Pulley clutches are used on belt-driven components to reduce engine drag and wear. Air conditioning compressors use a centrifugal pulley clutch, while cooling fans have a thermostatic clutch. Alternators may also have a pulley clutch to maintain high output at lower RPMs. Failure of these clutches can cause overheating, poor fuel economy, and reduced gas mileage.
Many vehicles have pulley clutches to allow some of the belt driven components to spin freely when not in use. The clutch works by engaging when the accessory is needed and disengaging when it is not. This can help reduce engine drag and improve fuel efficiency, while also reducing wear 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 can also be designed with pulley clutches, so they can continue to spin at higher RPMs than the engine itself for brief periods.
Air conditioning compressors are generally designed with a centrifugal pulley type of clutch. This type of clutch can naturally disengage the internal shaft of the compressor from the pulley when it rotates. An electrical signal can cause the clutch to re-engage the pulley to the shaft when the air conditioning is turned on. A low coolant level or faulty switch can cause the clutch to engage and disengage rapidly, causing unwanted noise and fluctuating engine RPM.
Belt driven cooling fans will often have a thermostatically controlled fan clutch. This type of pulley clutch is usually designed with a viscous coupling and contains a bi-metal strip that acts as a thermostat. If the engine starts to run too hot, the bi-metal strip can cause the viscous coupler to seize. This can allow the pulley clutch to spin freely before the engine has warmed up and then draw air through the radiator once the engine is warm.
Failure of a fan clutch can cause the engine to overheat or prevent it from reaching operating temperature. A fan clutch that fails to engage can cause the motor to overheat under heavy loads or when stopped. If the fan clutch gets stuck in an engaged position, it can lead to excessive noise, poor fuel economy, or an engine that never warms up properly. This can further reduce gas mileage and also manifest itself in cold temperatures at heater vents since the coolant in the heater core never gets hot.
Most alternators have a solid pulley, although some may be designed with an alternator pulley clutch. Rather than allow the unit to disengage when not in use, they are often designed to allow the alternator to spin freely at high RPM when engine RPM is reduced. Since an alternator will tend to produce a lower amperage output at lower RPMs, this can allow the alternator to continue at a higher output when engine speed is slowed down for a short time.
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