i have a 93 e150 when i go up hill the a/c stops blowing out the dash vents and starts blowing out the defrost vents please help driving me nuts

 

You've got a vacuum leak somewhere. The mechanism that operates the vents are controlled by vacuum. Best way to tackle this it to open up the dash and check all your vacuum hoses from the motor to the vacuum actuators in the dash. Hope this helps!

 

Did you ever find out the problem and able to fix it? I'm having the same problem with my 1997 E-150 4.6L with front and rear A/C