a hydraulic motor system minimizes wasted power by using a plurality of fixed displacement hydraulic motors to drive a drive shaft in a cooperative manner . the motors are selectively switched into operation in response to variations in fluid pressure . as a consequence the hydraulic fluid acts upon a motor system having an effective combined displacement for producing a predetermined shaft rotation rate at the volumetric flow rate which caused the pressure condition . the invention is disclosed as having particular utility for driving a cooling fan for an automotive engine .