I'm usually asking for help, but I had a success I wanted to pass on. I had the AC in my 1990 740 charged for the first time in years last year. As I was leaving the shop, I noticed the air didn't feel cold, but I kept driving. A few miles down the road, I was just about ready to turn around and take it back when all of a sudden it started working. Long story short, for the past year I have had issues with the AC working intermittently like this. Recently it stopped working altogether, so I pursued the issue. Weird voltage readings at the hot wire to the low pressure switch. Sometimes full voltage, sometimes just a few volts. Someone recommended checking the climate control unit. So I took the trouble to take out all the components required to do so, which includes pretty much everything below the control unit. Got the control unit pulled out, checked all the connections, even took the actual AC switch apart to inspect. I noticed there are two wires in the plug at the back of that switch. The blur/green one is hot, so I figured the other one (solid green) must be responsible for sending the power on when the switch is in the correct position(s). So I jumped these two wires and checked again at the plug to the low pressure switch. Nothing. So that led me to "somewhere between" the control unit and the low pressure switch plug. Following the green wire led me to behind the glove box. Removing the glove box revealed the green wire enters a relay. And that was the culprit. Oxidized male spade connectors. At first I took the relay apart to look for damaged circuits on the board, but didn't see anything, so just cleaned up the spade connectors and now it works like a charm. Hopefully, this information will be useful to someone else at some point, and save them some trouble.

 

That's a delay relay - it prevents power going to the ac clutch for about 7 seconds after starting the car.

 

Whatever it is, the oxidation on the spades was preventing it from doing its job.

 

OK, I'm going to have to update this thread. While cleaning the relay did result in the AC working right away, the problem has returned. Checking voltage at the disconnect near the compressor shows voltage there. I did find an intermittent ground and thought that might restore it to working status, but alas, no. So how do you check voltage AT the compressor? The hot wire goes into a little square block of some kind before it gets to the compressor. I have no idea what that is. But the main problem is I don't see any place where there is a bare wire to check for voltage at the compressor. Bottom line is apparently there were three things going on, two of which are now resolved (for now), but I am back to no AC, and Florida is already getting warm in the afternoons.

 

You can stick a sharp probe into the wire to check the voltage.

 

Unplug the connector and probe it with the AC system turned on. To check the compressor run a hot 12v wire to the compressor and listen for the clutch to engage.

 

Of course! (I forgot you can do this) So I checked the voltage in this way close to where the hot wire goes into the compressor. Good voltage there. Could it be the clutch? I realize it could be low pressure, but I had no indication the system was losing pressure. It either worked fine or didn't work, there was no on/off/on/off you get with low pressure. Is there a way to test the clutch?

 

Another update and related question: As it turns out the clutch coil died. Can anyone explain to me how in the hell the clutch comes off this compressor? The center bolt comes out. Great. Now what? My a/c guy pointed out that it requires a special tool. So I've borrowed the loaner from 2 different automotive outlets but the metal on their tool is so soft they cannot perform the job. Either that or I'm doing something wrong. Whatever the case, I'm extremely frustrated. Does anyone have any light to shed on this problem?

 

I use a tool like this one

With that puller you will need to find the appropriate bolts (and washers) to screw into the face of the outer clutch. Remove the bolt in the middle holding the outer clutch plate to the compressor shaft, install a longer one to push against. Mount the circular 3 bolt plate, screw the center bolt on the puller to push on the longer bolt you installed earlier. That will pull the outer clutch plate off. Then a snap ring holds the rest of the clutch on - you may need a different puller to get that pulley/bearing off. The electromagnet is then exposed and is held on with 3 Phillips head screws - that are usually very tight/difficult to get out - How much of this were you planning to replace? All 3 parts of the clutch? A word of caution - many times the flange that holds the clutch breaks off the compressor when trying to remove the pulley. Then you have to replace the compressor.

 

I appreciate your reply. Unfortunately, my brain just can't quite make sense of the process you describe.
Here are some shots of the pieces I'm using from the set I borrowed and how I've tried to use them. The part I've labeled with a number 1 is threaded into (onto?) the outer threads on the clutch. But when force is applied by the bolt (2) that screws into (1), (1) just starts stripping threads and starts tilting at an angle. This is obviously not the right way, but without pictures, I doubt I'll be able to understand how it's done.

 

Use whatever set of 3 holes that are threaded, either blue or yellow. Probably the smaller ones to mount the circular disc (from the kit I linked to earlier) with three bolts and washers (the 4 or 5 mm bolt heads will fall through the slots) You will probably have to source metric bolts (home depot). One of those screws in the kit fits in the circular disc and pushes against a longer bolt you put in the middle. For the tool you show - The inner screw that you show might be too fat, and the outer screw might work - perhaps you could use the tool you pictured with a slightly longer bolt in the center (replacing the one that clamps the outer clutch down) As shown you might be "mushrooming" the compressor shaft, pressing directly against it on the inside of the threads, deforming the shape - and destroying the compressor (shaft) With the tool shown, and a longer bolt in the center, when tight give the big bolt on the end of your tool tool a whack with a hammer, then tightn more and another whack - if set up properly that may break the outer clutch plate loose.

 

Okay, I'm starting to get the picture. I think I'm confused about what should be happening at the center. Removing the center bolt makes sense. But I still don't quite understand what you're supposed to be pushing against in the center. Now that I think about it, is it that something threaded, with the same diameter as the center bolt, has to be threaded into the hole the bolt came out of?

 

is it that something threaded, with the same diameter as the center bolt, has to be threaded into the hole the bolt came out of?

YES From earlier "Remove the bolt in the middle holding the outer clutch plate to the compressor shaft, install a longer one to push against"



Use this tool (with 3 bolts/washers you purchase ) and the center bolt that screws into it. Press against a longer center bolt in the compressor shaft (that you have purchased and installed) . That will pull the outer clutch plate off.

 

Got it! Thanks.

 

Make sure to check the front seal and the bearings on the unit. Usually as parts start to fail the whole unit should either be rebuilt or replaced. Good luck

 

Wondering if the part shown (red arrow) is essential. It's part of the existing wiring, but not on the wire on the new coil. I'm just trying to figure out if I need to cut the wire on the new coil and butt-join it.to the existing wire in order to retain this part, or if it really doesn't matter.

 

Further update: replaced the faulty coil and worked great --- until it didn't again. Checked power starting at the compressor working backwards and it brought me back to the relay. Jiggled the relay and it works again. So...