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What causes the ac vents to change from floor to panel and back by themselves

(2004 Dodge Ram 2500)
in Potwin, KS on May 12, 2014
with ac/heater running
Displaying 1 answers & 1 comments
Popular Answer
on May 13, 2014
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.
on May 13, 2014
do you know if there is more than 1 vacuum check valve,in the system
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