A hydraulic control system for automatic transmissions, comprising:
- a hydraulic pump for pressurizing fluids;
- a torque converter for transmitting engine power to the automatic transmission;
- a pressure regulating valve connected to the hydraulic pump, together with a damper clutch, for varying line pressure by a damper clutch control solenoid valve controlled according to a duty ratio when converting a drive mode and driving at high speeds;
- a reducing valve connected to the hydraulic pump for reducing line pressure;
- a manual valve operated by a shift select lever for selectively feeding pressure from the hydraulic pump to a drive pressure conduit at a "D" range and to a fifth friction member via a reverse pressure conduit at an "R" range;
- a shift control valve connected to the drive pressure conduit for feeding drive pressure to a plurality of shift valves by operation of two shift control solenoid valves which are controlled to ON/OFF states or controlled according to duty ratios by a transmission control unit according to vehicle speed and throttle valve opening;
- a 2-4/3-4 shift valve for feeding drive pressure to a first friction member at first, second and third speeds of the "D" range and to a third friction member at a fourth speed of the "D" range and exhausting pressure from the first friction member when third-to-fourth and second-to-fourth speed shifting;
- a first pressure control valve controlled by a first pressure control solenoid valve for feeding drive pressure to the second and third friction members at the second, third and fourth speeds of the "D" range;
- a second pressure control valve controlled by a second pressure control solenoid valve according to a duty ratio for feeding pressure to the first friction member at the first, second and third speeds of the "D" range and to the third friction member at the fourth speed of the "D" range by the 2-4/3-4 shift valve;
- a 1-2 shift valve for feeding pressure to the second and third friction members and for feeding drive pressure from the 2-4/3-4 shift valve to the third friction member at the fourth speed according to second speed line pressure of the shift control valve;
- an end clutch valve for feeding pressure to the third friction member according to fourth speed line pressure of the shift control valve;
- and
- a 2-3/4-3 shift valve for feeding pressure to the actuating conduit of the third friction member at the third speed according to third and fourth speed line pressure from the shift control valve and for releasing pressure of the second and third friction members when third-to-fourth, fourth-to-second, fourth-to-first and third-to-second speed shifting;
- wherein the first pressure control solenoid valve is Controlled to an ON state to prevent drive pressure from being fed to the 1-2 shift valve at the first speed of the "D" range and to feed drive pressure from the second pressure control solenoid valve to the first friction member via the second pressure control valve and the 2-4/3-4 shift valve.