Regardless of which air direction button (dash, floor, etc.) is pushed the direction changes from one to another by itself. How to fix this?
It might help to tell us what year Voyager.
Chrysler HVAC systems were prone to this type of problem for many years, for some strange reason. I believe that the problem no longer exists on their new cars, but it did plague many Chrysler owners, back in the '90s.
I can recall more than one auto magazine commenting that when they accelerated or climbed a hill with the Chrysler-produced vehicle that they were testing, the air flow would suddenly be diverted from the floor or dash setting to the windshield defrost setting. Then the driver would have to manually reset the direction of air flow.
Apparently these systems are susceptible to changes in engine vacuum, and when reduced engine vacuum is detected, the controls go somewhat screwy. It is possible that a Tech Bulletin or two might exist–assuming that Chrysler ever figured out the source of the problem. You might want to ask at the dealership’s service department if there is a Tech Bulletin regarding this particular problem.