[scbaguy]

my 99.5 F-250, 7.3 all stock

when the engine is at idle, and the tranny in park, it blows cool air. When the tranny is in gear, and the engine is turning 1900+ RPM it blows cool air. When the tranny is in gear and the motor turns less than 1900 rpm it blows warm/hot air, even if it is turning 1700rpm while going 65mph down the road.

What is this? Low Refrigerant? Possible vaccume line issue?

where to start?

 

[short_stuff]

Sounds like low refrigerant. Or a clogged internal filter. OR a combo of both. I would take it to an Auto AC shop and have them purge the system, change the dryer and internal filter and charge it up. Ford's double o-ring seal for refrigerant lines has always had an issue where the pressure of the internal line flattens the o-ring a little and form a leak. You could try charging it yourself, but the new systems are far more sensitive to the newer freon pressure than the older stuff.
I am assuming, of course, that the ac compressor is coming on. But it should have a low-pressure sensor that will keep it from kicking on if the line pressure is too low. jmho.

 

[toothless]

Sounds like A/C clutch is disengaging. Mine is doing the same thing, but I haven't fixed it yet.