what causes the ac vents to change from floor to panel and back by themselves on 2004 Dodge Ram 2500

with ac/heater running

Asked by for the 2004 Dodge Ram 2500
If vents are controlled by vacuum a loss of vacuum would cause servos to go to a rest position and usually air only comes out of floor and defrost vents. Diesel engines need a vacuum pump because they unlike a gas engine have no or very little vacuum.
do you know if there is more than 1 vacuum check valve,in the system