Compressor not engaging can be the result of many things, bad clutch coil, excessive air gap, bad low/high pressure switch, low refrigerant charge, faulty control head, faulty relay being the majority.
To test the system requires a $100,000 dollar test tool.... NOPE, you can test most of it with a few very simple items. Namely a test light and a few feet of jumper wire.
First thing is to actually TEST the fuses, this is easy to do. Open the fuse box and use the test light to actually make sure that the fuses that have power, have it on both sides, you just touch the metal tab at the top of the fuse, if the light turns ON check the other side. The light should come on for both sides, one side only = bad fuse.
For the relay, an easy test is to put your finger on it and have someone turn the AC on, you should hear and feel it click. If you don't then either the relay is bad OR the control side is not getting a signal. Now if you do feel it click you can skip a couple things like the pressure switch test, it has to be passing power for the BCM to turn the relay on. BUT just because it clicks doesn't mean that it is passing power to the clutch. The easiest way to test that is to swap the relay with a working/new one. You can pull it and test the pins but that can get ugly if you probe the wrong pin.
To test the air gap, start the car, turn the AC on HIGH, now CAREFULLY reach down and tap the front of the clutch plate. If the clutch has excessive air gap this may be enough to turn it on and make it work. If it comes on the air gap on some clutches can be adjusted by using a puller and removing the plate, then removing a shim under the plate and reinstalling it. Others are not adjustable, you just replace the clutch.
Clutch power test - If that doesn't work - Unplug the clutch and use the test light to see if you are getting 12 volts in the connector. IF you are getting 12 volts but no clutch engagement it is likely a burnt out clutch coil. Replace it and you should be OK. 90% of the time you can get a complete compressor for less than a new replacement clutch. IF you have the ability to discharge and recharge the system it can be easier.
Limit switch test - If you are not getting 12 volts to the pin you will need to test the limit switch. In your case it is a single switch that responds to both a low (under 40 psi) or high (over 400 psi) and is in the high side line. If things are "normal" the switches are both closed, to test if it's the problem, simply unplug the connector and jumper the two pins. If the compressor comes on , the switch is faulty OR the charge is low. As the gauge you put on went into the red and most of those the red area is over 50 psi the charge should be high enough that the switch is closed.
If you do all these and still have no AC operation then you might want to take it to a shop as the issue could be the control head or the BCM and testing those takes a scan tool.
Images (Click to enlarge)
Friday, August 11th, 2017 AT 11:22 AM