a test stand for testing hydrostatic drive units which have a variable displacement hydraulic pump in a closed fluid circuit with a hydraulic motor and are characterized by a motor output speed and a system fluid pressure . the test stand includes a power source for energizing a hydrostatic drive unit , a speed control for controlling the level of motor output speed , a pressure control for controlling the level of system fluid pressure , sensors for detecting the instantaneous level of motor output speed and system fluid pressure , and logic circuits for providing a command signal responsive to the sensed instantaneous levels to the speed control and the pressure control .