Why does the air change from the dash vents to the defroster vents upon acceleration in your 1994 Dodge caravan?

Answer 1

In the 1970's, Dodge operated the vent/heater controls from engine vacuum. There were vacuum lines that went from the engine compartment into the passenger compartment to operate the flaps that directed air wherever you wanted it to go. With the engine off, there would be a "default" or "relaxed" position that the flaps would return to. When idling or when driving under almost any condition except hard acceleration, engine vacuum is high, and moving the controls to something other than the "default/relaxed" position would cause vacuum to be transferred through different actuators, thereby moving the flaps and redirecting the air. Under hard acceleration, engine vacuum goes way down. With no vacuum or very little vacuum, the flaps would return to their default position. It's possible (likely) that Dodge still used the same system in 1994. If your vents quit working completely (and your engine starts running rough), its likely that there is a vacuum leak somewhere....probably a broken/cracked/dry rotted rubber hose.