Auto A/C Terms - Glossary
Clutch Field - Clutch Coil

Every automotive A/C compressor clutch assembly is made up of three distinct components. The clutch hub or clutch plate is the front most component that will engage or click when the compressor engages. The pulley utilizes a bearing and is mounted on the front head of the compressor, turning with the belt attached. Behind the pulley is the clutch field or coil. This is the component that makes the magnetic field to engage the compressor clutch.

The coil is really just hundreds of windings of wire with an electrical connection. When electrical current is applied to the coil or field, it creates a magnetic field that will pull the front clutch plate or hub into contact with the pulley. The clutch plate is attached directly to the main shaft of the compressor. Once the clutch plate is engages and contacting the pulley, the compressor starts to turn.

Clutch fields or coils can and do fail. A failed clutch coil can be difficult to diagnose. If any of the hundreds of windings of wire break, the coil can not complete the electrical circuit and is unable to create the magnetic field. In a lot of cases, the clutch field or coil will work when under hood temperatures are moderate. However, increased under hood temperatures cause components to expand. That can expose small breaks in the coil winding causing the compressor to disengage. When that happens, the A/C cooling performance is lost.

The only reliable test is to assure that there is full battery voltage available at the clutch. Anything less than full battery voltage could mean that magnetic field being created is not strong enough to engage the clutch. If full battery voltage is available and any ground circuits on the coil are confirmed, then the coil is at fault and will have to be replaced.