A constant speed drive system which is automatically controlled according to the load of the pump, fan, etc. driven by the system. Control of the system is achieved by means of a pump or compressor and a diaphragm-actuated spool valve, wherein a reference point or pressure in a spool valve is matched with a pressure, proportional to the speed of the load or output shaft, acting on a diaphragm in the spool valve. The spool valve, in turn, modulates the pressure to a clutch actuator for a clutch located between a constant speed motor and a shaft driving the requisite load.