How to Tell If You Have a Bad AC Relayby Lee Sallings
The relay in your car or light truck's air conditioning system controls the compressor. By using a relay, the high-current electromagnetic compressor clutch can be controlled by a low-current switch. Testing the relay requires the use of an automotive circuit tester because many of today's computer-controlled cars and trucks use the computer to turn the relay off and on. Using a simple test light may damage the computer circuits.
Locate the relay in the power distribution box, usually found under the hood, and unplug it. Inspect the wiring and the plug for heat damage and repair as needed. Notice that two of the four wires going into the plug are a larger size than the other two. These are the higher-current wires going to the compressor clutch. Turn the ignition key on and test for power on the high current wires. One of them should have battery voltage with the key on. The other is ground. If no voltage is found, check the fuse, and if no ground is found, check the compressor clutch.
With the engine running, and the A/C controls switched to max cooling, test the remaining two wires for power and ground. If power is not found on one of the two smaller wires, check the fuses. If ground is not found on the remaining small diameter wire, the indication is a control problem. Possible causes are low refrigerant, a faulty pressure switch, a faulty control switch or a faulty computer. If power and ground are found here, move to the next step.
With the engine running, and the A/C controls in the max cooling positions, test for power at the compressor clutch electrical connector. If power is not found at the clutch, the relay is faulty. If power is found, suspect a bad ground, or a defective compressor clutch.