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1997 Lumina - AC compressor wont engage
07-24-2012, 12:28 AM
Post: #1
1997 Lumina - AC compressor wont engage
Hello all;

First off, I have done extensive searching online the past few weeks and have yet to find a posting on any site that accurately depicts the problem I'm having, so if something has been posted before please forgive me as I did look but may have missed something! Some info..

I am in the process of selling my 1997 Lumina. In case it matters, the motor has been replaced with a 3.1 out of a 1998 a few years back, but everything else matched so I don't think it affects anything here; please correct me if I'm wrong.

Long story short, AC Compressor will not engage. If I jump the AC relay underhood the compressor will engage, as it will if I shoot 12v to the compressor itself. However, and maybe I'm testing something inaccurately, I havent been able to find any native power running to the compressor with AC on any of its settings. I have also tested the AC Pressure Switch and havent found any power source to it; nevertheless, I did replace the switch since everyone claims it is this that causes the issue. I then replaced the dash temp control unit- the car does have dual climate; still no result. The only other guesses I got were that the compressor itself was bad; therefor, at the recommendation of my shop, I replaced the compressor, evac'd and refilled the system to spec- still, no operation.

Temp gauge in the car works correctly so I imagine the ECT works as well, although I may be mistaken?

To the best of my knowledge from my limited research the AC in this car is operated by the PCM, which must read quite a few different signals 'in the green' to shoot power to the compressor. As I recall, this includes cooling fan operation, engine temperature below a certain value, throttle position sensor below a certain value, IAT below a certain value (?) and perhaps more... My best guess is that some or all of these values hitting the pcm are past their given threshold and thus the PCM isn't shooting a signal to the compressor to engage. Does this sound plausible? Also, is there any way to test this? Can I chek the PCM and see what signals are/are not correct, to save me the trouble of replacing EVERYTHING and simply hoping it works when I'm done?

I'm literally ripping my hair out at this point, as I have already bought and installed the compressor I have invested too much money to really throw in the towel just yet. I would really REALLY appreciate some advice or help here- I can't explain HOW thankful I would be, as nobody I know (professional or otherwise!) seems to know what might be the issue, or even how the PCM operates the AC in this car.

If you have any other questions for me please ask- sorry for the small book I've written but I wanted to be as thorough as possible!
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07-24-2012, 09:15 AM (This post was last modified: 07-24-2012 09:36 AM by Rupe.)
Post: #2
RE: 1997 Lumina - AC compressor wont engage
Oh joy, a can of worms for breakfast!

You should have been on guard when someone told you to replace the compressor if you already knew it would work with a jumper wire. That said, let's revist a few things.... and keep in mind that the "engine transplant" may be suspect just due to the nature of that operation.

Most GM cars use the ECM / PCM to read a variety of inputs before allowing things to happen. If you follow a wiring diagram you will also see the PCM doesn't supply "hot / 12 volts" to much of anything because that would mean a LARGE power wire and a LARGE fuse to feed that stuff. Instead the PCM usually switches the ground leg to the component's relay. You are correct in presuming the inputs on throttle, vac, engine temp, and trany for the AC operation, which kicks the AC off for a few seconds while climbing hills or passing in traffic.

That said, the places I would look are at anything that was different from the original engine. (presuming you did that yourself and know what to look at) Double check all grounds to the body, engine, etc, just due to the fact that this car is 15 years old and there's bound to be a few rusty spots. If you come up empty have the PCM scanned with something fancier that a code reader and you can watch the various functions in real time. You will be reading vac, throttle position, trany gear, and AC functions. IOW, if you see the PCM shows the AC off then you know it's an input function. If the PCM shows the AC is on (but it is not) then you know it's on the output side of things.

BTW, most sensors / inputs that would hold the AC off will only do so for a matter of seconds as a normal function. Anything that will hold it off for longer would probably trigger a check engine light. (like a high reading from the second temp sensor) By the same token, the PCM does not know if the electric fans are working. It only knows that it sent a command to the fans and the engine temp is normal. What I am driving at is sometimes you need to observe things and read between the lines. This is where a scanner helps you see what the computer sees and allows to know what it's trying to do.

20 years ago I was working on an 86 Chevy wagon and I found a bad ground to the AC relay right at the point where the wire went into the relay socket. Since that was a molded socket I wound up extending the wires and installing new crimp terminals then mounting the relay to the harness with a zip tie. It wasn't pretty but it worked for another 10 years.

Edited to add: In my experience GM switches the ground for the AC relay via the PCM but that relay switches the hot to the compressor. Soooo, as you are poking through your testing with a test light you may need to reverse your thinking on where to clip the wire of the test light to verify an open circuit. Sometimes it's better to have 2 test lights so you can watch more than one thing operate... and sometimes it's better to use a meter. Mostly this is a matter of mind set and what you are comfy with personally.

Let us know how you make out.
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