A/C turns on once, then won't turn back on again.
 

For what it's worth, mein Fraulein's air conditioner would turn on once then when I moved the switch it wouldn't come back on again.

It originally was the switch position and I had to loosen the screws and reposition so it would click... Did that, then the intermittent problem started again. So I eventually figured out that the terminals were corroded after hours of going back and forth between the pressure switches and the evaporator switch under the plastic sheet in the cowl area.

I pulled the switch to better inspect. (And now that I have done that, I won't do it again. Leave the switch mounted. Trust me.) But anyway, the terminals were darkened so I used some sandpaper and brightened them back up, put on a little white lithium grease, tightened up the female spade crimps on the wires, and now it works fine. I can turn it on and off repeatedly and it works like a champ.

The Fraulein is really coming along nicely. Glad to be driving a 928 again after all these years.

Glad it was an easy fix and did not require replacement of the relay soldered on the ventilation control circuit board.

Good 'tip' there Vegas. Don't forget to rotate the 100 some fuses once a year under the floor board and you should be on the road 100%

well, I thought that solved it all. But the problem came back again. I wound up discovering that there is a pressure switch under the plastic cover in the cowl. It gets "stuck", which I also discovered when I smacked it and the a/c came back on. So... seeing how there is a low/high pressure combination cutoff down at the receiver/dryer, I jumpered the connection in the cowl. Now the a/c turns on and stays on, regardless of where it is in the cycle.

Replace the ps switch.