Electronic control for a hydraulic system to drive an auxiliary power source is provided, with an application as a system for controlling the operation of a hydraulically driven AC generator. The system includes a hydraulic pump, a hydraulic motor drivably connected to the generator, and a fluid circuit for circulating fluid from the pump to the motor and back. The feedback circuit contains a feedback conduit to feedback the motor. The system also includes a proportional servo control valve assembly for controlling the fluid conduits and a control circuit for controlling the control valve assembly to thereby control the flow of fluid to the motor. Sensors for measuring the operating parameters of the system and an operator interface module are both able to influence the operation of the system.