An electroviscous fan clutch (10) is employed as an accessory drive mechanism within a truck or auto­mobile. A shaft (36), either driven by the engine or driving a load (31), carries a clutch member (14) arranged for fluid engagement with a body member (12) for the transmission of torque therebetween when electroviscous fluid is disposed in an operating chamber (45) defined by complimentary shear surfaces (40 and 44) in the clutch and body members. The clutch and body members are electrodes, connected to a source of high voltage potential (86), which establish an electrostatic field within the operating chamber to selectively vary the characteristic viscosity of fluid flowing there­through.