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Jim my understanding is that the ‘79 (and earlier?) had its AC relay in the CE panel, as you’re seeing, but that was changed to a smaller (and apparently under-rated) relay at the hvac control unit itself in 1980 and all later years.
jschiller Thanks for pointing out the Morehouse info. I will add it to my next order from Roger.
I don't understand the reference to homepage "left side menu". What am I not seeing?
I posted the link to the frame that the chart is displayed in to reduce the clutter. The home page can be reached by clicking the hyperlink at the top "928 Specialists Online Store" Then you get the left side menu. Sorry about the confusion.
jschiller Got it, thank you. That site and menu is one I had forgotten. I sold my last 928 in 2017 and bought another in May. Lot of re-learning to do.
jschiller rjtw More help required. I understand the engine needs to be turning to check compressor. Does it need to be running to check for voltage at the switch at the cowl? Then to confirm after disconnect at the compressor, where do I check voltage? At the cowl freeze switch or at the compressor lead?
Last edited by Jim Chambers; 07-08-2024 at 02:08 PM.
Hi Jim,
I *think* with the key in run position (but engine off) and with the AC controls engaged you should be seeing 12V at the compressor if all is working... BUT... I haven't verified that on the car or by looking at the diagrams. You're now beyond my level of expertise with the AC system... until only just recently it was just about the only system on the car that I had never touched in my nearly 40 years of ownership. The good news is that there are many experts here (and many many past threads) that can help.
I've updated post #16 above with some additional links for "further reference" that I found useful. Check out the first two links in particular -- the first is a thread with many other useful links, and the second is a link to a PDF document from user Eplebnista that has an entire how-to checklist about debugging AC issues AND with links to Porsche WSM documents.
I'm sure you'll be able to track things down and find it's not really that bad once you get into it!
Jim my understanding is that the ‘79 (and earlier?) had its AC relay in the CE panel, as you’re seeing, but that was changed to a smaller (and apparently under-rated) relay at the hvac control unit itself in 1980 and all later years.
'80 is the first year of the automatic AC head.
Originally Posted by Jim Chambers
Preliminary check: Compressor not turning with engine running and AC "on". Meter at freeze switch shows 9v with engine off.
If there is at least some voltage at the freeze switch, but not enough to engage the clutch, it is fairly easy and unobtrusive to add an external relay. (As was done by the PO of my car.) This is a good idea to preserve a "good, soon to be bad" AC head relay.
PorKen thank you. That may be the answer as I appear to have 9v at the switch. What is the ABS terminal which is not in the picture and does the fuse go between it and the relay? Do you know what relay I need to do what you suggest? And where to find it?
Need to test more. Still not sure how to test with disconnect at the compressor. I know I sound too green to do much, but I do have some skills and experience!
Last edited by Jim Chambers; 07-08-2024 at 05:34 PM.
Jim, 1980 was also available without the automatic AC option, and this means the AC button in the dash carries all the current of the compressor clutch, there is no relay in the circuit. I can share the 1980 specific wiring diagram if this is your situation. If you have automatic AC, others have already pointed you in the right direction.
Your 80 does not have ABS. You would have to run a wire to the jump post with a 1/4" ring terminal. Regular 53 relay (EG. horn). (Jump post unscrews with a 11mm wrench.)
On your 80, to test the compressor you can run a temporary wire from the jump post to the black wire on the idle speedup valve attached to the strut brace over the engine. This wire goes directly to the compressor clutch. (The brown wire is ground.)
Andrew Schauer Not sure what automatic AC is. How do I tell? I notice some later cars have temp settings on the control. Is that automatic AC? My car has no temp numbers.
Can you send an '80 diagram to jimchambers41@hotmail.com. My manual set stops with '79. The only thing I know for sure at this point is that the compressor does not turn when system is on.
PorKen I see the black wire at the idle valve. I will do as you suggest. However, I am confused by the existence of two wires, one black, attached to a clip on the timing belt housing above the compressor. That black wire also seems to go to the compressor. Is it fed by the wire from the idle speed valve?
If you do not have the optional automatic AC, then you do not have the relay in question.
The black wire comes from the low pressure switch on the side of the accumulator in front of the radiator and goes to the compressor and that valve to raise the idle speed 50 rpm.
If you can pull off the low pressure switch wires, under a rubber cover, and bridge them, this will test if the system is low on freon.
PorKen Andrew Schauer Very helpful. So, I have no relay at the head unit. Do I understand that if pressure is low (low freon) the switch will prevent the clutch from engaging? If I read the diagram correctly, voltage goes from the AC switch on the dash to the temp switch and then to the air valve and clutch?
07-07-2024, 10:02 PM
