I have a 99 F150 that will only work in the A/C mode when you are at an idle and while the fan is turned to high , you switch the control form off to either A/C or Max A/C. Also, when the A/c is working, if you accelerate too fast you will hear the blend door clunk and you are now in the heat mode (with compressor engaged). You will then need to come to a complete stop, turn the control to off wait a few seconds thatn turn on. I replaced the rehastat on the control panel. What do you think? Blend door motor?

 

Sounds like a blend door/vacuum problem. Do a search here, lots of valuable info on blend door and vacuum problems.

 

Indeed, sounds like a busted blend door.

Dash has to come out to replace. Big job, $700+ at a dealer. PITA but doable by persistent owners.

Steve