Abstract: A hydraulic drive control system for driving a generator or alternator or the like at a constant RPM regardless of variations in the output load or the input load comprising a driven pump having a variable volume and variable pressure output connected in a closed system to a hydraulic motor which will maintain a constant RPM output, regardless of the RPM's of the driving means, which is coupled to the device to be driven. Connected between the pump and the motor is a volume control line having a volume control valve which insures that the volume of fluid to the hydraulic motor will be constant thus providing a constant line RPM output of the driving motor, and a pressure regulating line connected between said pump and said motor to regulate the pump pressure output to insure that the motor RPM is constant although the load on the motor may vary. The volume control valve regulates hydraulic fluid between the pump and motor regardless of the pressure and volume output of the pump.