i have a 1990 420SEL. If i turn the heater on while driving, you get nothing but cold air. if i am stopped, or better yet, shut the engine off, restart, then turn the heater on (in that order), nice hot air, until you step on the gas, then it will start to get cold again. sounds like a vaccuum problem to me. are there check valves, or door actuators that might not work when engine vaccuum is low? I don't want to throw parts at it, like a new control unit $$$$, if it might be something else. Also related, the AC blows out the defroster and all the vents, instead of just the AC vents, or the floor vents, etc. any ideas?

I'd like to know the answer to this question, too...

George Murphy helped me get mine straightened out; he is expert in these systems. I found out about him in the Mercedes-Benz Club of America magazine. He rebuilds the control boxes, too. He is somewhere in Tennessee, he has a web page: Performance Automotive, or something like that. I am not advertising for him, but help is help when you are too cold or too hot in one of these things.