The air conditioning clutch relay transfers power, like a switch, from the interior A/C controls of an automobile to the A/C compressor, engaging the compressor clutch. Checking an A/C clutch relay is a process of eliminating possible connection failures. While performing the checks, do not leave the relay bypassed for any length of time because the A/C compressor is designed to cycle off and on, running continuously.
Locate the Relay
Locate the air conditioner clutch relay by tracing the wire connection from the air conditioner compressor plug-in to the relay. In most cases, the relay is mounted on the fender well or firewall. The relay in a typical A/C control installation has a three-wire connection. One wire is a ground, one wire is the input from the interior control panel and switches, and one wire is the power feed to the A/C compressor clutch.
Using a 12-volt test light, connect the alligator clip to the negative terminal on the battery. Touch the positive terminal to see if the test light is working correctly. Unplug any relay terminal wiring with the interior car A/C controls in the "off" position. Check all three wires with the test light; with the engine running, the wires should all be inactive. With the engine running, turn the A/C control or switch to "on" at the dashboard. Check the three wires again. The wire from the controls should show "hot," but not the ground or the A/C compressor feed. This test confirms that the interior A/C control switches are functioning. Now plug the relay back in and disconnect the two-wire feed at the A/C compressor connection. One wire should be "hot," if not, the relay is not sending power to the A/C compressor. If it is, then the relay is functioning correctly and the A/C clutch is bad.
Bypass the Relay
To bypass the relay, plug the A/C back in. Return to the relay. Make a small 6-inch jumper wire and strip both ends. With the wiring plug still plugged in, touch the jumper wire to the hot side relay lug, from the controls as already identified and from the relay lug that feeds the A/C compressor. This will bypass the relay and engage the A/C compressor and confirm that the relay is defective.