HYDRAULIC CONTROL SYSTEM FOR TRANSMISSIONS

The hydraulic control system is applied to transmissions in which the high speed range is obtained by supplying oil to a first one of at least two hydraulic servos and discharging oil from a second hydraulic servo. The low speed range is obtained by discharging oil from the first hydraulic servo and supplying oil to the second hydraulic servo. This hydraulic control system comprises a hydraulic pressure source, a pressure regulator valve, a governor valve, a throttle valve and first and second shift valves. The hydraulic control pressure supplied from the hydraulic pressure source and regulated by the pressure regulator valve is directed through the first shift valve to the first hydraulic servo and is also directed through the second shift valve to the second hydraulic servo. The governor pressure developed by the governor valve, which conforms to the vehicle velocity, and the throttle pressure developed by the throttle valve, which conforms to the engine output, are applied to the first shift valve. The throttle pressure, moreover, actuates the second shift valve. In addition, the hydraulic pressure of the first hydraulic servo is applied to the second shift valve to oppose the throttle pressure. Thus, the oil discharge starting time of the second hydraulic servo at the time of upshift is delayed in accordance with the hydraulic pressure that is supplied to the first hydraulic servo. The degree of the time delay also is changed in accordance with the magnitude of the throttle pressure. The second shift valve forms a variable capacity accumulator chamber at the point at which the hydraulic pressure is applied from the first hydraulic servo. An orifice for reducing the rate of hydraulic pressure increase at the time oil is supplied to the first hydraulic servo as well as a check valve for enhancing the rate of hydraulic pressure decrease at the time oil is discharged from the first hydraulic servo are provided in an oil passage between the accumulator chamber and the first shift valve. By virtue of the orifice and the check valve, the gradient of the pressure change due to the oil supplied to the first hydraulic servo at the time of upshift is reduced. The degree of this gradient reduction changes to conform to the magnitude of the throttle pressure. Furthermore, at the time of downshift, the second shift valve is operated in response to the oil discharge from the first hydraulic servo so supply oil to the second hydraulic servo, and therefore the downshif