A/C only works on accessory
 

Hi everyone, need some help. All of sudden, my a.c. only works in my 83 911 sc when the key is turned to accessory. When I start the car, the a.c. stops completely. Turn off the engine, put the key in accessory, and it works again. Diagram shows that the ground wire (yellow) for the smugglers box relay goes to the starter!? Can anyone explain the reason for that to me, please? Problem seems to be that yellow ground wire, as when I ground the wire out, a.c. works fine. Does the starter provide a ground when the engine is running? I don't remember the a.c. working on accessory before. Thanks!

have not looked at diagram but yes, the starter could provide the ground,. it does this so that when you start the car the AC cuts off.

I get that, but why would it stay off when the engine is running...

The yellow wire is not the relay ground. It's designed to shut down the A/C compressor when you are cranking the engine. Key to start, powers starter solenoid, puts +12V to the yellow wire, which puts +12V to pin 86 of the relay. If you have the A/C switch on, then you also have +12V to pin 85 of the relay. +12V on both sides of the coil=no current, so the relay shuts off. It's part of Porsche's special little way of triggering some relays backwards...

To test that system, the yellow wire at the relay should only be hot when the key is to "START".

I'll tell you a little story, it may or may not apply to this, but it sounds similar to me.

I recently got the AC working on my 72 hotrod, Blucille.

I was in the middle of a 1300 mile rally in distant parts. Suddenly, when I got out of the car, I noticed the AC blower would stay on, with the key off and removed. The cig lighter would stay on too. I found this behavior very odd, checked the relay in the smuggler's box, all was ok. The shop that has helped me in the past theorized the electrical portion of the ignition switch....I doubted that since not ALL the accessories were staying on.

So, after the rally I dug in. I found the one wire that provided power to the AC blower was tied to the cig lighter at the fuse box. The wire had been chafing on the backside of the fuse box, to fuse #1, which was constantly powered. So, when the ignition switch would turn off, the power from fuse 1 would still be available to that circuit. I insulated the wire, rerouted it, and the problem was completely solved.

In other words, if this is a new problem, start with the pieces that were recently touched, and then look at common failure points, like the fuse box.

Ok, after a good 8 hours work....power comes in on the black. Doesn't work at all with yellow disconnected.....works with "yellow" pin grounded. So, left the yellow wire off, grounded the pin, and A/C works fine. Thanks everyone for the ideas. Its going to be that way for the time being.