Question about Cars & Trucks
The hub turns but as soon as an attachment is connected it stops turning.
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Posted on Jan 02, 2017
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With the hublocks in the
AUTO position, the 4x4 ESOF system uses timed vacuum sequences to lock and
unlock the wheel ends when switching the instrument panel MSS between 2WD
and 4x4 modes. A high vacuum level (222 mm [8.75 in] Hg and greater) is applied
to the hublocks to lock the internal spring mechanism that engages (locks) the
hublocks for 4x4 mode, and a lower vacuum
level (114 to 184 mm
[5.9 to 7.1 in] Hg) is applied to unlock the spring mechanism that disengages
(unlocks) the hublocks for 2WD mode.
The vacuum system is
actually pretty simple. On the passenger side firewall is an electric vacuum
pump. There is a vacuum line coming off of it that goes over to a vacuum
reservoir. Also connected to the reservoir is a T fitting with two lines from
it. One line goes forward into the cab to the heater controls. The other line
enters a loom and goes to the PVH (Pulse Vacuum Hublock) solenoid. This
solenoid controls vacuum to the automatic hub locks.
The first tests I would do are pretty simple. With the key in the RUN position, unplug the line at the pump. The pump should start running immediately. Put your finger over the nipple and the pump should stop immediately. This verifies your pump is good.
Once you've done that, I'd hook the pump back up, leave the key in RUN and watch the system for awhile. Maybe 10 to 15 minutes. The pump should run long enough to pull a vacuum in the reservoir and all the lines and then shut off. If the system holds and the pump doesn't come back on, then you've essentially verified that the reservoir, the lines going forward to the heater controls, and the lines going to the PVH solenoid are all good and not leaking.
If the pump cycles back on regularly, then you have a leak somewhere in those lines or the reservoir and you need to start tracing them out to find it. Start at the reservoir, disconnect a line on the pump side, plug it and see that the pump pulls down a vacuum and holds it. Reconnect that line and move down to the next connection and do the same thing. Continue this procedure until you've traced down all the lines. If you get to a point where the pump continues to run, or cycles on and off, then you've found a leak in the last section of line you've tested.
If all of the lines up to the PVH solenoid test good, then you're left with the solenoid itself, the lines from the solenoid to the hubs, or the hub seals. This is where a good hand vacuum pump with a gauge comes in handy. You can attach the pump at the line going down to the hubs, pump up about 14 PSI of vacuum and see if it holds. If it does, then the lines and the hub seals are good. If it doesn't, then start working down towards the hubs, isolating lines to see if they hold vacuum. It is common to find cracked lines going down to the hubs this way. If you get all the way to the hubs, attach your pump to the vacuum nipple on the back of the hub, pump up about 14 PSI and see if it holds. If it doesn't, then that hubs seals are bad and need replacing.
If, in the end, all of your lines and hubs hold vacuum properly, you are pretty much left with the solenoid itself. Attach your vacuum gauge to the line after the solenoid and have someone switch to 4WD. The solenoid should apply about 14-15 PSI for up to 60 seconds. Then have them switch back to 2WD. The solenoid should apply about 7 PSI for about 60 seconds. If neither of these happens then your solenoid is likely bad.
You may want to check the center pin of the solenoid wiring connector for power.
Sep 02, 2009 | 2004 Ford F250
May 20, 2009 | 2003 Ford F250 Super Duty Crew Cab
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