bobzdar

Yes, I did a search, did not find anything matching my issue, which appears to be electrical.

When I bought my car back in Jan ('86 951), the a/c wasn't working (shocker) but I didn't pay it much mind and figured I'd get to it when it got hot out. Well, the time has come, so first thing I did was charge the system up by jumping the compressor. This worked fine and I now have cold a/c, good start. However, when hooked back up correctly, the compressor does not turn on at all. My a/c button works, stays in and lights up. The rest of the climate control unit works fine (and I swapped to one from another car that was working just to be sure), I also swapped the a/c relay and replaced fuse 29 to be sure neither were the issue, no change. Next I jumpered the low pressure switch, no change, it's just not turning on. Where to look next? I couldn't find any obvious issue with the wiring, so maybe to the fuse box next? I get 12v on pin 85 on the relay but nowhere else with the a/c on, which doesn't seem right, but not sure were to look now. Any suggestions where to look first?

944Ross

Did you jumper the snowflake switch contacts?

veleno

What do you mean by "you charged the system up by jumping the compressor?"

bobzdar

No, but I swapped the whole climate control unit for a known working one to rule out any issues with that unit.

bobzdar

I jumped the clutch wire to Pos power to get it to engage and charged the system. Nice cold air. Then I hooked the compressor back up properly and it will not turn on at all.

Jfrahm

I'd check the pressure again to see see if it is high or low. If it is OK you can jumper the AC relay to see if that works.

Also make sure it is an AC relay and not some random relay, a non-AC relay will blow the fuse.

The AC switches are not super robust so I'd verify the replacement one isn't bad.

-Joel.

bobzdar

The replacement switch worked in the other car, so unless I broke it somehow when I R&R'd the unit, it should be fine. Which pins do I jumper the a/c relay with? I tried but could not figure out where the power should go. I swapped the a/c relay with one from the other car that was working, so pretty sure it's the correct relay. If I can get the compressor working by jumping power in the fuse box, that will narrow it down a little.

Jfrahm

My relay actually had toasted contacts in it, and they were also misaligned in production. With the cover off I sorted it out. The relay package has a diode in it.

I do not recall the pins for this relay particularly but it's usually 30 to 87, please verify with the diagram on the relay itself. The outer side poles are the coil and on the relay you can see the winding with a DMM.

-Joel.

PeteL

This is a long-shot. But once, I could not get my A/C to come on. Through some trouble shooting, I became convinced that it was the control panel. Ordered a new one. But when I pulled the old one, the back had come off the harness plug. I snapped it back together and everything started working. Was lucky to be able to return the A/C control panel. With these cars at this age, electrical problems are frequently issues with the connections, vice the components.

bobzdar

Well, I spent a couple of hours working on it this evening and no real progress other than I'm fairly certain I'm not getting a signal from inside the car as pin 30 on the relay isn't showing any voltage, which should be the input voltage to trigger the compressor if the a/c is on. It looks like the relay is always on unless the DME tells it to shut off the a/c, it's just not getting any voltage to the relay. That leaves either the a/c switch/climate control, the freeze protection switch, or the wiring. I'll start with the climate control first and double check operation of the switch and move on from there I guess. I pulled the fuse box to make sure there were no burnt wires or anything out of the ordinary and all looked fine. I also pulled every single fuse to make sure none were blown.

phlfly

May be compressor has high internal resistance, so it shut off to prevent damage the system. I would try different compressor.

bobzdar

Does anyone know exactly what the freeze protection switch looks like? I think I have it narrowed down to that as the a/c switch works (I tested the resistance open and closed and it read properly, plus it's turning on one of the hvac box vacuum solenoids when depressed). There's just no power getting to the relay. I tried jumping positive power from the battery through pin 87a on the relay which leads to the low pressure switch and then the compressor and bam, nice cold a/c and the compressor stayed on, so it's definitely not the pressure switch or compressor or the wiring from the fuse box to the compressor.

I'm going to check the output of the climate control unit and make sure voltage is getting out of it, then I'm going to hunt down the freeze protection switch as my guess is that's either bad or disconnected.

bobzdar

Cannot figure this one out. No power coming out of the cc unit on pin a8, which should feed the compressor. I took the cc unit apart as I have a newer one with replaceable switches and the switch checks out via multimeter, so it's either the cc unit or a power delivery issue. I know the cc unit is getting power as every other function works perfectly and the ac switch lights up when turned on. Is there something I'm missing? Do I have two bad cc units, including a known good newer model I pulled from another car? Doesn't make sense, and the Porsche wiring diagrams show nothing on the cc unit operation, so I don't know if there's something else that tells it to turn on the compressor besides the switch or not...

phlfly

Btw what is low pressure depended on outside temp for R12?

I looked on this link http://www.ackits.com/aacf/ptchart.cfm and shows at 80 F the low pressure port should be 18 psi? To me it's low, it should be at least 25 psi, when compressor is on and should never exceed during cycle ~ 55 psi before kicks on again.
Any ideas about that chart?

Jfrahm

Careful if you use a DMM to test switches, they can fail under load but look OK on the DMM. Sorry I never delved deep into the CC head.