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.
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....
*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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