The AC in my 2004 F150 supercab is now blowing warm air when at highway speeds. At low speeds and at idle the air is much cooler. Any Ideas?

 

Possible cracked or broken vac line to your AC/HEAT control switch? And low vacuum to the switch causing the vent to let in (hot) outside air when it shouldn't? Then at low speeds it works because you have higher vacuum. Vacuum's always higher at idle and usually at low speeds. That's my first guess anyway.

 

Thanks for the information. That is something I would have never guessed. I will look at it ASAP.

 

Let us know what you find.