need some help for my 420 SEL
 

the car runs fine (temp. OK), until i turn on the AC. As soon as the AC starts working and the car sits at a light for few minute the Temp starts to go UP. While driving the temp drops back to normal. I do not see the fan in FRONT of the radiator come on at all. Could that be the problem? And if thats it what relay should i look to replace? Any suggestions would be greatly appriciated:(

The aux. fan has a low and a high speed. It sounds as though you are not receiving the low fan speed(for starters). Check out Arthur Dalton's comments in this thread:

http://www.peachparts.com/shopforum/showthread.php?threadid=43267&highlight=fan

Accordingly, the low fan speed(the first fan speed that would kick in) is controlled by a pressure switch on the a/c systems receiver drier. I'm assuming that your a/c is supplying cold air.

There have been posts here about bridging connections off of the receiver-dryer in order to do problem analysis. In addition, there are countless posts on the fan issue.

I'd be suspicious of a pressure switch on the receiver-drier; then again, it could be many other things.

More ideas will follow.

Check out the SEARCH facility.