A/C blows out the defrost vent
 

Could someone with some knowledge on my trucks a/c system please help out? I own a 98 f150 4x4 lariat that wants to blow cool air out the defrost vents. With the a/c in either a/c or max a/c postion whenever you accelerate the air stops coming out the front panel vents and starts coming out the defroster vents. When you let off the gas the a/c returns to the normal vent. Could someone explain what is going on with this system.
Thanks very much

I had an Aerostar van that did the same thing, step on the gas and loose the air from the vents. I took it to the dealer, it was under warranty, and they said it was a vacuum leak. Vacuum is used to acuate the door that selects where the air comes out, and with no vacuum, a spring returns the door to defrost, the only position required to be in a car (need to keep the windows clear).
In my case, the leak was not in a critical line so it didn't effect engine performance enough to notice.
Hopefully you still have some warranty left, if so take it in and have them look at it, or you can try to find it yourself.

Had this problem in a Chrysler before the Exlpedition. There was a vacume check valve that leaked, and allowed the vacume to leak out of the HVAC distribution system, the damper doors defaulted to blowing out the defrost vents, only. New check valve fixed the problem. I am guessing Ford must have a similiar setup.