S420 AC Works great, quits, works again, quits again
 

My wife's 1989 S420 AC has started to fail at times. I had the freon checked and it's fine, the R-12 system has started to quit working. One moment it's cooling at 42 degrees, then it quits. The next time I start the car the AC works great. Two days pass and everything is fine, I head in to town and the AC quits again cooling. The compressor does stop spinning when the AC stops working. As long as the Compressor is working, the system works great. Yesterday the temp outside was 85 degrees as I started it and it did not work. Later while the car was still hot and the AC works great.

98 ML320 with same problem
 

Rod,

Ive got the same problem with my wife's ML. I have been working at narrowing the problem. Same symptoms...Ive gotten several possibilities from an evaporator temperature sensor, to the control unit...Those are two possilities you can check.

I thought it might have been the control unit, but from what I see happening now, the AC will act up even when I dont mess with the controls at all. I want to know if anyone knows how to test the evaporator temperature sensor? If there is a way to test it.