How do I repair a 1999 F 150 blender door?

Blend Door Problem

Your problem sounds like the common blend door problem on the Ford Expeditions, and Ford Explorer. The door is on a vertical axis and when it breaks, it can swing back and forth randomly giving heat or AC or nothing. The difference on acceleration is the force placed on the door swinging it one direction.

You can check the operation by fully opening the glove box, removing the actuator motor on top of the plenum box and examining the blend door axle. Pictures of the common fail signature are available on the heatertreater.net web site. There are several "free" fixes that mostly are worth what you pay for them. The solution on the web site is solid and proven over hundreds of vehicles and will resolve the problem once and for all.