Hi, all-
I just had my A/C checked out by a mechanic of unknown quality. He tells me that, although the fuse and relay are OK (and the A/C light goes on in the control unit when the button is pressed), my A/C compressor is not getting any current. He tried to bypass that and activate the compressor directly, and couldn't get the compressor to work that way, either. The good news is that the system holds pressure (it must have a very slow leak, since there is no refrigerant).

Questions:
What is causing the compressor to not get any juice? What's the most likely problem there? The control unit in the dash? Some random wire is off/broken? If the wiring is messed up in some untraceable way would it just be unbearably ghetto to wire up a separate switch somewhere?

Do I really need a new compressor? What's a good one that doesn't cost a ridiculous amount of $$ (I've heard of substitutes). While I'm at all this... R134a or R12 (hope that doesn't start a holy war).

Thanks!

-Mark
'86 928S

 

He probably bypassed the pressure switch to get the compressor running. With the coolant charge extremely low, the switch prevents the compressor from running to self-destruct.

It's more likely that you have a leak, but the clutch/compressor may need replacing. The material costs might seem expensive, but good labor won't come cheap.

Better to find a shop that proudly displays the "MACS" symbol.

http://www.macsw.org/macs.asp?mfurl=/repairshops/

borland
90' S4, Slate Metallic

 

If the "mechanic" applied a good 12 vdc line directly to the compressor clutch, and nothing happened, then either the clutch is bad or there is no ground connection. If the clutch engaged, and there were terrible noises, the compressor is bad. If the clutch engaged, and nothing happened, either the system is empty or the compressor is bad.

If there is no refrigerant in the system, the compressor clutch will not engage, as there is a low pressure switch on the receiver/dryer.

A good mechanic can check for 12 vdc at the anti-freeze switch under the plastic shield at the base of the windshield. and then at the low pressure switch at the receiver/dryer.

The compressor system is NOT complicated - a competent mechanic who is even passingly familiar with the 928 should have no problem in diagnosing the problem.

There is less reason to go to R-134a now than there was - the cost of R-134a has gone up sharply (from $4 a can to $10-15 a can), so there is less difference in cost between R-134a and R-12. I would not suggest anything other than one of those two. If you can get the system fixed properly, with no or minimal leaks, then I would go with R-12 - if you can find an honest shop to do the work correctly at a fair price.

If you will send me an email ( to tech@928gt.com - not a private message on RennList), I will send you a paper on the 928 HVAC system.

 

Thanks for the advice and help, guys. Borland-- I'd love to do that MACS thing, but there don't seem to be too many in NY...

Here's what I am left to wonder: he said he could tell the system held pressure (perhaps he said vacuum... he did say there were no leaks). So does that mean the low pressure switch was tripped, and the clutch should have engaged? Or no? I am wondering about the electrical connections to the clutch. Apparently nothing at all happened when he applied the 12v line directly to the clutch, although he was able to activate the fan (which I had never seen run, BTW).

I think the key is to check for 12v at the anti-freeze switch and then the low pressure switch to give us an idea of where the problem is. Any likely spots for, say, wires to get corroded or damaged?

I agree 100% about the R12.... makes perfect sense.

Thanks again..

-Mark

 

Mark,

The normal procedure is to first hook up a gage and see if the system has any pressure. R12 at room temperature is about 75 psi. So, if you have anything less than 75 psi, it's all gas and no liquid (very low).

There are a number of reasons that compressor isn't getting electrical power to the clutch, so I won't go there.

The Haynes "Automotive Heating and Air Conditioning" book is a good reference on the subject. Suggest you read up on the topic. I think Wally is not clear, when he says "a good mechanic", as automotive A/C systems are a specialized field of mechanics. Any other mechanic is just not good enough to know what he's doing wrong.

If you want to fix it yourself, you really need technical resources get the job done. If you have the Work Shop Manual, that's a good start.

borland

 

Well, I finally got around to start troubleshooting the 'no power to the compressor' condition. Since the fellow I took it to at least claimed he had tested for power as far as the compressor itself, I wanted to start there instead of at the antifreeze switch. A little backwards, I know, but I at least wanted to verify his claim that the compressor was bad. So I got out my trusty multimeter and popped the hood.

One problem: I can't *find* the A/C compressor electrical wire! I know that it's been mentioned that it's "in front of the right cam belt cover" (John Pirtle's nice write-up with pictures of everything else! ), but there doesn't seem to be anything there. I tried looking at the compressor itself from both the top and the bottom, but didn't see anything at all (I could follow the pressure lines, but no electrical wire). I even tried just feeling for it with my hand, but... nothing.

Maybe the chronic overheating experienced from riding in my non-A/C 928 in the NYC summer has affected my eyesight (or brains.) Am I just blind?

I am going to go and take a picture of it and see if I can post it here in case anyone wants to help. Thanks!

 

Mark:

The wire is between the cambelt cover and the air pump, kind of behind the radiator hose. I believe it has a plastic cover over the connector. Good luck.

 

OK, I'm pretty sure I got it... it was zip-tied to the block, and kind of turned so it was hard to see. There is a plastic connector on it which basically fell apart when I touched it. There is a small black wire leading from this connector back toward the compressor.

The wire itself coming from the harness is in pretty bad shape-- the outer insulation is worn through in at least two places and the bare wire is showing in at least one place. I can fix that easily enough.

If all is working OK, I should read 12.x V when I check there, right? Does the car have to be running or just in the 'on' position (with the AC button depressed)?

If I run a +12V wire directly to that other wire going to the compressor, with the engine running, the compressor should start, right? If that doesn't happen, I assume either the compressor or the compressor clutch is bad...?

Thanks

 

You are working on it in the 'voltage injection' mode. With the engine off, disconnect the black wire connection to the AC clutch and apply 12V directly to the wire leading to the compressor clutch. You should here a "thwack" sound, and see the clutch pull in to the compressor pully. If that happens, then you need to troubleshoot the system normally using the methods outlined in the Nichols/Pirtle articles.

One of the high failure items is the relay in the AC control head. If your wire is frayed to the compressor, this will also need to be looked into. You may want to start with the electrical service instruction about cleaning every fuse, and checking every relay in the car.

Do a search on "AC" or A/C" problems and you will get hundreds of hits. Many of which will describe things to check and places to find information.

Doc 90GT

 

Thanks, doc. I've got several articles, including Wally's excellent HVAC paper, to work from. I wanted to check that compressor clutch first, though, since that's what the mechanic who tried to fix it thought was the problem. I know he checked some of the electricals, but I was suspicious. Anyway, yes, if the clutch works I have the rest of the troubleshooting stuff here at hand... including the relay replacement/modification tip from the nichols site.

Thanks!

 

I try to respect all mechanics at the outset but w/o knowing exactly what he did, it's hard to know how he can say it holds pressure. Unless he has a professional (expensive) leak detector he can't know unless he pulled a vac overnight. If he did that he either released the freon (illegal) to pull the vac and didn't charge you for recovering it properly and /or put it back in w/o reclaiming or he didn't pull a vacuum in the first place. How long did he have the car?

Maybe he did all kosher things. At any rate IMHO you need to know if there is any leak and fix that, otherwise you are wasting $$; then pull vacuum and see if it holds to verify problem fixed, then recharge with clean r-12. You say it was empty, but slow leak. My '85 has no leak, OEM permeable hoses will require occasional charge, but mine has only needed it once in 20 years. Mine may be an exception, but you need to know why yours was empty. Only a professional can advise you about replacing the dryer and expansion valve; if sys was not "empty" and/or open for extended period, dryer is prob OK. And you already know about the power/pressure issues.

 

Thanks, Steve-
Here's what I know: I took it to this guy I found in the yellow pages who claimed to specialize in AC service. Fine. The AC system was found to be essentially empty when I arrived (just a slight hiss when manipulating the adapter, then nothing.) So there was no freon to release. He hooked the system up to his pressure tank and dialed up the pressure to [something I forget]. He left it like that for about an hour with no change in the pressure level. He seemed to feel that this indicated that, whatever the leak was, it was probably slow enough that at least there was a chance that a charge would be worthwhile. This hour was when he "tested" the system electrically, telling me that he "jumpered the compressor". I now know he didn't do that exactly; he probably applied power somewhere else (?) and didn't see the clutch engage and the compressor work, leading him to believe the compressor was bad.

Earlier today, I located the compressor wire and ran a +12vdc wire directly to it. The clutch engaged with a very satisfying clunk. I detached the wire and started the car, then went back under the hood and briefly engaged the wire again. The compressor turned very nicely, with no noise or fuss. Yay.

At this point I realized that without a charge, or at least pressure, I couldn't test the pressure switch. So as soon as I got out of work I went back to this fellow (he's very nice, but he seems to despise electrical diagnostics entirely).

We repeated the pressure test for about 30 minutes and he told me that made him think the system integrity was probably OK. He then pulled a vacuum for about 30 minutes (I was there for this part) with no change in the level. He said that was pretty encouraging too. So I had him add a charge of R12 along with stop-leak and UV dye. While he charged it, he had me start the car and then run my 12vdc wire to the compressor. When I did that, the air became cold... extremely so, actually. So for the first time in my 3yrs of 928 ownership I felt air conditioning.

Now it remains to me to figure out why the system is not working from the switch. I know the clutch and compressor are OK, so that leaves the pressure switch, the antifreeze switch, and the control head (relay). Whichever is most difficult (or expensive) to fix will probably be the culprit But I am pretty sure I can figure that out after reading Wally's paper.

I know that without longer-term testing there's a decent chance that the refrigerant will leak, but I am hopeful that it will take a long time to happen. Besides, I couldn't think of an easier way to find such a leak than to add dye to the system. I do regret that I may be venting the stuff to the air and that it's going to cost me $$ to recharge afterwards, but maybe I'll get lucky.

 

control head relay is the likely suspect

 

Mark,

1) Get a 12 vdc tester - a test light is cheap, Harbor Freight sells a multimeter for less than $5 on sale.
2) Carefully (it is very thin and brittle) remove the black plastic shield under the hood at the base of the windshield. Find the evaporator anti-freeze switch near the center of the car. It will be a 2" block with two wires and one metal tube.
3) Turn the ignition switch on, put the lower level on Defrost. The blower should run at full speed. Test the wires on the block for 12 vdc to ground - that is, hook the test light clip to a good engine or chassis ground, use the point to check for power on the wires (same general ideal with the multimeter). There should be power on both terminals. If there is no power on either terminal, the relay in the A/C control circuit may be faulty. If there is power on one, but not the other, the anti-freeze swith may be faulty. If there is power on both, go to the next step.
4) Find the receiver/dryer (round metal can) between the radiator and the grille. Find the low pressure switch - the one down on the side of the receiver/dryer. Pull the harness connector off the low pressure switch, and check for 12 vdc to ground on one of the terminals - there should be power on one of the terminals. If there is no power on either terminal, there is a faulty connection (possibly at the plugs on the bottom of the Central Electric Panel) or a broken wire between the anti-freeze switch and the low pressure switch.

 

Sorry I haven't followed this up in a few days; that darn "real life" thing intruding on 928-world again.

Anyway, this is what I've since found:
With the a/c set to max cool (engine on or off), I was getting voltage on both terminals of the antifreeze switch, but it was low, even with the engine running (9-10.5 vdc). Not enough current to even light a test light (a taillight bulb), either. If I ran a +12v wire from the jump start terminal to the antifreeze switch, though, the compressor would engage and I would get nice cooling. This seems to mean that the problem is upstream of the antifreeze switch... meaning the control head, most likely the relay.

Looked up the relay repair diagram from the Nichols site (thanks, guys!), got out the solder, and went to work. Installed the new relay from Radio Shack; no problems there. Connected the unit electrically. When pressing the lighted AC button in the car, I could hear (and feel) the new relay activate. ("Success!" Says the optimistic voice in my head). Check the compressor-- it's not turning. What the...? Pull the terminals off the antifreeze switch. Found the one from the dash- Same readings as before-- about 10vdc with the car running, doesn't light the test lamp.

Now I am stumped. What could be causing *that*?

My temporary solution for the road trip I was taking that day (so as not to upset the girlfriend too much) was to leave my test wire from the jump start terminal in place on the antifreeze switch input. A/C ran great the whole time. I did have to open the hood to shut it off, though.

Any ideas? Is it something else inside the control head? Not that I want to, but is there a reason why I can't run some sort of switch in between a known 12v constant power source and the antifreeze switch, into the cockpit <cringe>?

Thanks,
Mark