Here's a little problem with my ’84 300D that has me stumped. The climate control system works perfectly, except that the air-conditioning compressor works intermittently. I have ruled out the pushbutton controls, as I have temporarily swapped them with another unit that I know is good. I have traced the problem to the relay (controlling the compressor) in the engine compartment. If I tap on the relay, the compressor will come on. I slightly crimped the electrical conductors in the socket to assure good contact. I also replaced the relay, assuming that the old one was sticking. The behavior was the same as before however. I swapped the relay with another one, just to be sure that I had not received a defective new one. Again the behavior was the same, the air-conditioning compressor will come on when I tap on the relay with my finger.

I checked for continuity in the wiring leads and they are fine. I pulled the relay out and checked the voltage going to the coil (terminals 85 and 86) -- it was between 12.6 and 12.7 V. so it seems as though there's enough voltage to trigger the relay, but perhaps not enough amperage? Strange. Has anyone else ever come across this problem?

I looked in the service CD, but the wiring diagrams are pretty useless -- apparently they are scanned into a PDF file, but the resolution is really lousy. I noticed in the wiring diagrams that there is an "electronic switching unit for temperature control". Question: is the AC relay supplied with voltage via the “electronic switching unit for temperature control”, in which case this may be the culprit? Or is the voltage supplied directly by the pushbutton unit, in which case I don't know where to turn to next.

Any information would be greatly appreciated!

Thanks everyone!

RTH