Ok, so I was idling in a parking lot today, with the a/c on normal, cold, but it was on low fan speed. Well, I start to drive away, and switch my fan speed from low to high, then after a while i went back down to low. And when I did that, it didn't pump out any air. I toyed around with the controls for a bit, and started to think that maybe the switch has gone out. But then I noticed when I accelerated that the a/c would work like normal, but only when I was going fast, or accelerating quickly. Now I'm thinking that a vacuum is loose somewhere. Does anyone know where I should start looking?

By the way, it's a straight 6, '96 f150 reg cab.

 

look at the damper door that switches from heat to cool it may be stuck you also could have a bad blower motor