The a/c on my 1987 300E is acting strange. It works fine on start up but sometimes when I shut the car off and re-start it, the compressor does not engage. This does not happen all the time but it does happen on occasion. Usually when I re-start the car the compressor will engage and work as it should. If it doesn't engage on first re-start it will on the second re-start. Has anyone had this problem? The compressor is about 18 months old and the refrigerant level is correct. I am looking for suggestions where to start. Could this be the climate control unit? The ignition switch? The speed sensor on the compressor? The relays? Thanks in advance for your suggestions.

Lots of possibilities here; you might consider the following thread. I also wonder if there could be an intermittent short in the climate control?

http://www.peachparts.com/shopforum/tech-help/16397-91-500sl-c-compressor-will-not-engage.html