Abstract: To permit mechanical override of an electrical command system to control deflection of the swash plate or eccentricity of an axial or radial piston pump, for example upon failure of the electrical control system, a group of valves 5-8 are connected to an electrical control unit 23 and an additional control valve 4, 4' with a manual control lever 18', 30' is provided, the deflection of the swash plate 1 or eccenter ring 28 of the pump being sensed by a position transducer 22 moved by a linkage which includes the manual control lever. In normal operation, the manual control lever is maintained by spring pressure to deactivate the control valve but, under emergency conditions, the manual control lever will operate the first control valve 4, 4' to admit hydraulic control fluid to position the swash plate, or eccenter ring, respectively, the consequent movement of which is fed back through the linkage to the manual control lever.