Abstract: The present invention provides a method of controlling thermal energy within a fan drive. The fan drive includes an input member rotating at an input speed, and an output member attached to the fan and rotating at a fan speed, with a viscous fluid disposed therebetween. The fluid selectively transmits torque between the input member and the output member such that the fan speed is a function of a controllable parameter of the fluid. The parameter is continually controlled by a programmed controller. The method includes the steps of pre-determining a threshold temperature for the fan drive, with the threshold temperature approaching a maximum desirable temperature for the fan drive, and adjusting the fan speed as necessary to maintain a fan drive temperature below the threshold temperature. The controller may regulate fluid volume or fluid viscosity, or any other controllable parameter of the fluid.