Starting yesterday on my 02 f150 the a/c would vent out the defrost vents instead of the normal vents. This happens only when I am accelerating or increasing the load on the truck. As soon as I back off to a normal driving speed, it will return back to the normal vents. Any Ideas??

 

You have a vacuum leak. The hvac system in the F-150 will default to defrost if the controls lose vacuum. There is a hose that runs through the firewall near the battery. Good luck!

 

Thanks John for the info. I'll start chasing it.

Gene