Pierpressure

Couple weeks ago the AC in my 88 S4 stopped working. I am lucky enough to live near Jim Doer and took it by his place for help in diagnosing the issue. He isolated the issue to the relay in the control head and we followed Dwayne's write up to swap in a replacement from Radio Shack. This seemed to fix the issue and I was enjoying my AC on the way home when there was a puff of smoke from the sliders on the control head and the compressor stopped running.

I figured there was an issue with my HvAC controller so I ordered a rebuilt unit for 928 international. It showed up last Friday and it seemed to do the trick for about 5 min until the same thing happened. I spoke with Mark at 928 Int today and he thinks my compressor is the culprit.

Should I replace or is there any other components which could cause this issue? I am comfortable fixing the relay but not sure where to go to diagnose the problem.

MainePorsche

As you've smoked more than one 'new' relay - I would agree with Mark. Someone who knows that exact local cause within the compressor will tell the reason why, but your compressor is drawing to much current for the little relay to handle. Search Wally Plumley (WallyP) who has posts on employing a second relay to send current to the compressor. The HVAC compressor needs only to supply the small amperage to the relay to activate the coil.

jcorenman

Shorted windings in the compressor clutch coil. A short can bypass part of the coil, decreasing resistance and increasing current. The increased current will more-or-less make up for the bypassed windings, but the increased current will toast the relay.

In theory you can just replace the coil (which is available separately-- search 928 Int'l for "A/C clutch coil"). The pulley needs to come off, which may require a puller, but if that can be done in situ then it would save having to evacuate and recharge the system.

Alan

...? The HVAC relay just drives the compressor pulley clutch, you should be able to swap this without changing the whole compressor. A functional issue in the compressor itself (over pressure etc) should not affect the clutch coil current though... So its likely just the pulley/clutch/coil

I'd supply the compressor pulley clutch line with power via a fuse and see what current it takes with a series DMM in 10A ammeter mode. You can simply connect to it at the freeze switch.

Alan

SeanR

When you put in the new unit from 929Intl did you install the fuseable link at the freeze switch? It's included just for this reason.

Pierpressure

It did not come with the fuseable link and I did not become aware of adding one until I came across a thread where Greg Brown recommended it. Unfortunately this was after I fried the second unit. Should have used the search function to do more research before throwing expensive new parts at the problem.

worf928

FYI: 3A is the compressor clutch draw.

Jfrahm

I use an aftermarket fog light relay mounted near the jump post to reduce the load on the relay in the HVAC head. You drive this relay with the signal from the HVAC head and then run a fused wire from the jump post through the relay to the compressor. It's pretty easy and should save the CC head. You can find a simple relay with a mounting ear for use with aftermarket for lights or whatever in your local auto parts store.

The AC clutch wire gets pretty beat up and chafed and might also be shorting out, maybe someone has put in a 10A fuse on that circuit and you are turning CC heads into expensive toasters.

-Joel.

MainePorsche

You would not have known about the current draw causing specific failure of the in unit relay without prior knowledge such as seen in Greg's post on it. Sounds like you assumed it was a failure of another component in the unit.

Jadz928

Great feedback!
I hadn't yet considered the clutch coil as the culprit, however I did test full circuit to the comp (just not amp draw) I can certainly put 12 volts on the coil and test amps.

When I went thru the system, I found the HVAC head was re-wired to direct power (not relay pwr) from another place on the panel. Fuse #6 for A/C had been out of commission due to a bad terminal (and had been bypassed)
It was my plan to replace the fuse block and inspect the back of the CE panel for shorting.

I was focused on that end of the problem, but now will focus on both ends. Poor poor HVAC head relay stuck in the middle.

andy-gts

when MO was around here (now in california) he pulled the board and said the problem was porsche was running too much current through the board and frying it. INSTEAD OF a relay he soldered a thicker segment on the board and it never blew again…..

Pierpressure

I am trying to apply 12v from the jump post to the compressor with a lead with an in line 7.5 amp fuse and it blew. Is there a recommendation on how big a fuse to use? I chose the 7.5amp because that is what is in the CE panel at the ac position.

Alan

If it takes more than 3A steady state it is fried - time to replace it ... culprit found I think.

Do test the line with an ohmmeter - it could be that there is a direct short, should settle out at more than 4 ohms - if it is virtually zero - look for another cause

Alan

dr bob

If the clutch circuit is drawing more than than 5 Amps, you need to find out why. Testing with bigger fuses isn't a good plan. Instead, use the resistance/Ohm meter function on your multimeter to determine the resistance of the clutch coil circuit. From memory mine read somewhere in the 4 to 6 Ohms range. To blow the fuse, yours would be in the <2 Ohm region.

The circuit for the clutch coil passes through the 14-pin connector, a few feet of the front-of-engine harness, to a male spade connector breakout very near where that harness passes the oil dipstick tube. There's a female connector there on the lead to the clutch coil itself, with the lead passing through the support brackets for the air pump and the compressor, a screwed-down clip on top of the compressor itself, then into the clutch coil from the rear. The ground lead comes out of the clutch at the same place, and has an eyelet terminal on it that fits under a top cover screw. On lead from the harness, it passes through metal pieces that might rub through and short to ground. It's dark, tight and dirty in that area in front of and on top of the compressor, so some o the inspection will be by feel. If there's a rub-through, you can probably add a layer or two of good heatshrink to it. Just remember that you don't want to make the wire so rigid that the stress point for any flex is localized to a short area of the wire.

I have that harness connection gathered and coiled with a cable tie, and also have a cable tie for the main harness where it passes behind the dipstick tube. The little wire from that gathered pigtail bundle is tied in the bundle, then the section to the coil is loose as it snakes down through the brackets and supports. My goal is to let the harness and all the attachments sit and move with the engine so no flex points and wear in that area.

Alan

This is true only of the very early cars ('78/'79) that have no relay for the clutch...
On all other models this is false and there is virtually no loading on the slider switch so no sense modifying it - the whole issue is that the load was movd to the integrated relay contact6s - which are also insufficiently sized for the load.. ergo they moved the problem elsewhere but didn't solve it...

Porsche screwed up here in both cases - and should always have mounted a full size (replaceable) relay on the CE panel for the clutch - it was just silly not to. IMO the correct fix here is really just to do exactly that - add a new relay on CE - there is always at least one spare slot. The relay for the clutch should also be individually fused - since it's at high risk of mechanical damage during servicing - makes sense this is located on the CE panel too.

Alan