Control system for converter lock-up and simultaneous pressure reduction

Abstract

A transmission control system is provided for use with a transmission assembly having a torque converter which couples a power input member with a transmission speed ratio section. A lock-up clutch is provided to selectively bypass the torque converter and couple the input member directly with the transmission speed ratio section. Some systems provide converter lock-up and lower transmission control pressure resposive to positioning of the transmission speed selector valve while others require additional complicated controls to operate the lock-up clutch of the converter and reduce transmission control pressure relative to the vacuum level of the prime mover. In the subject arrangement, a single valve (112) that is responsive to the output speed of the transmission (10) simultaneously actuates the lock-up clutch (32) of the torque converter (14) and lowers the operating pressure level of a dual stage pressure relief valve (72) by directing a fluid pressure signal to an actuating chamber (106) of the dual stage pressure relief valve (72). This arrangement ensures that the output speed of the transmission is operated at a sufficient low torque level before the lock-up clutch is actuated and furthermore does not require numerous complicated controls to lower the transmission control operating pressure and actuate the lock-up clutch.