The AC pulley, or air conditioning pulley, is located on the front of the air conditioning compressor and drives the compressor with help from the engine's accessory drive system. Older versions of this pulley, found on vehicles prior to the mid 1980s, used a V-belt; since then, the system has used a multi-ribbed serpentine belt to drive the system. The AC pulley is attached to an electronic clutch, which allows the pulley to spin freely when the AC unit is not in use, but, once the compressor is activated by the AC switch, the clutch engages and the pulley drives the compressor.
There is rarely ever an issue with the AC pulley itself, however, the bearing that the pulley rides on typically requires occasional replacement. The electronic clutch system that works with this pulley can be heard engaging as a noticeable clicking sound when the air conditioning system is in use. A slight drop in engine speed is often found accompanying the clutch activation. An AC pulley bearing requiring replacement can often be indicated by a squealing sound coming from the AC compressor, as well as a noticeable wobble in the pulley itself as it spins with the vehicle's engine running.
A failed electronic clutch on a vehicle's AC compressor can lead to the freeze up of the AC pulley which, in turn, destroys the belt. This situation is dangerous due to the serpentine belt powering not only the AC unit, but the vehicle's charging and cooling systems as well. Typically, the pulley will freeze due to a malfunction in the electronic clutch or the failure of the pulley's bearing. Either of these conditions will be foreshadowed by a squealing or grinding noise emanating from the front of the AC compressor unit. This condition is commonly found only in vehicles that are driven with an inoperative AC system for long periods.
If a vehicle's air conditioning system is not working and will not be repaired, one remedy to avoid a frozen AC pulley from destroying the vehicle's belt system is to obtain a non-air conditioning-equipped fan belt. On a V-belt-equipped engine, it is often possible to simply remove the fan belts from the AC pulley. On this type of system, the air conditioner commonly uses a dedicated fan belt running off of another engine accessory to power the air compressor. This dedicated belt or belts can be removed, effectively disabling the air conditioning compressor with no further action required.