I own a 94 F150 and recently when I have my AC running on MAX AC or normal it will quit blowing air out of the vents. When it does stop I can still hear the fan turning, but the vents are shut and the air is going no where. It usually does this when I'm going up hill or when I get above 70mph. I messed with it today and was looking at that valve thing underneath the dash and turned the switch to floor, Max ac, normal ac, defrost and vent. When I would turn it to Max ac it would move reall slow to open up. But when I turned it to normal AC it would move a lot faster.

Sorry for the long post, but I'm really not sure on what is wrong.

Thanks.

Maybe I should put this in the HVAC forum? Sorry if I put it in the wrong one.