Most of it is vacuum. You have an electronic control unit that sends electric signals to a vacuum block that opens and closes the doors in the heater box. I would think that you have a fault in the vacuum system. I would test the VOLUME of vacuum coming from the booster hose. I have seen the line clog up from time to time (hint)>

------------------
Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM