Automatic transmission with anti-rollback band brake pressure control

Abstract

A control system for a vehicular automatic transmission includes a speed change mechanism including an output element that is connected to the drive shaft of a vehicle to transmit power, and a rotary element connected to the output element. A control system for the speed change mechanism is also provided. The control system includes a hydraulic servo actuated brake for retaining the rotary element to check the rotation of the output element in a vehicle roll back direction. The brake includes a band brake that establishes different application forces depending on whether the brake operates by a self-energizing action or a de-energizing action according to the rotational direction of the rotary element. The band brake is set so that the rotary element rotates during backward rotation of the output element in the direction of the self-energizing action, and is set with a first oil pressure necessary for retaining the rotary element during backward rotation of the output element caused by the roll back of the vehicle. The first oil pressure is lower than a second oil pressure necessary for retaining the rotary element during operation of the band brake by the de-energizing action. The control system further includes an oil pressure feeder for feeding the hydraulic servo of the band brake with an oil pressure which is equal to or higher than the first oil pressure and lower than the second oil pressure.