I just noticed something I should have noticed before, and it explains some of my confusion. In reading more closely the Heating & Air FSM troubleshooting procedure I've been following for the magnet clutch not engaging (Diagnostic Procedure 6 on page HA-53), I happened to see "Manual" in the upper right corner of each page for that procedure. I feel stupid for not noticing that before, and it explains why I was confused about not finding the thermo control amp by the glove box (which I mistakenly thought was another name for the A/C auto amp). It also explains why I was having trouble matching up some of the pinouts shown in that diagnostic procedure.

So far, I haven't found an equivalent procedure for the auto air, but I think it's pretty much the same all the way to the final steps of checking the thermo control amp. I'm guessing the self-diagnosis for the auto air replaces this procedure for the manual air, and that I can follow the procedure on page HA-96 to at least partially check out the a/c auto amp connections. I do see the circuit diagrams on pages HA-90 to HA-94 as well, which should help if wiring diagnosis is needed in case the BCU swap doesn't solve the problem.

 

Ok, I got the replacement BCU today, and swapped it out, feeling fairly confident this had to be the problem. Nope! I was pretty disappointed, to say the least. I checked at the A/C relay again just to see if I could repeat the previous readings, and sure enough, with the relay removed, I had 14V across the coil side of the relay. I'm pretty stumped at this point, but it seems the remaining possibilities are the ECU, or some faulty wiring. From what I understand, the ECU is not such an easy swapout, since it can be tough to match, and it has to be reprogrammed by the dealer? I'll do some testing of the wiring between ECU pin 12 and pin 2 of the A/C relay, and maybe also see if I can test to be sure the ECU has a good ground. Beyond that, as much as i hate to admit defeat since I feel like I've been through most of the possibilities now, I'm about ready to throw in the towel.

 

Hi again guys,

I was reading some more about the ECM to see about diagnosing it, and I think I came across something. Would appreciate some confirmation on this from anyone knowledgeable. I was focusing on pins 12 and 21 of the ECM, since these specifically had to do with the A/C. I noticed on pg. EC-98 of the FSM that pin 21 of the ECU should be 11-14V with AC switch off, and ~0V with AC switch on. From my earlier post where CMAX had me test at pin 1 (dark grn/blk) of the triple-pressure switch (which connects to pin 21 of the ECU), I note that I had ~11V on pin 1 regardless of AC on or off. Could this indicate the problem? I should have flagged this before, and in fact did question in an earlier post why the pressure switch reading wasn't changing with a/c on/off, but I had my understanding of the switch operation backwards and terminals reversed. I wrote this artifact off later when CMAX didn't seem concerned about the test results, and figured it must be normal, but maybe not?

The one thing I will note is that in the FSM it does note that the "Engine is running" for these tests. When CMAX had me run the tests, he specifically said not to crank the car. Not sure if that would make a difference or not, although there are places where it simply says "Ignition switch on". I'll rerun these tests, but my daughter's gone with the car now, so it'll be a couple days before I can confirm. Also in looking back at CMAX's tests, he said I should see ~28 ohms on pin 4 (lt. grn/red) of the pressure switch, and I read about 400 ohms. Again, CMAX didn't seem overly concerned about it, so I didn't worry too much either. Pin 4 of the pressure switch goes to pin 5 of the auto amp, so if this was the problem, then I was really unlucky and got another bad BCM since I did swap this out with no success.

I'm really struggling with this one, so any insights are greatly appreciated. I think I have a reasonable test plan when I have access to the car again, and if I fail there, I'm giving up and will seriously consider a "hotwire" switch.

Test plan:
1) Confirm the voltages at pin 21 of the ECU. If not as expected, bad ECU (since push control unit already swapped out)? If voltages as expected, optionally (since previously tested already) confirm voltages at pins 1 & 4 of pressure switch.
2) Test voltage at ECU pin 12 (with and without relay in place). If voltages as expected at ECU, then fix wiring issue. If not as expected, check pin 12 of harness connector for possible short. If no problem with harness, then bad ECU?

 

Holy crap...you’ve been working on this issue for almost a year now! Do you love this car? Sell it and buy one with working A/C.

 

LOL... I'm almost there. It's my daughter's car, and she'll soon be able to afford a new one since she just graduated. Funny you say that, since I tried to talk her into just bypassing the A/C when the old compressor locked up, realizing A/C wasn't my strength. Little did I know the compressor/freon would be the easy part.

It hasn't been continuous, as I took a long layoff over the winter/spring months. But I'm bringing it to conclusion one way or another here very soon. It's almost a matter of pride now, but I admit I'm close to defeat.

 

I get it, hopefully you’ll get this thing beat. Good luck.

 

Ok guys, I think we have some definitive conclusion here now, and something I really should have arrived at a long time ago. It's difficult when doing things piece meal and having to break off for long chunks of time while the daughter had the car away. But really the test that CMAX had me do earlier on should have clued me in much earlier. I did in fact query about the behavoir in question, but because of my lack of understanding of the system operation, I wasn't confident enough to conclude a definite problem there.

Pg. EC-98 of the FSM provided the acid test- pin 21 of the ECU should be 11-14V with AC switch off, and ~0V with AC switch on. I performed the test with engine running this time, just to be sure. There were so many wires in the way, I couldn't quite get to the pin socket itself, but I was able to probe the green/blk wire of pin 21 very close to the ECM socket, and get 14V with ignition on and A/C off. Then cranked the car and turned the A/C on, and got no change. So looks like I have a bad ECM. Thinking back, I did not unhook the battery when replacing the compressor, and I'm guessing when I removed the pressure switch to replace the drier that I must have toasted the ECM and somehow blew that fuse too. Is that a likely possibility, or am I reaching here?

Given what I'm seeing, the other only likely possibility would be the push control unit, but since that's already been swapped out with no success, it seems it's down to the ECM. Anyone disagree or have other ideas?

If not, then my next question is what's my most cost-effective way of fixing this? I was wondering if I could install a switch to ground for pin 21 of the ECU, but I'm thinking that's probably a bad idea and may cause some other problems? If I have to swap out the ECU, then I think the replacement has to be carefully matched, and then reprogrammed. Does the reprogramming have to be done by a dealer, or are there more cost-effective options?

Thanks for any advice!