A/C Compressor Clutch

When the air conditioning is off, the pulley spins freely on its bearing and the compressor does not pump. Many modern compressors have no clutch and vary their output internally with a control valve instead. The gap between the clutch plate and the pulley is set with shims and affects whether the clutch engages reliably.

Current through a coil creates a magnetic field that pulls the clutch plate against the pulley, so the compressor shaft turns with it.

A clutch that does not engage can be caused by a blown fuse, a relay, low pressure, a failed coil or an excessive air gap. A noisy pulley bearing is heard even when the air conditioning is off and can damage the belt if it seizes.