Heat/air issue with F-150


I have a 1999 Ford F-150 with the small V-8, and I have had issues with my heat and air. Late last summer, my air would stop blowing cold air, and start blowing heat. Then this past winter, the heat did not work at all, but now that it is warming up again outside, the heat works (only when it is warm outside). The A/C only works periodically. Any suggestions as to what causes this or how to fix it would be appreciated.


when you have the AC on, after initial start up, if you touch the temp control knob does the AC 'disappear"

if you shut the truck off, then restart, does the AC act right, until you touch a knob again?

does the fan blow consistently from all setting/ vents as selected?

and lastly, but probably most importantly, is the engine temp gauge working, and is it in the normal range, with no major fluctuations?


For the AC, turn it on, open the hood and let us know if the compressor clutch is engaged or not.



Do you have plain knobs for controls or the fancy-pants electronic thermostat?


Odds are it’s a broken blend door.

See this link & join the crowd.


Fortunately the fix is pretty simple.


It is plain knobs


Check to see if the compressor is engaging for starters. If it is, and you still get hot air, it is probably the blend door. It cold be the temp control pot (it is connected to the temp control knob) if you are really lucky. Try it in positions other than full hot and full cold to see if there is a difference. Sometimes you can get a dead spot.


The AC runs ok for a while, and then will switch to heat. It usually happens when I stop, then start again. The fan blows consistently from all vents, and the temp guage is working fine. It always stays low.
Concerning the heat, the core warms up, but no hot air comes out of the vents.


now for the stupid question…where do I find the compressor clutch? I am new at this repair gig, but am willing to learn.


so you’re saying it changes temp, up or down, with NO input from you? that sounds like a blend door malfunction.

it could be caused by either a vacuum problem, or an actual blend door mechanical break (a bracket, lever or connecting rod)