A fan clutch is attached to the front of an engine on rear wheel drive cars and trucks only. Bolted to the water pump between the water pump pulley and flange the clutch fan turns at engine speed and is driven by the serpentine belt. This important part of the cooling system is used to pull air though the radiator when the car is stopped or moving below 35 MPH. The fan clutch is design with either an electronic controlled activation device or a temperature sensitive spring at the front of the clutch to tell the clutch when to turn on and engage to the input shaft.
When the clutch engages the fan blade starts to operate pulling air though the radiator to cool the engine down. Most fan clutches are filled with silicone grease which will leak out when the fan clutch goes bad. Because the fan clutch is not used when the engine is cool it will disengage which helps increase gas mileage and engine performance. A main housing, input shaft, input shaft bearings and a clutch shoe is used to make up the fan clutch which then bolts to the fan blade and water pump. A cooling fan can turn either clockwise or counterclockwise depending on the manufacturers design.
One of the most common problems with a fan clutch is when the internal clutch stops working or wears out allowing the fan to freewheel which will cause the engine to overheat. When this occurs the fan clutch must be replaced.