Control system for hydraulic transmission

In a control system for a hydraulic transmission for a vehicle powered by an internal combustion engine having an air intake controlled by a throttle valve, the improvement comprising, in combination:

• a lower speed hydraulic engaging element for driving engagement with a lower speed transmission element, a higher speed hydraulic engaging element for driving engagement with a higher speed transmission element, a control valve assembly movable between a lower speed position and a higher speed position, an oil draining line connected to the lower speed hydraulic engaging element when the control valve assembly is in its higher speed position, said control valve assembly being provided with an orifice and a drain oil control valve arranged in parallel with said orifice, said drain oil control valve being movable toward closed position with an increase in the throttle valve opening during shifting between said lower speed hydraulic engaging element and said higher speed hydraulic engaging element, said drain oil control valve having a shape for throttling the rate of oil flow therepassed in an intermediate throttling position between open and closed positions, and means for causing said drain oil control valve to be moved to its open position when oil pressure fed to said higher speed hydraulic engaging element increases to a predetermined pressure which varies in response to the opening of said throttle valve.