I have an 04' F250 FX4 6.0 and everytime i switch to 4X4 using the fly switch my AC goes from the front vents to defrost for a short period of time, has anyone else had this problem and if so how do i fix it?

 

4x4 solenoid on passenger front fender or a vacuum leak

 

Your automatic locking front hubs use vacuum to lock and unlock. On the passenger side firewall you'll find an electric vacuum pump. It pumps down a vacuum that is used by the hubs and by the HVAC.

If you find the vacuum pump and follow the vacuum line, it'll go to a vacuum resevoir where there will be a T. One side of the T will go forward into the cab, the other side will follow a wiring loom around to the PVH (Pulse Vacuum Hublock) solenoid. Another line will come out of the PVH solenoid and head down to the front axle where it T's out to the hubs.

This is where you really need a hand vacuum pump and a gauge. You need to connect the vacuum pump to each hub individually and pump down a vacuum and see if it holds. If it doesn't, the hub seals are bad. If both hubs hold vacuum, then you need to start working your way back up the line, connecting your hand pump and pumping down a vacuum until you get to the spot where it doesn't hold.

If you never find a vacuum leak, then the problem is most likely the PVH solenoid itself and it needs to be replaced.

 

I had the same problem on my F-250 (Gas), I built a smoke machine (YouTube) and found that m vacuum reservoir had a 1/16 hole in it, now my A/C works as it should.