Fred - thanks for the reply. I also think that I have a short somewhere but have been unable to locate one. A problem is that the Mercedes maintenance CD doesn't have a wiring diagram that matches my 91 300E. The CD says that I have a A/C control relay while I actually have a MAS control unit, so I'm checking for shorts in the blind. No sign of varmints or belt slippage.

I'm going to try to isolate the problems (A/C compressor and fault code 17 for the oxygen sensor) for a while longer, but I probably need professional help. Does anyone know a technician in the San Diego area that can be recommended?

Walt Lake