Hydraulic control system for vehicle automatic transmission

A hydraulic control system for an automatic transmission, comprising:

• a hydraulic pump;
• a regulator valve for regulating hydraulic pressure generated from said hydraulic pump constant;
• a reducing valve for reducing hydraulic pressure regulated by said regulator valve;
• a pressure control part including first to fourth pressure control valves which are controlled by first to fourth solenoid valves, respectively, to independently simultaneously control a plurality of friction members for application or release;
• and
• a shift control part including an N-D control valve, a first-to-second speed shift valve, a second-to-third/fourth-to-third speed shift valve, and a fourth-to-fifth speed shift valve, all of which receive control pressure from the pressure control part and feed hydraulic pressure to the friction members;
• wherein the first-to-second and second-to-third/fourth-to-third speed shift valves are respectively provided with control ports to which hydraulic pressure, which is used for applying a friction member in fourth and fifth speeds, is fed, and wherein the control ports are connected to a line which is provided with a shuttle valve.

A hydraulic control system for an automatic transmission, comprising:

• a hydraulic pump;
• a regulator valve for regulating hydraulic pressure generated from said hydraulic pump constant;
• a reducing valve for reducing hydraulic pressure regulated by said regulator valve;
• a pressure control part including first to fourth pressure control valves which are controlled by first to fourth solenoid valves, respectively, to independently simultaneously control a plurality of friction members for application or release;
• and
• a shift control part including an N-D control valve, a first-to-second speed shift valve, a second-to-third/fourth-to-third speed shift valve, and a fourth-to-fifth speed shift valve, all of which receive control pressure from the pressure control part and feed hydraulic pressure to the friction members;
• wherein said first-to-second speed shift valve includes a first valve spool and a first spring for biasing the first valve spool, and said second-to-third/fourth-to-third speed shift valve includes a second valve spool and a second spring for biasing the second valve spool, the coefficient of elasticity of said first spring being larger than that of said second spring.