The actuators are vacuum operated electrically controlled servo valves. If the ACC is putting out the proper electrical signal then the valve is faulty. When the defrost vents fail the fail open for safety. The rest of the system, the valves fail closed to route the air flow to the defrost vents for safety.

That is why my amateur guess says it's the ACC Unit. I don't know if the valve can fail and stick closed, if the servo was once replaced with the wrong valve (or if it is even possible) or what. I am going to tackle my ACC System this year and want to learn as much as possible beforehand.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si