I have a 97 F-150 and this seems to work as described by 4Xpy4 .
With key on run, I selected defrost/floor/etc while listening to doors open/close to make sure things are moving as expected for air flow. Then starting engine with control turned to Off. Putting temp control all the way to coldest then turn to A/C or Max A/c. seems to work. Will probably have to replace temp control at some point though.

 

I posted the solution we found to intermittent A/C problems on a 97 F-150.

https://www.f150online.com/forums/hv...ml#post5196564

 

Hello!
Wondering if it is possible to bypass the temperature control switch on my 1998 F150. I realize it is a potentiometer/rheostat type switch. Just wanting to know if there is a way to bypass this. I can't get any heat right now and the switch feels awfully loose (no resistance).
Thanks in advance for any help offered!
Mike

 

Check to see if the **** is stripped on the shaft.

 

The **** fits tightly on the shaft of the selector switch it's just when you rotate the control it feels like it's not making contact inside the switch.

 

Then replace the control.