One thing to check is to see if the compressor tries to come on. Look at it with a light and have the wife hit the Defrost button. If it cycles, you are facing a speed signal problem or broken wire. IF not, you are looking at an input problem.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM