Probably a bad CCU . CCU puts voltage on the appropriate vac valve for diff functions. If no voltage out of the CCU the system goes to defrost mode because in Germany they want defrost if the heat system electronics or vac system fail. Do a search on CCU to see about possible resolder repairs. Used CCUs can be found also.

__________________
85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K