Failsafe hydraulic control system for vehicle automatic transmission

摘要

A hydraulic control system for a vehicle automatic transmission, in which hydraulic pressure of a pressure source is supplied through a corresponding one of hydraulic lines to each of engaging means when achieving different gear ratios, and a plurality of shift devices are coupled respectively to the lines for switching supply and discharge of the hydraulic pressures to and from the respective engaging means in accordance with running conditions of the vehicle. At least one valve device is coupled to at least one of the lines. The valve device is movable between a supply position where hydraulic pressure is suppled to a corresponding one of the engaging means and a discharge position where the hydraulic pressure suppled to the engaging means is discharged. A plurality of pressure detectors respectively detect hydraulic pressures in the respective hydraulic lines. When the number of engaging means having applied respectively thereto hydraulic pressures higher than their respective predetermined values is equal to or greater than a predetermined value, a switching device switches the valve devices to move same to its discharge position, thereby avoiding locking of the transmission mechanism. A pressure regulating valve device is interposed between the pressure source and a high speed engaging means which can achieve shifting into a high gear ratio.