Possible A/C clutch issue

[markagsmith5]

I recently had my A/C re gassed and was working fine. Hopped in the other day after completing a dash recover. A/C now not working. I did run the various tests described here [viewtopic.php?t=357] and discovered the only issue was the external temperature thermistor which read 470 ohms, whereas the other 2 read 10,700 and 10,500. Found a replacement at the local electronics store and replaced. Now reads about 9,700, so presumably close enough. I checked the voltage at pins 30 and 86 of the A/C relay with the snow flake switch depressed, both 12v. Also noted that the cooling fan started when the snowflake switch was depressed as well as an increase in idle speed. However the A/C clutch was stationary. Removed the A/C relay and jumpered between pins 30 and 87a. Checked the low pressure switch and both sides read 12v, still no A/C clutch movement.
Would I be safe to assume that the A/C clutch is faulty?
Does the high pressure switch come before or after the low pressure switch?
How can I test it as the wiring to it is fixed in place and sealed?
Many thanks for any help.

[Tom]

Sounds like the clutch is bad. The signal starts at the climate control unit, goes through the de-ice switch, then to the relay, then to the low pressure switch, and then to the compressor clutch. The high pressure switch controls the fan but I don't believe it can stop the compressor from coming on -- see schematic below. If both sides of the low pressure switch have 12v, then the compressor should have 12v. Pretty safe to conclude the clutch is bad, but to rule out harness issues, you can disconnect the connector going to the clutch and check that you have 12v there when the A/C is supposed to be running and/or connect 12v to the compressor side of that connector and see if the clutch engages. Pin 29 of the DME is tapped into the 12v wire going to the compressor to let the DME know when the A/C is on (see the G50 tag below). So, if your idle changes when you turn on the snowflake button, that suggests Pin 29, and therefore the compressor, are both getting 12 volts and that the clutch is just dead (again, assuming no harness issues).

IMG_2142.JPEG (3.81 MiB) Viewed 791 times

[dr bob]

My very similar clutch coil has a static resistance of about 6 Ohms. You can test for that at the relay (system off, engine off...) to determine the health of the coil itself and all the wiring and devices between.

There's a coil ground lead that, at least on my car, is connected to the compressor body at one of the screws for a manifold plate. If that's not connected to ground, you'll see the symptom you describe with 12V on both sides of the pressure switch with key on. The resistance check I described above will identify that missing connection. You can go to the stub cable connector by the compressor, and test for coil resistance to ground there, and if it's way higher than the ~~ 6 ohms I measured on mine, you've localized the open to the clutch coil and its connections.

Of course, my first-diagnostic-step is almost always to go back through the work you've done since it worked last. Things work until they don't, and the mission is to find out what happened or what you did during that time.

Considering the very recent "re-gassed" effort, I'd extend my search to include the reason for that, and if that reason was solved. Old gas leaked out? -All- the leaks fixed? What is the actual pressure in the system? If the pressure switch is no longer held closed by refrigerant, the compressor won't run. Your diagnostic of 12V on both sides of the switch points away from low refrigerant charge as a symptom, but it's still worth checking just to be sure. Replace the switch in the circuit with a jumper, then listen for the clunk of compressor clutch pulling in when AC is selected. No need to start the engine, but key must be in 'run' position (II) to do this test.

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Tom added the pic of the current flow diagram while I was typing. It looks like the G17 AC Relay is there for isolation/suppression of the reverse EMF from the clutch coil when it opens. The diagram also shows that the pressure switch is the last element in the clutch coil feed, after the AC suppressor relay. Your test of battery voltage at both sides of the switch points harder towards Tom's diagnosis of a bad clutch coil. Still, the check for continuity from the black/red at pin 4 in that AC relay socket (relay removed), or from E45 or F15 (still black/red) where it passes through the CE panel. Measure resistance to ground with your DMM, and you should see the coil resistance. Key is off for these tests. If the connections are solid and the clutch coil still shows open, then the coil is the the culprit.

FWIW, these clutch coils are amazingly stout. The suffer when there's a LOT of heat, but you usually see other symptoms of that heat before the clutch fails.

My amateur nickel is telling me the refrigerant has leaked out. To collect the nickel if I'm wrong, send a SASE and I'll drop the nickel in and return it.

[Tom]

My very similar clutch coil has a static resistance of about 6 Ohms. You can test for that at the relay (system off, engine off...) to determine the health of the coil itself and all the wiring and devices between.

There's a coil ground lead that, at least on my car, is connected to the compressor body at one of the screws for a manifold plate. If that's not connected to ground, you'll see the symptom you describe with 12V on both sides of the pressure switch with key on. The resistance check I described above will identify that missing connection. You can go to the stub cable connector by the compressor, and test for coil resistance to ground there, and if it's way higher than the ~~ 6 ohms I measured on mine, you've localized the open to the clutch coil and its connections.

Of course, my first-diagnostic-step is almost always to go back through the work you've done since it worked last. Things work until they don't, and the mission is to find out what happened or what you did during that time.

Considering the very recent "re-gassed" effort, I'd extend my search to include the reason for that, and if that reason was solved. Old gas leaked out? -All- the leaks fixed? What is the actual pressure in the system? If the pressure switch is no longer held closed by refrigerant, the compressor won't run. Your diagnostic of 12V on both sides of the switch points away from low refrigerant charge as a symptom, but it's still worth checking just to be sure. Replace the switch in the circuit with a jumper, then listen for the clunk of compressor clutch pulling in when AC is selected. No need to start the engine, but key must be in 'run' position (II) to do this test.

-------
Tom added the pic of the current flow diagram while I was typing. It looks like the G17 AC Relay is there for isolation/suppression of the reverse EMF from the clutch coil when it opens. The diagram also shows that the pressure switch is the last element in the clutch coil feed, after the AC suppressor relay. Your test of battery voltage at both sides of the switch points harder towards Tom's diagnosis of a bad clutch coil. Still, the check for continuity from the black/red at pin 4 in that AC relay socket (relay removed), or from E45 or F15 (still black/red) where it passes through the CE panel. Measure resistance to ground with your DMM, and you should see the coil resistance. Key is off for these tests. If the connections are solid and the clutch coil still shows open, then the coil is the the culprit.

FWIW, these clutch coils are amazingly stout. The suffer when there's a LOT of heat, but you usually see other symptoms of that heat before the clutch fails.

My amateur nickel is telling me the refrigerant has leaked out. To collect the nickel if I'm wrong, send a SASE and I'll drop the nickel in and return it.

Great info on the clutch coil!

My initial reaction was the same as your nickel bet, but I didn't mention it because he said "[c]hecked the low pressure switch and both sides read 12v, still no A/C clutch movement." If 12v is getting through the low pressure switch, that would mean 12v is getting to the compressor (absent harness problems) if I'm reading the schematic right. Now I suppose the refrigerant could have escaped and the switch is just bad, but that wouldn't explain why the clutch is not engaging when 12v is applied to it. That said, to your point, I've never come across a clutch coil that went bad on a 944. As you say, there is a little ground wire that grounds the case, but I don't know of any servicing that requires you to mess with it. If that got cut of disconnected, for reasons unknown, than that could indeed explain why the clutch isn't working. Or the coil just went bad. It's getting to the point where you can't even trust 37-year-old West German parts anymore.

[markagsmith5]

Thanks Tom and dr bob for your quick responses. I'll do the tests as detailed above, although I don't have any way to test the refrigerant pressure apart from taking it back to the mechanic.
As a bit more background, the A/C had not worked since about 2005. I hadn't bothered to get it looked at as I barely drove the car. After retiring in 2021, I have done a lot of repairs and maintenance to get the car back to good running order. As part of that I took the cover off the A/C relay and found the coil wire was broken. I soldered that back together and then had the A/C re gassed back in August this year.

[markagsmith5]

Have measured from pin 4 (87a) on A/C relay to ground, also from both pins on the low pressure switch to ground, all read 162 kOhm. I'll put the car on the hoist later today, remove the engine protective plates and test at the compressor. Hopefully is just a wiring issue.
Many thanks again for your help guys.
PS dr bob, don't have any nickels in Oz, could send you 20c, but suspect the postage may be a bit more than that

[Tom]

Have measured from pin 4 (87a) on A/C relay to ground, also from both pins on the low pressure switch to ground, all read 162 kOhm. I'll put the car on the hoist later today, remove the engine protective plates and test at the compressor. Hopefully is just a wiring issue.
Many thanks again for your help guys.
PS dr bob, don't have any nickels in Oz, could send you 20c, but suspect the postage may be a bit more than that

Not sure what to make of 162k ohms. Insofar as one side of the low pressure switch should go directly to the positive wire of the coil, it seems inconsistent with dr bob's spec, but I'll let him weigh in on that.

I'd probably bypass all the rest and just test for 12v at the compressor when it 'should' be there. If you have 12v, then you've narrowed it down to the clutch (either faulty or not grounded). Your back-story, however, does not encourage me to bet against dr bob. A/C seals dry up when they sit for years without refrigerant, so it's pretty common to fill up an old system and have it work for a short time until all the gas leaks past the dried-up seals. I'd place my bet based on how confident you were that you had 12v on both of the low pressure switch wires.

[markagsmith5]

Tested at the compressor and got a reading of 117 kOhm at the red arrow to the ground point next to it. Rather dodgy looking bit of wire there.

ACCompressor1.jpeg (92.07 KiB) Viewed 752 times

[markagsmith5]

Also getting 12v at red arrow.

[dr bob]

Had to print out Tom's diagram picture. so I can use my crayons to follow the circuit. Thanks Tom!

Your 117k ohms through the clutch coil confirms Tom's diagnosis of a failed clutch coil.

Deciding whether to replace just the clutch or go for the whole assembly is a question that might depend on some longer-term consideration. Depending on the nature and duration of the prior failure, internal corrosion in the compressor itself is certainly a consideration. If the system was open to atmosphere all that time, my instincts would lead me to a whole compressor. I don't know the exact compressor your car uses; the one on my 928 is a rather common Japanese compressor, with only the little manifold plates making it specific to the application. I can buy any 'specific' compressor and just swap my own manifold plates to fit my car. If you plan for a conversion to R-134a, a new compressor with the correct shaft seal and o-rings for the new refrigerant. New compressors come with PAG oil in them, which won't work with R12, should you go for a new replacement.

Meanwhile, take a hard look at the wires from the existing clutch coil, looking for a repairable break. My memory is that the wire itself may be steel, so choose your repair technique as appropriate. I've soldered steel wire before but it takes the correct flux (acid rather than rosin core). You can use a modern silver-solder blend meant for copper plumbing in a pinch. Then a bit of heat-shrink tubing to finish the job.

Let us know how it works out for you. My car is a month or so from winter hibernation, with maybe a month left before the roads get contaminated with a deicer slurry and become unusable for civilized aluminum cars. Predicted high temperature for today is about 45ºF or 7ºC. No plans to use the AC the rest of this season here. But your summer is almost upon you already. I haven't visited QLD yet except as transit. But I did end up in Darwin in your north for a few July weeks, and used the car AC daily on the way to and from a project. Hot and hotter seemed to be the seasons there.

Stay cool!