Both.

The actual door position for airflow out the center duct requires vacuum application. Since both the defrost door and the center duct are on one command vacuum line a single vacuum failure gives the spring loaded default of no center vent and full defrost.

The decision to place vacuum on that circuit is in the hands of both the pushbutton control unit and the temp control unit (on pre-86 units).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician