An electronically controlled magnetorheological fluid based fan drive assembly 60 used to increase the rotational speed of a coupled radiator cooling fan. Introducing a magnetic field within a working chamber 99 between a drive ring 74 and an output member 86 increases the viscosity of a magnetorheological fluid by changing the state of the fluid from a free flowing liquid to a semi-solid state. The shear rate of the magnetorheological fluid can be increased to create additional torque to drive the output member 86 and coupled radiator fan at a higher rotational speed to cool the engine coolant flowing through a closely coupled radiator. The magnetic field is induced by directing a flow of electrical current through an electronic coil 62 coupled within the fan drive assembly 60, and an electronic control unit coupled to the electronic coil 62 controls the amount of electrical current flowing through the coil 62. This maintains the temperature of the engine block within an acceptable temperature range at a particular engine speed.