[SDDan]

I just got my truck back from the dealer for same problem (warranty Repair).

Here is what the tech wrote on the repair:
....Inspect air distribution system repair vacum leak in line to heater control valve. Run system , vacum pump blows fuse from overheating due to constant running. Replace pump and Fuse. Air works fine except when 4x4 used defaults to defrost again, replace 4x4 vacum selenoid.

To check to see if Your 4x4 is engaging put the switch to 4x4 and drive the truck enough to see if 4x4 engaged. If you have a leak, you may still have enough vacum to engage 4x4, because mine did. But, the A/C would default to the defrost when you engaged 4x4. If your not blowing fuses or 4x4 does not engage I would say leak in line and or 4x4 vacum selenoid.

Sometimes I miss my manuel hubs and transfer case shifter on the floor I had in my '95.

 

[SDDan]

Well it was the dealers third shot to fix this problem, all under warranty so it did not cost me anything, can't help there. The dealer didn't even show the cost of parts they replaced under warranty.

Since I didn't try to fix myself I can't give ya first hand mechanical knowledge on this one.

There is one fuse and one circut breaker that was replaced, I'll check the owners manel and see if I can figure which fuses. On mine the Blower worked fine so it is not any of the fuses/ breakers for the blower. A leak in the vacum line to my heater control valve started all of my problems as I understand it. And not all Super Duties have a heater control valve (located on upper heater hose).

A pinhole in any vacum line will cause a leak. As I understand it the most common are the lines to your hubs.

Maybe one of the Techs on here will answer up.