4WD has left the chat

[GhostRing]

Don't know how far we'll get with this, but worth a try...

1998 Limited, push-button 4WD with 2WD lo switch
beyond that, essentially stock drivetrain.

A few months ago - while driving, pressed the 4WD button, light goes on.
Moments later, a high speed grinding noise occurs. I quickly stop,
assuming it's an actuator being sticky. Re-engage and it's fine
again. Use 4WD regularly and no further issues. Until Sunday.

Out snow wheeling, get nice & buried. Have buddy snap-strap me backwards
and once moving, there is a horrible popping/shuddering from the front end.
I figured I somehow snapped a CV or crunched the R&P, but after a some forward
progress, the noises started to subside. Eventually even went away (sort of)

I was now in E-locked 2WD even though the green 4WD light is on.
It does NOT FLASH. (This is why searching for this is kind of tricky.)
Several epic winch sessions later, I got home and here's what I know.

Upon inspection, all the vacuum hoses & wiring appear plugged in on
TC and front diff actuators. Engine off, key on - hear the click and
front driveshaft stops spinning freely when 4WD button pushed.

Essentially, the whole system is engaging, but no power is getting to
the front. 4LO shifts fine, my 2WD switch acts like always.

In motion, occasional click/thunk somewhere up front, so not all is well.
The CVs still seem fine, but when I spin driveshaft in 2WD, it is not
perfectly smooth - it tightens slightly for about 10-20 degrees or so.

Explain like I'm a dummy what I should be testing/inspecting.
I'm going to put whole rig in the air and see what spins when.
Broken ADD shift fork? Pull CVs? Maybe one broke way inboard?
Could this actually be the TC "telegraphing" symptoms to the front?

Sorry if this is a bit long-winded, and I'm hoping it doesn't involve
pulling the whole diff yet, but any input would be greatly appreciated!

 

[2TH4IST]

Maybe the actuator motor on the tc isn't engaging all the way.
Or bad u joints in the front d.s.

Could also be the add fork sticking and partially engaging the collar and your hearing it grinding. If it's cold and there is gunk or super thick oil the shift fork might get stuck (think to much stiction). Same goes with the 4wd actuator on the tc.

Your doing the right thing by getting all 4's off the ground and check what works and what doesn't.

I'll throw this out to left field, could there have been water in the vacuum lines for the add? Could have frozen not letting it engage. But kinda sus that the 4wd light is on.

Hopefully it's a easy fix.

 

[phattyduck]

Drain the front diff into a clean drain pan and see what metal bits come out. Sounds like a front diff issue.

-Charlie

 

[GhostRing]

Checked functions with rig off ground.

2WD: Engine off, front shaft spins smooth with driver-side wheel. Pass side free spins. No perceived TC or front diff noise.
Engine running + Trans in D, activated 4HI/LO sounds like the ADD is getting vacuum, but nothing seems to engage.
4WD light stays on and both the driveshafts spin - but no front tire movement, and a whirring from about center of axle assy.

I'd like to think a mashed R&P would render this almost undrivable in any condition. Or at the very least be horribly crunchy in 4WD, yes?
I'm going to drain the front and see what falls out. Also want to pull the ADD to see if anything is left.
By hearing the ADD activate, is it safe to assume the VSV solenoids are functioning correctly?

REALLY hoping that's all it is and that I can chuck my spare ADD in there and soldier on....

 

[phattyduck]

Sounds like the ADD mechanism is broken - likely busted teeth/splines where it meshes.

I think that can be replaced without opening the diff, and hopefully any metal in the fluid hasn't ruined the diff.

-Charlie