So... did I actually blow up my 2-pinion front diff?

[Pulgasari]

Had fun at CLCC's Fall Crawl last weekend, but towards the end of the trail, I noticed my 4WD struggling in places where it shouldn't. The rear seemed to be doing fine, but it sometimes felt like there was no power at all going to the front. Made it back to the campsite after getting unstuck by a 200 a few times, everything felt fine. Later, when I packed up and left, a terrible grinding noise came out of what sounded like both front axles. The grinding summoned ~15 cruiserheads, and after a few minutes of diagnosing, the consensus was that my 2-pinion 1999 front diff was blown. We completed a RWD conversion in ~2 hours and I drove another 2 hours home with the center diff locked without any issues.

I already have a few leads for replacement front diffs, but before I spend $2-4k to get a replacement back on the truck, I want to be 100% sure that the issue is actually with the front diff and not something else. I'm creating this thread to document the experience, make sure my diagnosis procedure is sound, and hopefully help someone else in a similar predicament in the future.

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Full sequence of events:

1. Friday, Oct 24
   1. Arrive at Fall Crawl, shift transmission into N, shift transfer case into L, shift back into D (I have not done the Pin 7 mod so this automatically locked the center diff)
   2. Ride on a few trails, it's a bit more difficult than previous years since I now have an open rear diff instead of the OEM LSD (still need to finish wiring the locker)
   3. Notice some intermittent clicking when turning, truck has done this occasionally in the past, but it's not consistent and not especially loud
   4. Inspect CVs visually, boots are still intact and there doesn't appear to be any significant amount of leaking grease, @imperium checked them for play and everything seemed normal
   5. Notice that I'm needing to back up and bump the truck way more than I usually need to, even when I'm not on especially muddy terrain and all 4 wheels are on the ground
   6. Get stuck a few times, including going downhill, can't move forwards/backwards, need to get yanked out by the 200 in front of me
   7. Finally make it back to camp, shut the truck off and go to bed
2. Saturday, Oct 24
   1. Wake up, decide not to wheel vehicle and ride along with others during the day
   2. Get back to camp, turn vehicle on to inflate tires, turn vehicle off, transmission/transfer case not shifted
   3. Later, pack up belongings into vehicle, turn power to ACC, shift transmission to N and transfer case to H, then transmission back to P, go to campfire
   4. Return from campfire with intent to drive home, start the vehicle
      1. No abnormal noises when the vehicle is in P
      2. With brake pedal pressed, shift to R, hear a terrible grinding noise that sounds like it's coming from the front similar to a bad transfer case shift for around 1-2 seconds, shift back to P
      3. Shift to N, no grinding noise, shift transfer case to N then back to H, trying to push the transfer case shifter as far forward as it will go
      4. With brake pedal pressed, shift to D, hear the same grinding noise for 1-2 seconds
      5. Shut off vehicle, return to campfire and ask for help
   5. Several people arrive and assist in diagnosing the vehicle
      1. Start vehicle
      2. Pull center caps off front wheels, pull dust covers off front wheel hubs, shift from P to D/R (transfer case in H) and back a few times while folks listen to grinding noise
      3. Noise does not appear to be coming from one wheel specifically, it's determined that the front diff is broken
      4. Shut off vehicle
   6. Emergency RWD conversion
      1. Remove aftermarket transmission skid plate and pull front driveshaft
      2. Jack up front of vehicle, for each side, remove the flange, grind off grooves in flange, reinstall flange with grooves removed
      3. Reinstall wheels and aftermarket transmission skid plate
   7. Post-RWD conversion
      1. Start vehicle, shift from P to D/R (transfer case still in H), immediately notice very similar grinding noise, even though the front driveshaft has been removed
      2. Shift transmission to N, shift transfer case to N, possibly to L, then back to H
      3. Shift transmission to D/R, noise is gone
      4. Manually press the center diff lock button, drive around campsite at around 10 MPH, notice no abnormal noises
3. Sunday, Oct 26
   1. Drive vehicle ~100 miles home with the center diff locked at 55 MPH, sometimes going to 65 MPH, notice no abnormal noises

Known issues:

• Around 3 months earlier, at Cruisers on the Rockies, my wheel bearings were pretty loose, while repacking them at camp, it was determined that my drive flanges were worn down and on their way out; I have an OEM set sitting in my garage (along with a new wheel bearing kit) but hadn't installed it yet since I didn't have the time/space and everything drove well
• I've always heard some inconsistent clicking when turning the vehicle at full lock since getting it 3.5 years ago, even after replacing the steering rack and ball joints and both CVs etc around 2 years ago
• While wheeling the truck on Friday, I did sometimes bounce a bit while giving it gas (needed a bit more momentum since I was open/open with no ATRAC), but I was not beating on the vehicle hard and would brake as soon as it was apparent that I wasn't going to move forward

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Maybe it's not the front diff?

Spent a lot of time thinking about the incident yesterday, and even though the noise ultimately went away after the RWD conversion, I'm still not 100% convinced that the grinding noise was actually the front diff breaking.

• I never heard an individual bang or other single noise indicative of a catastrophic failure
  • Another guy with a 2000 100 (so, 4 pinion front diff) broke his front diff on the same day, I believe his went out while wheeling and it was almost immediately obvious
  • My rear differential catastrophically failed 6 months ago (got the truck undercoated, they sprayed over the rear diff breather, the seal blew and I heard a hiss and saw white smoke coming from under the vehicle when it happened), but I didn't observe anything similar with my front diff this weekend
• When making it back from the trail, I didn't hear any noise indicative of the differential being broken
• A similar noise continued even after the front driveshaft was removed and it was physically impossible for the front diff to spin
• The vehicle did not move after the center diff lock was implicitly disengaged by shifting the transfer case from L to H;
  • I don't think I even attempted to move the vehicle after hearing the noise
  • I didn't press the CDL button all weekend until after the RWD conversion was completed, so maybe the center diff lock was still partially engaged even though the light was off in the dashboard?

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Diagnosis steps:

My plan is to do the following when I have time/space.

1. Drain front diff fluid, check magnetized drain plug, inspect for signs of metal shavings
   1. If there aren't many metal shavings, I'm assuming that means the front diff is probably good?
2. Reattach front driveshaft, start vehicle, shift from P to R/D, check for noise when idling
   1. Now, I know that the transfer case is fully in H and not making noise, so any noise I hear should be coming from the front diff?
3. Remove ground-out drive flanges and install new OEM ones, check for noise when idling in R/D, check for noise when moving in R/D, disengage center diff lock and see if vehicle moves
   1. If I get this far and the vehicle moves without any noises, I'm assuming I'm in the clear?

 

[2001LC]

Sorry no time to read now. So many have been covered.
Did you check to make sure hub flange to FDS axle "snap rings" still in place, under grease cap. It is the first place I'd check.

 

[Pulgasari]

Sorry no time to read now. So many have been covered.
Did you check to make sure hub flange to FDS axle "snap rings" still in place, under grease cap. It is the first place I'd check.

We did, they were still in place, but very thin. The ring itself was not stretched out, but it had some play along the axle. We had to remove them to replace the flange. When reinstalling the flange, we replaced both with thicker spares from a Cruiser Outfitters kit.

 

[Trunk Monkey]

Didn't go through all your text, but it's basically if the c-clips are there, it's your diff.

 

[TahoeZ]

Agree with above. If the clips are there it sounds like your diff.

Where are you located? I have a used 4.3 ring and pinion you can have. Free. Just pay shipping or pick up in N Tahoe.

 

[Pulgasari]

Agree with above. If the clips are there it sounds like your diff.

Where are you located? I have a used 4.3 ring and pinion you can have. Free. Just pay shipping or pick up in N Tahoe.

Thanks for the kind offer, I’m in the DC area, will let you know if I end up needing it.

The reason I think it might not be the diff is that I heard the same or a very similar noise when we first shifted into R after pulling the front driveshaft. With the front driveshaft disconnected and the vehicle not moving, shouldn’t it be impossible for the front diff to make any noise?

After shifting the transfer case from H to N to L to H for the 3rd or 4th time that evening, the noise finally went away.

It’s possible that I have both a broken front diff and messed up the transfer case shift to H the last time, but there weren’t any symptoms of front diff failure when the vehicle was actually moving.