Problem with shift on the fly

I have a 2001 F-350 with electronic shift on the fly. When hubs are in auto and I shift button to 4H the front wheels do not engage. When I put hubs in lock 4WD works fine. Does the auto function work off of
a vacuum, and could I possibly have bad vacuum lines, etc? What should I look for?
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On Feb 6, 3:15 pm, columbia snipped-for-privacy@suddenlinkmail.com wrote:

Those vacuum hubs are notoriously unreliable. Do the vacuum check as previously mentioned, but you will probably need to have the seals changed inside the hubs.
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When you say the seals inside the hubs should be changed, are you talking about the O rings? Thanks, Keith
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Yes your hubs work off of vacuum. When you push that button there is a solenoid that shuts off.
The solenoid is usually mounted on the bulkhead on the pass. side. There will be two vacuum lines running to it, and a 2-pin electrical connector. It might have gone bad and it might not be shutting off, or it could have corroded. I would unplug it and use a DVOM to verify no 12v with the 4WD off. Then do a vacuum check to see if the solenoid is operating. Just connect the vacuum tester to the output side of the solenoid and turn 4WD on and off. You should see full vacuum (no less than 10"Hg) there when its on, and 0psi guage or atmospheric pressure when its off. All that solenoid does is allow a vent to atmosphere when the solenoid is off and blocks atmosphere when its on.
Ford Tech
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When testing this vacuum system, it is important to know how it works.... The SuperDuties use a "pulsed vacuum" system. To lock the hubs in, the module commands about 10 to 12 inches of vacuum to the front hubs for about 30 seconds and then the vacuum is released and the hubs remain locked. To unlock the hubs, the module commands about 6 inches to the hubs for about 20 seconds.... this vacuum is also released after that time.... Many DIYers mistakenly think that vacuum is an all or nothing thing for this system.... Common failures include the vacuum lines and the PVH solenoid (located on the inside of the right fender) and, in the case of a diesel truck, the vacuum pump. The PVH solenoid has been known to fail but only to leak vacuum when at rest. Something often overlooked are the seals at the steering knuckles.... these should be serviced regularly along with the spindle bearings (the needle bearings inside the hub/bearing ass'y).
The systems is easy to troubleshoot using a hand operated vacuum pump....

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On Wed, 07 Feb 2007 01:59:33 GMT, "Jim Warman"

*boggle* Who designed this, Rube Goldberg? Ever hear of KISS?
What the [heck] is wrong with getting out and locking a set of manual hubs when it looks sloppy, then pulling on the transfer case lever when you actually need it?
--<< Bruce >>--
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