I very recently replaced an ACC switchover valve in my '85 300D. These are in the console behind the pushbutton climate control unit. After I had taken the valve out (leaving the vacuum line open), I decided to test the voltage going to that valve's electrical connector to make sure that the climate control unit was sending the right signal to the valve. The suspect valve was the one that controlled whether the leg flaps would open and let heated air out. After I found that manipulating the temperature dial would send voltage to that valve, I tried to turn the engine off but it wouldn't because of the open vacuum connection which I had not yet hooked back up. The engine stopped after I plugged the vacuum line with my finger.

My experience leads me to think that the problem you may be having occurs because a vacuum line or diaphgram downstream of the appropriate heating-related switchover valve may have a leak in it. When the climate control unit opens up the heating related valve to allow vacuum to activate a flap, the leak doesn't let the engine shut off. If the temperature dial is in the blue, then the heating-related switchover valve doesn't open and there is no leak.

That's my guess, but I am pretty new to this...so consider the source.

Good luck,
Dan