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.