onionpatchkid

I've got the car at a retired mechanics house and he's worked on 928's before but it's been a while. My A/C quit working. He's gone through the system but without the correct wiring diagram he's come to a stand-still. This is what we know:
The switch in the cab works correctly
The fan works correctly
The relay works correctly
The fuses are good
The A/C compressor will work if you by pass the power to it
The Freon is a little low
As far as we know the system is all original and has not been messed with.
Something is keeping the compressor from getting power and turning on through the system.
Is there some kind of safety switch, thermal, or solenoid that could be preventing the compressor from getting power when it's called for??? Help!!!

Speedtoys

Freon is "a little low"

What is a little?

Maybe the low pressure switch is doing its job, and thats standard AC stuff.

https://www.928gt.com/T-WallyHVAC.aspx

Mrmerlin

Your engine bay looks clean,
It ooks like you need a hot post cover a new stock air filter and a new coolant cap.
For the AC system,
add a can of refrigerant and see if it comes to life,
otherwise you could update the system to a Denso 80 to 88

The original Bosch system is not easy to fix as parts are NLA.
Also look at he lines see if any have a greasy dirt on them this indicates a leaking line,

Pay attention to the thin line where it passes next to the radiator
Greg Brown makes a repair line for this area as they all seem to leak at he swages

Petza914

Also, if the system is still original, it needs to be recharged with Freon, not any of the newer refrigerant options so make sure the guy working on it has old-school Freon. Changing to something else will require new seals and a bunch of other parts.

SeanR

Check power at the low pressure switch. If you have power to it but not past it, then it's doing its job. Add a bit of coolant and see if that wakes the system up.

onionpatchkid

I'm relaying messages from the guy working on it so bear with me. He did add the old style coolant. It pressure tests ok. If he bypasses the system and sends power to the compressor directly, it works and it blows cold air now. What he really wants is the electrical schematic for the A/C system for a 1979 Porsche 928. I was able to find one in a workshop manual online for 78-79 928's but he says the wiring in my car is different than the diagram and the numbers are different. Does anyone know where I can find the correct diagram online that I can download?
He says he can never get power to go to the A/C relay. He's not sure how the relay gets its power and from where. I have a spare relay and he's tried it but the relay never gets power.
He says the #9 pin to the compressor has a black wire. He wants to know what color the wire should be at the other end. He cannot get continuity on this wire. He does not know or has not checked the de-icer capillary, high pressure limit, low pressure limit. He needs the wiring diagram to do that.
Thanks for any help!

John DeHart

I had a similar issue.. check your freeze switch. I had to tap mine a few times to make it work... and it stopped working several times over a year. After a summer of constant beatings its been working ever since. You may also have to replace the micro switch in the head unit it (inside the car). A nice write up on the A/C system --> Here. I have also included the current diagram for review see sheet 18 of 23.

onionpatchkid

Will do. I picked the car up from the mechanic yesterday even though he hadn't figured it out yet because I wanted the car back and he was leaving town for a couple of weeks. On the way back, I turned on the A/C for the hell of it and it started working and blowing cold air! I drove for about 20 minutes and it worked, stopped working, and then started working again. I did read and send that article and the mechanic said it was really helpful but my system has some differences since it's older.
One thing I noticed last night though is the seems to have like 75 less HP now. A significant drop. I used to rev the engine up and the whole car would vibrate with power. It doesn't do that now. It definitely doesn't accelerate up to speed the same either. I don't know what the heck happened. I haven't told him yet either. I'm really bummed right now.

Petza914

If you turn the AC back off dies it feel like it used to? The parasitic drag from the A/C compressor running is significant. I can tell you from feral when the compressor is running and not just idling on every car I own - flat 6s, supercharged flat 6s, NA v8s, turbo V8s, supercharged v8s, etc. I drive my cars with the windows down and eco button pressed (or vent only mode on the 928) whenever possible, since they run so much better without the AC running.

onionpatchkid

The AC came to a grinding halt on Saturday. I think the compressor clutch fried and is metal on metal. Is this an easy fix that a handy guy can do himself or should I leave it to a pro? Could the clutch have fried because there's something wrong with the compressor and it's locked rotor? Here's a photo of the clutch.