I have a '94 F-150 (302)-Automatic; my problem is when I turn the Max A/C on, the air come throught the vent normally, then after a couple of minutes, I loose air flow at the vent and it seems to going to the defroster. I turn the control to normal A/C to get air flow back, then go to Max A/C and it works for a while, then the problem repeats. Help? Thanks!

 

Sounds like you have some vacuum troubles.
I would suspect a leaking vacuum line at the vacuum motor for the recirculation door. Be sure to check all the vacuum lines.

FordMasterTech

 

I would agree with masterfordtech. The reason it goes back to defrost is that is the default postion in a failure situation so that you can also have a clear windshield. Sounds like a leaky vacuum somewhere or possible a electronic control that is goofing up and causing it to go to the default defroster postion.