AC compressor disengaging
 

I have a 1987 300E with 120K miles on the clock. The car runs fine in every aspect except for the air conditioner. When I start the car and turn on the air the compressor engages and blows cold air. After about 15 to 20 minutes, the compressor dis-engages. If I restart the car everything is ok for another 20 minutes or so. Could this be an ignition switch problem. It is time for the belt tensioner and drive belt to be replaced. Could it be that the belt is slipping on the compressor? Any suggestions are appreciated.

Thanks in advance,
Donwo

Yes, the belt is slipping. Tighten it a bit if it looks to be in good condition. Replace it if not.

Don't forget to loosen the big 19mm bolt or you'll strip the threads on the adjuster.

I would bet that the a/c clutch coil is going bad.
It is electromagnetic. It could be getting hot and shutting down.
Attach a test light to the clutch wires and see if the light stays on when the a/c quits woking.