2000 ford f150 truck ac fan

My ford pickup ac has started to stop blowing air only when you accelerate from 30mi an hour to 60mi an hour. At low speeds it works fine. Once you accelerate to 60 and hold at constant speed the air comes out the vents fine. During the acceleration the air seems to be rederected to defrost or somewhere else. Why would acceleration affect air coming out the vents? ANY HELP WILL BE A THANKS KENT The dash airflow is controlled by vacuum. Acceleration increases the vacuum (lower pressure) which affects the dash setting. So you are getting a temporary redirection that eventually returns to normal.

Check with a Ford service manager to see if there's a TSB on this problem. Whether there is or isn't, Ford may recommend installing some kind of check valve on a particular vacuum line to deal with this situation.Car Repair Talk's forum.