Patent ID: 6848548

Claim:
Hydraulic control system for a vehicle transmission, connected to an output shaft from a hydraulic torque converter with a hydraulically operated friction clutch for locking up the torque converter and which has at least two hydraulically operated clutch units for shifting between the various gear speeds of the transmission, comprising a first hydraulic pump, which feeds, via a counterbalance valve, oil at low pressure to the torque converter; and a second hydraulic pump which feeds, via regulator valve means, oil at high pressure to first, second and third piston-cylinder devices, each operating an individual one of said friction clutch and clutch units; wherein the second pump is a variable high-pressure pump with variable displacement, which is controlled by a pressure-regulator valve, and control means are arranged to control the pressure-regulator valve to set the variable high-pressure pump at maximum displacement when the friction clutch is disengaged, and to a lower displacement when the friction clutch is engaged.