There is a thermal switch in the coolant jacket of the engine which senses the increase in temperature. It closes an circuit to a relay which in turn closes a circuit to the fan motor or motors. The current draw of these cooling fans is too high for the thermal switch to handle and thus the relay is used to handle that switching job. There is often a third component in the system that allows the fans to continue to run for a short period of time after the engine is turned off to prevent a loss of coolant caused by the spike in temperature if all air movement across the radiator was stopped when the engine is turned off. This device is called a radiator fan timer. The failure of any one of these components or of the fan motor will cause the fan not to turn on.
Jerry, June 2010
