4x4 Diff Question

R

RBBailey

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Jul 26, 2004
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Oregon
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What is it that causes the diff to stay locked after manually unlocking it after being off-road?

I seem to remember something about this being caused by "winding up" or spooling up of the differential? But it's one of those things I've never actually asked about or figured out.

Any 4x4 tech teachers out there?
 
p m

p m

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Apr 19, 2004
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yes, the "wind-up" can keep the center diff from unlocking (different tire diameter front/rear, or a lot of sharp turns in one direction). Occasionally, it can be "un-wound" by backing up a few (or a few hundred) feet; it is easy to achieve by jacking up one wheel (if your axle diffs are not locked).

I suspect more common is the locking/unlocking mechanism in the lever housing on its way south.
 
R

RBBailey

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Jul 26, 2004
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Oregon
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So, I'm not imagining it. I doesn't seem to be a "problem" with any of the trucks I've owned, but I do have to back up to unlock at least 50% of the time.

Why does it happen? Is it tension related?
 
Mike_Rupp

Mike_Rupp

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Mar 26, 2004
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Mercer Island, WA
Ben, think of it this way: when you unlock the diff and continue to drive the diff won't unlock itself until the tension is relieved because the front driveshaft and rear driveshaft want to travel different speeds because of turning or unequal circumference. Imagine the front tires being 33s and the rears being 32s. Now imagine the center diff is locked. The front and rear driveshafts are forced to travel the same speed, but the fronts want to go faster and that's where the strain / tension whatever you want to call it happens. Now, put the truck in reverse and all of a sudden that strain is gone and the diff can unlock since it's no longer under strain.
 
Mike_Rupp

Mike_Rupp

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Mar 26, 2004
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Mercer Island, WA
You really need to understand how a conventional differential works:

<iframe width="420" height="315" src="http://www.youtube.com/embed/gIGvhvOhLHU" frameborder="0" allowfullscreen></iframe>

At :50, you can see how the differential operates when each of the axles is travelling the same speed. In your scenario when the cdl won't unlock, the differential is completely locked up and one of the axles wants to make the differential compensate for the different speeds, but it can't unlock since the stress in the locking assembly hasn't been relieved.
 
R

RBBailey

Well-known member
Jul 26, 2004
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3
Oregon
www.flickr.com
Mike_Rupp said:
Ben, think of it this way: when you unlock the diff and continue to drive the diff won't unlock itself until the tension is relieved because the front driveshaft and rear driveshaft want to travel different speeds because of turning or unequal circumference. Imagine the front tires being 33s and the rears being 32s. Now imagine the center diff is locked. The front and rear driveshafts are forced to travel the same speed, but the fronts want to go faster and that's where the strain / tension whatever you want to call it happens. Now, put the truck in reverse and all of a sudden that strain is gone and the diff can unlock since it's no longer under strain.
Click to expand...

OK, thanks, that's pretty much what I figured just based on common sense. Just wasn't sure if I was missing something.

Would it be accurate then to say that when the drive train pressure is equalized between each of the wheels (prop shafts) the diff will come unlocked?

If it has issues coming unlocked, is this a sign of worn gearing or a lever problem of some kind?
 
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