The A/C compressor is driven by a belt from the engine crankshaft through the compressor clutch pulley. The compressor pulley rotates freely, without turning the compressor shaft, until an electromagnetic clutch coil is energized. When voltage is applied in order to energize the clutch coil, a clutch plate and hub is drawn toward the pulley. The magnetic force locks the clutch plate and pulley together as one unit in order to drive the compressor shaft. Refer to Air Conditioning Compressor Replacement .