I have a 2001 f150. My problem is when i have my ac on and the rpm gets towards the higher side the air switches from the main vents and starts to come out of the defrost vents. But when i let off it returns to the main vents. Any suggestions to the promblem? Thanks. -Kyle

 

I have the same problem on my 98 f150 4.6

 

Either your vacuum is leaking down (check all your vacuum lines) or 1) your actuators are getting weak or 2) Your blend doors are hung up. I think there is one or more threads regarding these doors not working right. Search for 'ac not working' or something like that.