My 2001 F250SD 5.4L is doing something odd. When I have the A/C on and accelerate, it shifts from blowing out cold air from dash vents and goes to blowing lukewarm air out from the defrost vents on top. As soon as I decelerate or tap the brake pedal, it switches back to the dash vents and blows cold again. Even minor acceleration switches the air from blowing out the dash vents to blowing out the defroster vents.

Any suggestions or is this a known issue?

Thanks in advance.

 

My guess is you have a vacuum leak somewhere, I think the ac system uses engine vacuum to operate the vents, when you accelerate it opens the throttle butterfly and you loose the vacuum, it has enough to operate it at slower speeds but that is it.