Abstract: An improved hydraulic auxiliary drive system for driving the normally non-driven wheels of a vehicle is disclosed. A variable displacement motor having a tiltable swash plate operable between zero and full displacement positions drives the normally non-driven wheels and a variable displacement pump supplies pressure fluid to the motor. A spring biases the swash plate toward its zero displacement position and a servo mechanism is connected with the swash plate for tilting the same. A first supply delivers pressure fluid to the servo mechanism to tilt the swash plate toward its full displacement position and a second supply delivers pressure fluid to the servo mechanism to tilt the swash plate toward its zero displacement position. The first supply includes a pressure control valve to control the pressure thereof.