Hydraulic control system for steplessly variable power transmission

摘要

A hydraulic control system for a steplessly variable power transmission includes primary and secondary pulleys, a belt connected with both pulleys and, hydraulic chambers for controlling effective diameters of the pulleys for changing a speed ratio steplessly. A speed ratio control valve controls the hydraulic pressure applied to the hydraulic chamber in accordance with a pilot pressure introduced into a pilot chamber provided in the speed ratio control valve means. A solenoid valve controls the pilot pressure applied to the speed ratio control valve, and a fluid pressure producing device produces a control pressure in accordance with a vehicle driving condition. A switch valve selectively connects the pilot chamber with either the solenoid valve or the fluid pressure producing device. The switch valve is actuated so as to connect the pilot chamber with the fluid pressure producing device when a deviation between an actual pilot pressure and a desirable pilot pressure is increased beyond a predetermined value. A stable speed ratio control therefore can be provided.