A viscous fan clutch (10) is typically employed in a fan drive mechanism within a truck or automobile. A shaft (12) driven by the engine, rotates a clutch member (24) within the fan drive. The clutch member is arranged for fluid engagement with a body member (42) for the transmission of torque thereto when the fluid is disposed in an operating chamber (70) defined by complimentary shear surfaces (30 and 56) in the clutch and body members. During relative rotation between the two members, fluid is pumped from the operating chamber to a storage chamber (76). Fluid is returned to the operating chamber via bleed ports (94) which are selectively restricted by an electromagnetically controlled valve (88). A control system (110) senses vehicle coolant temperature and actuates the valve as a function thereof.
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