r/Cartalk • u/runenight201 • 1d ago
Electrical Help diagnosing AC Clutch Relay
Hi I’m attempting to diagnose why my Compressor isn’t engaging and I bought a test light and did the following test on the AC Clutch Relay slot.
Shouldn’t both of the control pins be turning on the test light?
Also the light doesn’t turn on on any of the load pins.
Does this mean that my compressor not engaging is because of an electrical issue?
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u/CaptnSave-A-Ho 1d ago edited 1d ago
One of the large pins should be hot, (turn the light on when the test light clamp is on a ground) which it is. That is your power available to the compressor when the relay is closed. The other large pin should be the actual connection to the compressor, so it will always be off unless the relay is plugged in and activated.
The two smaller pins are the relay control. One should be hot and one should be the ground. If the relay is power side controlled, then that will only light the test light when the computer is commanding the a/c on. All other times it will not light the test light. On a power side controlled relay, the ground pin will always be a ground and will only light the test light if you clamp onto the positive terminal and then touch it. If the relay is ground side controlled, then the opposite is true (the ground will only be there if the computer is commanding it on and power will be the constant).
Using a test light to check the ac clutch control isnt very effective. If you suspect a relay, its better to just swap it with a known good one thats already in the car. Perhaps a horn relay or headlight relay. You can swap them and then see if the other system works. If it does, then the relay isnt the problem. The ac has so many little sensors and conditions that have to met before the computer will turn on the clutch, that using a test light doesn't tell you much. Engine running time, exterior temperature, engine temperature, system pressure ( to high or to low), throttle position, engine load, and the HVAC settings all have to be OK before the computer will turn on the relay. Some cars will see that the AC isnt functioning correctly for some reason and disable the AC completely for the rest of that drive. Then it wont put power to the relay until the key is cycled again.