Truck Repair: a/c, vacuum hoses, engine vacuum

Question
Have a 98 dodge pickup with 308.  A/C stops blowing out the vents when accelerating.  What could be the problem? Thanks  Randall

Answer
Under the dash, there are some air flow door controllers that are operated by vacuum.
When you open the throttle, the engine vacuum drops.
Usually there is a vacuum reservoir in the system, with a check valve between it and the intake manifold.

Either the check valve is bad, or one of the vacuum hoses or components has a leak, and loss of vacuum allows the internal spring in the door controller, to push the door to a different position.

After running the truck, with it blowing where you want it, shut the engine off, and listen for air leaking under the dash.

Van