and forces the 3-4 shift valve 1005 to shift the spool to the right end position. When the second solenoid valve 1012 is ON, the fixed level of line pressure from the reducing valve 1008 is delivered to the bypass control valve 1004 through a pressure line 1006 and, when the bypass control valve 1004 holds its spool in the right end position, further directed to the control port 1006a of lock-up control valve 1006 as pilot pressure through a pressure line 1107. This causes the lock-up control valve 1006 to shift the spool to the left end position. On the other hand, when the bypass control valve 1004 holds the spool in the left end position, the fixed level of line pressure is delivered to the control port 1003a of low-reverse shift valve 1003 as a pilot pressure through a pressure line 1105 and forces the valve 1003 to shift the spool to the left end position. Further, the fixed level of line pressure from the reducing valve 1008 is also delivered to a regulation port 1001a of regulator valve 1001 through a pressure line 1109. In this instance, the line pressure is regulated according to, for instance, engine load by means of a linear solenoid valve 1031 installed in the pressure line 1109. In this way, the line pressure is regulated according to engine load by means of the regulator valve 1001. A main pressure line 1100 leading to the 3-4 shift valve 1005 is held in communication with a first accumulator 1041 through pressure line 1110 when the 3-4 shift valve holds its spool in the right end position, so as to introduce line pressure into the first accumulator 1041. The line pressure delivered to the manual shift valve 1002 through the main pressure line 1100 is introduced into a first output pressure line 1111 and a second output pressure line 1112 in each of forward ranges, i.e. the drive (D) range, slow speed (S) range and low speed (L) range, into the first output pressure line 1111 and a third output pressure line 1113 in the reverse (R) range, and into the third output pressure line 1113 in the neutral (R) range. The first output pressure line 1111 leads to the first duty solenoid valve 1021 and delivers the line pressure to the same as control source pressure. The second output pressure line 1112 leads to both second duty solenoid valve 1022 and third duty solenoid valve 1023 so as to supply the line pressure to them as control source pressure. The first duty solenoid valve 1021 leads at its downstream to the low-reverse shift valve 1003 through a pressure line 1114 and, when the low-reverse shift valve 1003 holds the spool in the right end position, further holds itself in communication with the servo apply pressure chamber 44a of 2-4 brake 44 through a servo apply pressure line 1115 and on the other hand, when the low-reverse shift valve 1003 holds the spool in the left end position, further holds itself in communication with the pressure chamber 44a of low-reverse brake 45 through a low-reverse pressure line 1116. A pressure line 1117 branches off from the pressure line 1114 and leads to a second accumulator 1042. The first output pressure line 1112 further leads to the3-4 shift valve 1005 and, when the 3-4 shift valve 1005 holds the spool in the left end position, keeps communication with the lock-up control valve 1006 through a pressure line 1118. When the lock-up control valve 1006 shifts its spool to the left end position, it brings the first output pressure line 1112 into communication with the pressure chamber of forward clutch (FWD) 41 through a pressure line 1119. A pressure line 1120, which branches off from the forward clutch pressure line 1119, leads to the 3-4 shift valve 1005. The 3-4 shift valve 1005 provides communication of the pressure line 1120 with the first accumulator 1041 through the pressure line 1110, when holding its spool in the left end position and, when holding its spool in the right end position, communication of the pressure line 1120 with the servo release pressure chamber 44b of 2-4 brake 44. The duty solenoid valve 1022 leads at its downstream side to the control port 1007a of relay valve 1007 through a pressure line 1122 and supplies pilot pressure with which the relay valve 1007 is forced to shift its spool to the left end position. A pressure line 1123 branching off from the pressure line 1122 leads to the low-reverse shift valve 1003. The low-reverse shift valve 1003 provides, when holding the spool in the right end position, communication of the pressure line 1123 with a pressure line 1124. Branching off from the pressure line 1124 through an orifice 1051 is a pressure line 1125 which in turn leads to the 3-4 shift valve 1005. The 3-4 shift valve 1005 provides, when holding the spool in the left end position, communication of the pressure line 1125 with the servo release pressure chamber 44b of 2-4 brake 44 through the pressure line 1121. Further, a pressure line 1126 branches off from the pressure line 1125 and leads to the bypass control valve 1004. The bypass control valve 1004 provides, when holding the spool in the right end position, communication of the pressure line 1126 with the 3-4 clutch 43 through a pressure line 1127. The pressure line 1124 leads directly to the bypass control valve 1004 which in turn provides communication of the pressure line 1124 with the pressure line 1125 through the pressure line 1126 when holding the spool in the left end position. That is, the pressure lines 1124 and 1125 form a bypass pressure line in connection with the orifice 1051. The third duty solenoid valve 1023 leads at the downstream side to the lock-up control valve 1006 through the pressure line 1128. The lock-up control valve 1006 provides communication of the third duty solenoid valve 1023 with the forward clutch pressure line 1119 when holding the spool in the right end position and with the front pressure chamber 26a of lock-up clutch 26 through a pressure line 1129 when holding the spool in the left end position. Similarly, the pressure line 1131 branching off from the third output pressure line 1113 leads to the bypass control valve 1004. The bypass control valve 1004 delivers the line pressure to the control port of low-reverse shift valve 1003 as pilot pressure through the pressure line 1108 during holding the spool in the right end position, forcing the low-reverse shift valve 1003 to shift the spool to the left end position. The hydraulic pressure control circuit 1000 is further provided with a converter relief valve 1009 which regulates the operating pressure introduced from the regulator valve 1001 through a pressure line 1132 to a fixed level and supplies it to the lock-up control valve 1006 through a pressure line 1133. The fixed level of operating pressure is delivered to the front pressure chamber 26a of lock-up clutch 26 through the pressure line 1129 when the lock-up control valve 1006 shifts the spool to the right end position and, on the other hand, to the rear pressure chamber 26b of lock-up clutch 26 through the pressure line 1134 when the lock-up control valve 1006 shifts the spool to the left end position. In this instance, the lock-up clutch 26 is released or unlocked resulting from filling the front pressure chamber 26a with the fixed level of operating pressure and locked resulting from filling the rear pressure chamber 26b with the fixed level of operating pressure. When the lock-up control valve 1006 holds the spool in the left end position during locking the lock-up clutch 26, the lock-up clutch 26 can locks with locking force according to the level of the operating pressure generated by the third duty solenoid valve 1023 which the lock-up clutch 26 receives in the front pressure chamber 26a. As was previously described, in the hydraulic pressure control circuit 1000, the regulator valve 1001 regulates the line pressure with the control pressure supplied by the linear solenoid valve 1031 to a level according, for instance, to a throttle opening or position. This regulation of line pressure is made differently amongst different ranges. Specifically, the line pressure is regulated to a level higher in the reverse (R) range than in the forward respective (D, S and L) ranges and neutral (N) range. For this purpose, as shown by way of example in FIG. 14, a regulator valve 1001' is typically provided with an extra intensifying port 1001b' to which line pressure is introduced from a manual shift valve 1002' in the reverse range only as well as a regulation port 1001a' to which pilot pressure is introduced from a linear solenoid valve 1031'. Such an extra port 1001b' needs installation of an auxiliary spool and its guide sleeve to the regulator valve 1001' in addition to a primary spool 1001c', which always undesirable in the light of the number of parts and overall size of the valve. In contradistinction to that, as shown in FIG. 13, the regulator valve 1001 utilized in the hydraulic pressure control circuit 1000 has, on one end of its spool, a regulation port 1001a to which pilot pressure is introduced from the linear solenoid valve 1031 and, on another end of the spool, a reduction port 1001b to which line pressure is introduced from the manual shift valve 1002 through the pressure line 1135 in each of the drive (D), slow speed (S), low speed (L) and neutral (N) ranges. This structure is intended to regulate line pressure not to a level higher in the reverse (R) range but to a level lower in the drive (D), slow speed (S), low speed (L) and neutral (N) ranges, with a result of divided arrangement of the control ports 1001a and 1001b on opposite sides of the spool 1001c which eliminates the necessity of an auxiliary spool and its associated parts. FIG. 15 shows a controller 1200 for controlling operations of the valves 1011, 1012, 1021-1023 and 1031. The controller 1200 receives various control signals, such as a signal representative of a vehicle speed from a speed sensor 1201, a signal representative of engine throttle position or opening as an engine load from a throttle opening sensor 1202, a signal representative of an engine speed in revolution from a speed sensor 1203, a signal representative of a shift position or range selected by a shift lever from a position sensor 1204, a signal representative of a turbine speed (Nt) in revolution of the torque converter 20 from a speed sensor 1205, and a signal representative of temperature of hydraulic oil in the control circuit 1000 from a temperature sensor 1206, on the basis of which the controller 1200 controls the valves 1011, 1012, 1021-1023 and 1031 according to engine operating conditions and/or vehicle traveling conditions. |
