Transmission control system with modified back pressure accumulator

Abstract

A fluid pressure control system for controlling an automatic transmission which includes a gear transmission mechanism and several fluid pressure actuated friction engaging mechanisms, selective engagement of which provides various speed stages from the gear transmission mechanism. A line pressure control valve produces a line pressure which increases from a base pressure by an amount proportional to engine load. A throttle pressure control valve produces a throttle pressure which is proportional to engine load. A shift valve selectively switches supply of line pressure to one of the friction engaging mechanisms via a passage, according to the operational conditions of the vehicle. A back pressure accumulator is connected to an intermediate part of the passage. Back pressure is supplied to a back pressure chamber of the accumulator by a fluid pressure control means which receives supply of line pressure and outputs a pressure which, when engine load is medium, is approximately equal to line pressure, but which, when engine load is small or is large, is substantially lower than line pressure. Thereby, the performance of the accumulator for cushioning over abrupt engagement of the friction engaging mechanism is more suitably tailored to the torque characteristics of the engine of the vehicle, and gear shift shock is accordingly reduced.