Hi!

I have an '01 Civic. I had the AC compressor replaced. Now it runs (the clutch is engaged) whenever the car is on, even if the dash fan switch is off. Unless we pull the relay. I don't think the problem is the relay, we tried swapping that.

Any ideas?

The guy that did the swap says I need someone better at electrical than he is. Another garage I talked to said it was a known problem with third-party aftermarket compressors, and to swap in a Honda brand compressor. I have trouble believing that; I would think the compressor just gets power from the relay to engage the clutch or not, and something else decides whether or not to send that signal. Am I wrong?

 

You are correct in assuming that since it always stops when you remove the relay, it's going to be an electrical problem not mechanical.

The ECU drives the relay, when it is directed to by the control panel. All the wiring is "negative logic" which means that when a wire is pulled to ground, the circuit is active or "on".

Turn the key on but don't start the engine. Does the compressor engage immediately? If it does, either the ECU is bad or the wire from the relay to the ECU is shorted to ground.

If you have to start the engine to make the compressor engage, the ECU is probably working properly but the input (through the MCU) is stuck on. The next test in that case would be to unplug the pressure switch on the receiver and see if that makes the compressor stop.

 

Thanks very much!