2005 Ford F-150 • 120,000 miles

THE AC WORKS GREAT UNTIL THE TEMP SWITCH IS ADJUSTED TO WARM, THEN ITS STICKS IN ONLY HOT AIR. When temp outside reaches one-hundred degrees its begins to blow cold. There is no control on the temp from a/c.
it stays in the hot position with no adjustment.
Leland homan
June 28, 2013.

You need to check the blend air door. It determines if you get heat or AC to the passenger compartment. It is located near the heater core and should open and close based on the temp you are looking for. Check to see if that is working.