How to Test a Car's AC Compressor Clutches

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Testing your vehicle air conditioner's compressor clutch will ensure your vehicle stays cool and comfortable. If your AC fails to work or does not cool the vehicle efficiently, the compressor clutch may to be blame. If the clutch does not work properly the compressor cannot cool the air.

You can test the clutch with a light test to ensure the clutch is receiving power.

Turn the AC setting to the "off" position. Open the bonnet and locate the AC compressor. Follow the wire that goes from the AC compressor to the clutch relay mounted on the firewall. The relay will have a three-wire connection.

Connect the 12-volt light test to the negative cable on the vehicle battery. Turn the engine on. Touch the test light to each of the three AC clutch wires. All three wires should read inactive. Turn the AC settings to the "on" position. Touch the light to all three wires once more.

Unplug the wire feed at the AC compressor connection. Touch the test light to the wires on the clutch relay. The middle wire should read "hot." This shows the AC clutch is working properly and receiving power from the relay. If the wire does not have a reading, the clutch is bad and should be replaced.