I have had a number of problems with the A/C control on my 1988 420 SEL. If I have the heater giving warm air and then select a cooler temp, I can't get the heater to go back to warm without an ignition cycle. I also have a problem keeping the fan running in auto or low. Auto works ok when min temp is selected. The fan usually won't turn on initially in any selection, although if I move the ignition key slightly while tapping the brake it will come on. The A/C compressor will only work about half of the time, but when it does it cools just fine and usually keeps working. I replaced the push button control unit, but still have the problems. Any help would be greatly appreciated.

Had similar problems with 87 420SEL; replaced bits and pieces with no luck - and then took it to good MB mech, who replaced pushbutton unit, aux water pump and other items. Bottom line: W126 a/c problems are beyond capability of most home mechanics; having the right diagnostic tools is a must. Bite the bullet and take it to an expert.

Joe B.

REPLACE YOUR IGNITION SWITCH. IT IS DEFINATLY BAD!
AFTERWARDS IF ANY OTHER PROBLEMS OCCURE ,THE CAR IS EQUIPPED WITH ONBOARD SELF DIAGNOSTICS.
IT'S A SIMPLE MATTER TO TEST AT X11/4 BUT REMEMBER IT LOOSES MEMORY EVERY TIME THE KEY IS TURNED OFF.
MF