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Vacuum to hubs in 2wd

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Old 11-25-2012, 05:45 PM
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Vacuum to hubs in 2wd

Ive got a small vacuum to my front hubs when in 2wd. Noticed a thump thump noise going around sharp corners like i had my hubs in. Sure enough i pulled the vacuum line off and have a vacuum there. Looked in the tech folder trying to find a vacuum schematic but couldnt. Anyone else ever had this issue?
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Old 11-25-2012, 05:49 PM
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Steel IIRC thats Normal the Vac Part

High Vac when Hub is Locked
Low Vac In Free

But always some vac

Hublock Operation
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 signals are supplied to the hublocks by system components, including the 4WD control module or GEM,
wiring

harness



, solenoid, vacuum harness and vacuum seals. As a first step in service, eliminate obvious items such as loose wiring

connections, loose vacuum connections or damaged vacuum lines.


Hublock Engagement/Disengagement Time
With the hublock dials in the AUTO position, switching the instrument panel MSS to 4x4 HIGH or 4x4 LOW results in the high
vacuum level being applied to the hublocks, which lasts for at least 51 seconds (including a venting cycle). The hublocks should
engage during this time through the spring mechanism. Switching the MSS to 4x2 less than 51 seconds after the MSS has been
switched to 4x4 HIGH or 4x4 LOW will not interrupt the high vacuum level; instead, the lower vacuum level will be applied after the
high vacuum level cycle is completed.
With the hublock dials in the AUTO position, switching the MSS to 2WD results in the lower vacuum level being applied to the
hublocks, which lasts for about 15 seconds. However, the actual time required for the hublocks to disengage by spring force can
vary considerably due to the effects of driveline wind-up. Road bumps, vehicle speed, acceleration cycles or momentary reversal of
direction can assist this process. Switching the MSS to 4x4 HIGH or 4x4 LOW always overrides the lower vacuum level cycle and
results in an immediate high vacuum level and in locking of the hublocks.

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Old 11-25-2012, 06:09 PM
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Thanks benny. Wonder if my springs are getting weak. It thumps in both right and left turns. Ill pull the hubs off and take a look.
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Old 11-25-2012, 06:16 PM
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is the Axel catching and Locking in when it shouldnt be???? the hub may need to be taken apart and greased if so

Have you checked Ball-Joints and U-Joints and shock mounts ect.........
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Old 11-25-2012, 06:26 PM
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There should be NO vacuum to the hubs, except for the timed pulse after switching into and out of 4WD. The high vacuum is applied to lock the hubs, then it is released. The low vacuum is applied to unlock them, then it is released. If it isn't released, they are going to try to pull in.

If you are measuring vacuum there, then your PVH solenoid is probably leaking and needs replaced.
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Old 11-25-2012, 06:34 PM
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Quote:
Originally Posted by dchamberlain View Post
There should be NO vacuum to the hubs, except for the timed pulse after switching into and out of 4WD. The high vacuum is applied to lock the hubs, then it is released. The low vacuum is applied to unlock them, then it is released. If it isn't released, they are going to try to pull in.

If you are measuring vacuum there, then your PVH solenoid is probably leaking and needs replaced.

Yep Dave is right

Looks like vac is engauged for 51 seconds to engauge hub

and 15 seconds to Disengauge Hub


Did my Post look framiliar Dave????? It was yours

Been a Few since I have seen you here good to see you come back
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Old 11-26-2012, 12:10 AM
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Based on my experiences with auto locking hubs, one of the failures for inoperable hubs is the axle seal. If you had ball joints or hub bearing assembly work, there's a good chance the seal was not replaced and is damaged. The locking hubs themselves don't require alot of grease and too much grease can actually cause issues.
To test, apply vacuum at the fitting on the steering knuckle and it should hold vacuum. If it immediatly loses vacuum, chances are the seal is bad or the oring around the hub.
I did have one truck with a vacuum pump failure once.
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