System Controls: Defrost
WARNING: This page is about a different car, the 1994 Ford Mustang. However, it is still accessible from the selected car via links, so may be relevant.
With function selector knob turned to defrost symbol, all vacuum to system is cut off, and cable-controlled temperature blending becomes the only controlled variable. Outside air flows past evaporator core, through and/or by-passing the heater core, and out through defroster and demister outlets. Whenever a vacuum malfunction occurs in the system, operation automatically diverts to defroster only operation regardless of function selector knob setting.