I have an 05 F250 with a 6.0L that when I switch to 4x4 the vent switches to defrost. My hoses look find and you can hear the vacum engage under the hood. I don't know were to look. Also the AC blows cold on max but will not cool the cab.

 

took mine to the dealer for other problems and the same one. they replaced some seals in the front hub. so far it has been fine. just my .02 cents worth

 

it does that when you have a vaccum leak in the 4x4 system, there is a guy that has a good video on youtube on how to find where it is. either in the tubing or the oring in the hub. quick way to find out is to pull the tube off both sides and plug them with a bolt or nail then turn to 4x4 if vent is normal than the problem is in the oring in one of the hubs, put one side back on and test then do the other side to find out for sure, you can do the same with the tubing if its in that / isolate the side to narrow it down. hope this helps a little

 

DO you know the link

 

here is the link, hope it works, never posted a link before.
how to find and diagnose esof hub caccum leaks- is what the video is called,

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Holly crap---it worked

 

Thanks Bob, mine has been leaking for over 2 years and I've just never had the ambition to fix it. I just leave the hubs locked all the time. Good vid.

 

Mine too, just have to pull it in the garage one day and take care of it