Automatic transmission control system with holddown device

Abstract

An automatic transmission includes a gear transmission mechanism with several friction engaging mechanisms. Selective supply of actuating pressure to these mechanisms selectively provides different speed stages from the transmission, including a directly connected speed stage and an overdrive speed stage. A driver operated manual selector valve can be set to several forward ranges including a 'D' range, and produces a lower range pressure when any forward range except for 'D' range is selected thereon. A kickdown pressure is produced when the accelerator pedal of the vehicle is depressed beyond a predetermined amount. An electrically operated pressure control device produces a holddown pressure signal according to operating conditions of the vehicle. A direct/overdrive shift valve controls supply of pressure to the friction mechanisms, so as to engage the overdrive speed stage, or the directly coupled speed stage, according to a balance relationship between a throttle pressure and a governor pressure, and, when override pressure is supplied to an override port of this direct/overdrive shift valve, it is prevented from providing the overdrive speed stage. A downshift pressure selector valve supplies pressure to the override port when and only when there is present at least one of the lower range pressure, the kickdown pressure, and the holddown pressure signal.