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I have a 03f250 6.0 4x4. I use the 4x4 maybe 3 or 4 times a year. My truck has the shift on the fly, automatic 4x4. While using it a couple months ago, the a/c was blowing thru the defroster while in 4x4. Yesterday I had to use it and I could not get the electronic 4x4 to engage. Finally I got out in the mud and locked the hubs. By that time the 4x4 light came on and everything worked fine. When I was done, I unlocked the hubs and it took a while for the 4x4 light to go off. Like, longer than the 15seconds or whatever it saids in the book. Bottom line is: 1. I don't know if I am engaging the 4x4 correctly. 2. A/c is blowing out the defroster while in 4x4. ( I think this may be the vacuum pump, but I am unsure). If it is the vacuum pump, how hard is it to change and how much would a dealer charge to change it?
 

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This could be as simple as a vacuum leak. But the vacuum pump is under the hood above the passenger side front fender. There is also a vacuum solenoid right in front of the pump. When my solenoid went out the ac defaulted to defrost and that's the only way it would come out no matter what. So u may just have a vacuum leak somewhere. Hard part is finding it

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Most likely a vacuum leak. Loose hose at the right valve cover to the online heater valve, dry rotted hoses at the hubs, or leaking hub seals.

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I have had the same problem for several years. I have just make a habit of getting out and locking my hubs before I get into a situation where I need 4x4. You still engage your t-fer case with the shift on the fly knob.. Like they said vac system problem are hard to find and it could be anywhere.
 
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