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A fan clutch is a device found on an automobile's cooling fan. In most cases, the fan is attached to the fan clutch by four bolts. As the engine speed increases, the fan clutch disengages and puts the fan into a free-wheeling mode. With the fan clutch in this mode, it is not being turned by the engine, which saves horsepower and increases fuel mileage. Some types of fan clutches are operated by a thermostat and only engage when the engine reaches a specific temperature.
Prior to the 1970s, American-manufactured automobiles used a direct drive fan that turned continuously at engine speed. This type of fan cooled the engine well at low speeds or while idling in heavy traffic, but the fan actually robbed power from the engine at highway speeds. Air and wind coming through the vehicle's radiator caught the fan blades and slowed them down. This action caused the engine to overpower the fan, and the vehicle's driver was required to accelerate harder in order to maintain engine speed.
The thermostatically-controlled fan clutch is operated by a fluid coupling. When the engine temperature rises, the coupling engages. The most common sign of a fan clutch failing is an engine which becomes too warm or overheats while sitting in traffic—with this type of failure, the fan does not engage as the engine becomes hotter. In cold climates, a frequent sign of fan failure is a vehicle that will not reach operating temperature. In this scenario, the fan runs continually and never allows the coolant to reach the proper temperature.
The engine's fan does more than simply cool the engine. In the summer, the vehicle's air conditioning system is also cooled by the engine's fan. As the fan draws air through the vehicle's radiator, it also pulls air past the air conditioning condenser. When a faulty fan clutch is present, it typically will not pull sufficient air through the condenser to properly cool the system. The result is an air conditioner that does not get cold. As the vehicle comes up to speed, the air conditioning gets colder due to the flow of air over the condenser.
Periodic checks of the cooling fan should be made to ensure it is in proper working order. The fan should spin with slight resistance while the engine is not running. If the fan cannot be turned by hand while the engine is off, the clutch may require replacement. If the fan spins freely with no resistance whatsoever, the fan clutch may also require replacement.