1995 SL500 A/C compressor problem
 

I just replaced the compressor for a second time. First rebuilt made "whinning" noise after it was replaced, but the air was cold. Rebuilder sent replacement. I just installed it, along with new expansion valve and drier, with help from ASE certified tech. Air compressor will not cycle on even when the dash light indicates it should be activated. Also, before this second compressor was installed, the electric fans worked properly, ie. turned on when the engine coolant temp climbed toward 90C (hot outside) and when the a/c was "running" like it was supposed to. Now the fans are on from the onset of starting the car even with the engine is cold. Did we reverse a wire or have a switch go bad in the process of taking things on and off? This thing is costing me a lot of money and time removing and replacing twice even thought the part was covered, the labor is not.

1st thing to do is test the coolant temp sensor for the aux fans!

A shorted OR open circuit on that sensor WILL keep the compressor from coming on.