Just like the title says.

99 F150 XLT, 4.2 L V6

AC is cold at a stop or idling, but upon acceleration, HOT air comes out.
Just noticed this a few days ago when it was warm outside (first time running the AC this year).

Does anyone know what's happening here?

Thanks!

 

I think this may have something to do with vacuum. Check for vac leaks also while at cruising speeds, while letting up on the gas does the ac work. If so then it is probly vacuum related

 

No, vacuum controls only the selection of the inlet and outlet airflow.

I'd say the the OP has either a broken blend door or possibly a compressor clutch with an excessive air gap. Personally, I'd give the first possibility very high odds.

Steve

 

Could be low on freon, as you speed up this will cause RPM's to go up with the motor turning faster which causes the suction pressure on the A/C to drop, with a low pressure switch the compressor will not come on. Untill the motor slows down then the suction pressure will come out of the low pressure state and the compressor will go back to cooling

 

Not a chance.

A low charge will result in compressor short cycling even at higher engine RPMs.

Steve