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The 4x4 system works on vaccum and not presure. If this is not a Diesel you may have a vaccum leak. Check your vaccum hoses from the wheeles back up to the intake for cracks. If this a Diesel Super duty the 4x4 system works off a small vaccum pump out under the hood on the passanger side right over the tire. There is no pressure it is vaccum. Run the A/C and see if the A/C is blowing up on the windshield or out the vents were it should. Change it to the floor and see if it changes, If it does not change the vaccum pump is not working. Check your fuses first. If they are ok check for voltage at the the vaccum pump. Check your ground as well. If you have voltage and the ground is ok the pump is bad.
are you using shift on the go? if so I have had the same problem but only when i am already in some really sloppy stuff...i found out i have to manually lock the hubs, most people don't think that, but when in slop you have to be prepared...
A snap ring holds the lock assembly. Get a set of snap ring pliers to help remove them. Then remove the caliper and disc , There should be 4 bolts on the back to remove. Then it should just comes right off.. Be careful it is quite heavy. Check for an ABS sensor wire if equipped. And vacuum hoses on hub .
Hey, I also have a 90 F-250, manual locking hubs. My repair manual says to 'be sure to apply a light coat of high temp. wheel bearing grease to the splines of the axleshaft and the components of the locking hub.'
Had the same problem on my 2001 Super Duty.
Solution: leaking vaccume line to the auto locking hubs. It seems Ford saw fit to tie the two systems together. Once I fixed the lines to the hubs my vents worked properly.