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Chipped Pinion Gear?

 
Old 05-16-2019, 07:16 AM
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Default Chipped Pinion Gear?

My stock 3.42 rear with barely 20K miles on it started making a strange noise when coasting the other day. Its a clicking / knocking sound that is in sync w/ the driveshaft. You can't hear it when accelerating, or driving at a steady speed above 20 MPH, but as I slow to a stop (clutch in), I hear it. Thinking it might be the U-joint, I raised the rear axle off the ground and checked it out. The noise is definitely coming from the rear end and you can only hear it when the drive shaft is rotated in reverse or the rear tires are rotated forward (similar forces on the gears as when coasting). Would a chipped pinion gear cause this noise? Or maybe a pinion bearing? I can't detect any abnormal play in the pinion shaft (I didn't disconnect the driveshaft to check this). Could a bent torque arm change the pinion angle and cause this? Any advice would be appreciated.
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Old 05-16-2019, 02:56 PM
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The noise you describe and conditions under which it happens sounds similar to the sort of chatter that's typical of a Torsen LSD (especially as they age). Except that you have a '98, which should have come stock with an Auburn. When you rotated the tires with the rear off the ground, did they both spin in the same direction, or opposite directions like an open rear?

The only reason I'm questioning this is that there was an update to the 1998 owner's manual (second edition) issued late in the '98 model year which calls for the updated Torsen LSD fluid recommendation. This could suggest that perhaps some very late '98 cars received the upcoming '99+ Torsen unit rather than the standard Auburn. That is just speculation on my part, I've never actually encountered a '98 which came with a Torsen but I've always wondered if perhaps a handful of very late-build '98s might have. If my memory is correct about '98 WS6 build totals, then your ASC conversion number would put yours very near the end of '98 production - so this potential situation could apply to your car and perhaps have something to do with the noise you're now hearing (hence the reason I've brought it up).

If you're already certain that you have the Auburn, then I apologize for the side track here.
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Old 05-16-2019, 06:15 PM
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Thanks for the feedback.

Yes, both tires rotated in the same direction. I didn't try putting the trans in gear to measure how much torque it'd take to get the tires to spin

Indeed, mine is a very late '98 build, it didn't get delivered to the dealership until late Aug '98, shortly before the '99s started showing up. It was delayed due to the M6 shortage that year that was the result of a strike. I've changed the fluid to Mobil-1 synthetic at around 5K miles, but didn't look close enough to see if it was a Auburn or Torsen diff. My owner's manual indicates it's the 2nd edition, but it calls for SAE 75W-90 Synthetic Gear Lube & 4 oz. of Limited Slip additive, which sounds like pretty standard stuff to me. In any case, I'll be pulling the cover off soon to take a closer look, I'll make note of which type of differential it is too. I assume that the Auburn is a clutch-type diff w/ 4 coil springs or an S spring between the side gear clutch packs, and the Torsen is NOT (something different)?

I do realize that if something has broken, it will be a great excuse to upgrade to s stronger rear. However, I had other plans for the $3K or so it'll cost to do that this summer.
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Old 05-16-2019, 11:29 PM
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Originally Posted by JohnnyBs98WS6Rag View Post
Thanks for the feedback.

Yes, both tires rotated in the same direction. I didn't try putting the trans in gear to measure how much torque it'd take to get the tires to spin

Indeed, mine is a very late '98 build, it didn't get delivered to the dealership until late Aug '98, shortly before the '99s started showing up. It was delayed due to the M6 shortage that year that was the result of a strike. I've changed the fluid to Mobil-1 synthetic at around 5K miles, but didn't look close enough to see if it was a Auburn or Torsen diff. My owner's manual indicates it's the 2nd edition, but it calls for SAE 75W-90 Synthetic Gear Lube & 4 oz. of Limited Slip additive, which sounds like pretty standard stuff to me. In any case, I'll be pulling the cover off soon to take a closer look, I'll make note of which type of differential it is too. I assume that the Auburn is a clutch-type diff w/ 4 coil springs or an S spring between the side gear clutch packs, and the Torsen is NOT (something different)?

I do realize that if something has broken, it will be a great excuse to upgrade to s stronger rear. However, I had other plans for the $3K or so it'll cost to do that this summer.
If both tires are rotating in the same direction when the rear is unloaded, then it's most definitely an Auburn. No need to even pull the cover for that. The Torsen units are torque-sensing and will operate as an open differential when unloaded (tires would spin in opposite directions with the rear off the ground, just like a standard open rear).

However, the recommendation of 75w90 synthetic in your 2nd edition owner's manual is actually not correct for the Auburn rear. This was the recommendation for all '99+ V8 examples due to the Torsen update, but the older Auburn rears were originally spec'ed for use with 80w90 conventional lube (plus the same LSD additive, which technically isn't even required for the later Torsen LSD as the additive is a friction modifier designed to be used with a clutch-type LSD such as the Auburn). The 1st edition '98 manuals (such as what came with my 3-23-98 build date car) still show the original 80w90 conventional lube recommendation.

I don't know why GM changed the fluid recommendation at the end of the model year when they were apparently still issuing the Auburn LSD on examples as late as yours, but some folks have found that the stock Auburn doesn't always perform properly with synthetic lube (especially the aftermarket synthetics). Obviously you've had that Mobil 1 lube in there for quite some time without any prior issues, but I've encountered several folks who've had problematic Auburn LSDs after switching to synthetic. Noise and/or poor operation could be the result of lube selection, but that wouldn't explain why it took so long to show up in your case.
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Old 05-20-2019, 06:01 AM
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Although if the Torsen is buggered....turning one wheel can cause the other to turn the same direction.

When I used a Ford Racing Torsen in my 8.8, I had noticed this behaviour for a while when doing work at the car. When it eventually broke completely.....the internal gears all had stress cracks so could not turn or move freely, hence it behaved the way it did when jacked up.
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Old 05-20-2019, 01:09 PM
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Originally Posted by stevieturbo View Post
Although if the Torsen is buggered....turning one wheel can cause the other to turn the same direction.
That's interesting, I didn't realize this was a potential failure mode for a Torsen. When yours behaved this way, did it also make any noises like the OP describes?
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Old 05-20-2019, 01:40 PM
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Originally Posted by RPM WS6 View Post
That's interesting, I didn't realize this was a potential failure mode for a Torsen. When yours behaved this way, did it also make any noises like the OP describes?
No noises that I could hear, although with a 4-linked rose jointed setup.....it isnt the quietest in other regards anyway.

It only came apart after a few incidents of 2-1 shifts...instead of 2-3 shifts lol....faceplated makes both of those shifts very easy. In my case the last time I did it, it ripped the crownwheel flange off the diff basket.
Hence the entire thing had to come apart.

But all of the internal gears had stress cracks...this would make it harder for them to all mesh and turn smoothly...but ultimately they should still have been able to operate fine. Perhaps a little better even as it would have been like having some pre-load so one tyre couldnt become totally unloaded as such.

That particular diff had been in my car for quite a few years and had lots of use, so all things considered it did hold up very well.
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