Jim Chambers

A week ago the AC worked exceptionally well, almost too cold on an 80 degree day. Today, as I was leaving a store parking lot at about 90 degrees, I switched it on and it blew only hot air. The fan comes on and the AC button lights but no cool air. Where do I start trouble shooting?

gazfish

Could be a failed relay on the head unit, lots of info here on the problem/fix, or you could be out of gas.

Jim Chambers

Apparently my search skills are weak. What do I search to find problem/fix? And what is the head unit?

jschiller

This thread is probably the granddaddy of all HVAC relay threads and has spun off a dozen or so follow-ups.
https://rennlist.com/forums/928-foru...re-w-pics.html

Note the Radio Shack relay that Dwayne used is probably NLA but many of the follow-up threads give part numbers for substitutes.
For example:
https://rennlist.com/forums/928-foru...ubstitute.html

Jim Chambers

Thank you jschiller. Dwayne's write up gives me a headache. Since the fan works, I believe that eliminates the issues of fuse and one of the relays on the board. There is another relay labeled compressor (speed limiter). Could that be it? Clearly I need to spend more time checking, but it appears the compressor is not compressing. However, it is 101 here at the moment and near 90 in the garage so nothing until morning. I'd almost rather do without AC than tackle the head relay fix.
I just got the car in May and was thrilled that the AC worked and worked well.

jschiller

Here's a link to the original Dr Bob and Louie Ott writeup that Dwayne refers to:
https://www.nichols.nu/tip055.htm

Note that they are working on S4s so the exact steps may be a little different for your MY80. But Dr Bob's basic troubleshooting steps should still apply in your case:
Here are steps for quick diagnosis:

With the engine running and the AC switch on, look to see if the compressor is actually turning. If it's not moving, pull the plastic tray at the top of the engine compartment/base of the windshield, and locate the anti-freezing switch that hangs in the center on that little bracket. Use your meter to measure the voltage at one of the switch terminals to ground without pulling the wire connectors off. Relay good will give you 12v, a controller relay weak/bad will give you much less. If you find low voltage, double-check your diagnosis by pulling the connector at the compressor and try again. If the voltage comes back up with the clutch coil disconnected, the problem is definitely that controller relay.

BTW, I don't see a AC Compressor relay on the CE Panel layout, the last year that I see one is for a MY79. Are you sure you have a MY80?

Jim Chambers

Thank you, very helpful. I have a 4 volume WSM which came with my car. There are two CE panel illustrations, each with same page #, 1 says printed 1979 and the other has no date. Both show a compressor speed limiting relay (XI). I am certain my car is a 1980 US (CA) model. I will do the checks you described tomorrow. Is the "controller relay" the one in the head unit, accessed as Dwayne describes? Oh how I don't want to do that!

jschiller

Yes.

You can get a reconditioned head from Mark at 928 Int'l if you don't want to tackle the relay replacement:
https://shop.928intl.com/A_c-Control...3%2D060%2D05R/

Check with him to make sure 'reconditioned' means the relay has been replaced!

Jim Chambers

Thank you. I will report after I do the preliminary trouble shoot you provided.

Jim Chambers

Preliminary check: Compressor not turning with engine running and AC "on". Meter at freeze switch shows 9v with engine off.. Also, not certain (bad hearing), but with engine off and ignition/AC on, I think I hear a "click" when I connect the v meter to the freeze switch to get the 9v reading. It's too hot and I'm too lazy to do more today.

rjtw

Jim,
While I won’t hazard any guess on the cause of the issue, if in the end you DO track it down to the hvac controller/temp control unit in the dash, just a word of encouragement: it’s actually not a big job to remove it at all.

See my posts starting at around post 149 here:
https://rennlist.com/forums/928-foru...oses-83-a.html

While you’re in there, it’s a great time to replace the little light bulb that illuminates the hvac controls and sliders, and perhaps the sliders too (mine were wiggly from broken tabs).

Dr. Bob had a great writeup somewhere about replacing the hvac relay. Perhaps you can find it if that’s where you need to go (EDIT found it, see post #16 below). Personally, I would consider adding a relay down at the CE board in an unused slot (and documenting it for future owners) if and when I need to deal with it.

The HVAC system, like so many other systems on our cars, can seem really daunting at first but once you get into it you’ll see it’s it that bad at all. And it’s always a great joy to get things working as well as or even better than what they were!

Good luck!
Rick

Jim Chambers

Thank you rjtw.

Jim Chambers

jschiller I have figured out that latest wiring diagram in my factory WSM set is for 1979, published in 1978. That's why I see a relay on the CE panel. My car is an '80 but I haven't yet opened the CE, Can you tell me the changes between '79 and '80?

jschiller

I don't know enough about the early cars (usually referred to as the OB's) to tell you the differences between the two years but for immediate use, one of our stalwart vendors has published a relay chart which you can view online for your year at:
https://www.928gt.com/t-80fuse.aspx
Other years are accessible from the homepage in the left side menu.

Even better for long term ownership is the Jim Morehouse set of manuals available from Roger at 928srus and worth every penny:
https://928srus.com/products/jim-mor...0369784e&_ss=r

It has a Workshop Manual and the parts list for all years, and much, much more. Get it!

Jim Chambers

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?