The fan clutch is a small fluid coupling with a thermostatic device and controls a variable-speed fan. It ensures that the fan will rotate at just the right speed to keep the engine from overheating, and reduces drive to the fan when it is no longer needed. An automatic device frequently used in automotive cooling systems. When the engine is cool or even at normal operating temperature, the fan clutch partially disengages the engine's mechanically-driven radiator cooling fan, generally located at the front of the water pump and driven by a belt and pulley connected to the engine's crankshaft. This saves power since the engine does not have to fully drive the fan. However, if engine temperature climbs beyond the clutch's pre-determined setting, the fan becomes fully engaged, thus maintaining or lowering engine temperature.
The fan clutch has a fluid coupling partly filled with silicone oil designed for just that purpose. When the temperature of the air passing through the radiator rises, the heat alerts a bimetal coil spring to "uncoil" or expand. When it expands, it allows just a little more oil to enter the fluid coupling, so the fluid coupling starts to rotate the fan. If the air coming through the radiator is cool, the opposite happens; the coil spring contracts, the oil leaves the fluid coupling and the fan slows. Slowing the fan when it is not needed reduces fuel consumption, makes less noise and saves engine power. Most fan clutches are viscous or "fluid" couplings combined with a bi-metallic sensory system similar to that in a thermostat. Normally fan clutches are fairly reliable, but sometimes they fail.
A common symptom of fan clutch failure is overheating at idle. Bad fan clutches can also cause poor performance of the car's air conditioning system because the fan also cools the air conditioner's condenser, which is directly in front of the radiator.Another potential symptom if the fan clutch fails (always drawing air at a high rate)in a cold weather climate is that the heating system blows lukewarm air never delivering enough sufficient hot air.