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Many vehicles have pulley clutches to allow some of the belt-driven components to freewheel when not in use. The clutch functions by engaging when the accessory is needed and disengaging when it isn't. This can help reduce drag on the engine and improve fuel efficiency, while also reducing wear and tear on the component itself. The pulley clutch is most often found on air conditioning (AC) compressors, while a thermostatic variety is often used on belt-driven cooling fans. Certain alternators may also be designed with pulley clutches, so they can continue to rotate at a higher revolutions per minute (RPM) than the engine itself during brief periods.
Air conditioning compressors typically are designed with a type of centrifugal pulley clutch. This type of clutch can naturally disengage the internal shaft of the compressor from the pulley when it spins. An electrical signal can then cause the clutch to reengage the pulley to the shaft when the air conditioning is switched on. Low refrigerant or a bad switch can potentially cause the clutch to engage and disengage rapidly, resulting in an undesirable noise and engine RPM fluctuation.
Belt-driven cooling fans will often have a thermostatically controlled fan clutch. This type of pulley clutch usually is designed with a viscous coupling and contains a bimetallic strip that acts as a thermostat. If the engine begins to run too hot, the bimetallic strip can cause the viscous coupler to engage. This can allow the pulley clutch to freewheel before the engine has warmed up and then pull air through the radiator once the engine is hot.
The failure of a fan clutch may potentially cause an engine to overheat or prevent it from reaching its operating temperature. A fan clutch that cannot engage may result in the engine overheating under heavy loads or when stopped. If the fan clutch becomes stuck in an engaged position, it can result in excess noise, poor fuel economy or an engine that never warms up properly. This can further reduce the gas mileage and also manifest in cool temperatures at the heater vents since the coolant in the heater core never gets hot.
Most alternators have a solid pulley, though some may be designed with an alternator pulley clutch. Rather than allowing the unit to disengage when not in use, these are often designed to allow the alternator to freewheel at a high RPM when the engine RPM is reduced. Since an alternator will tend to produce a lower amperage output at lower RPMs, this may allow the alternator to continue at a higher output when the engine speed is lowered for a short time.