I have a 99 f250 sd 7.3 my heat system sometimes work normal but sometimes it blows cold air. I can have the heat on, and it works fine. turn it off and then turn it back on again and it only blows cold air. the temp of the motor never changes. I have looked behind the glove box and all looks normal. sometimes the heat will never come on and only blows cold air. any info would be helpfull
thanks

 

With the age of your truck the blend door controller may be fritzing out. Its not very expensive if thats what it is and can be replaced relatively easy on the super duty.

It could be as simple as a cracked control ****.

 

HomerWinzlow
is there a way to check out the blend door? It looks like it operate nml, when you go to off postion it goes full closed and when you go through each postion on the control **** it moves. What lets the heat come through is it the blend door?