JohnnyBs98WS6Rag

iTrader: (1)

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.

SPOILER ALERT: I'll save you some time reading this entire thread, but what probably started out as a fractured pinion gear tooth that caused this initial noise eventually resulted in that tooth breaking off and trashing the ring gear with it. An S60 from MWC is now on order to replace it.

RPM WS6

iTrader: (3)

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.

JohnnyBs98WS6Rag

iTrader: (1)

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.

RPM WS6

iTrader: (3)

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.

stevieturbo

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.

RPM WS6

iTrader: (3)

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?

stevieturbo

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.

JohnnyBs98WS6Rag

iTrader: (1)

Quick update: I finally got the cover off last night and confirmed that this is a standard clutch-pack type posi (multiple coil springs between the spider gears). No chunks of broken gear came out or could be found laying in the bottom of the housing. The magnet on the cover did have some metal "powder" stuck to it, but nothing alarming. The gear teeth that I could see looked OK, but I still need to jack up the rear and spin it to see the rest of the gears and see if I can determine what is causing the noise. Will update with those findings and some photos.

TA_Freak

I'll be interested to see if everything checks out ok, and you use the friction modifier with the conventional gear oil.

JohnnyBs98WS6Rag

iTrader: (1)

More (slow) progress: Got the axle in the air and was able to spin it around, all the gear teeth look fine. For some reason, I was unable to hear any noise sync'd to the driveshaft rotation like I could earlier. The rear U-joint seems solid. Next I'm going to disconnect the driveshaft and make sure there is no play in the pinion before I put it all back together.

I'm considering adding an aluminum "girdle" cover, do these really help these rears last longer? Or just a waste of money?

FirstYrLS1Z

iTrader: (4)

They've never been 'proven' to help, but they've never been proven to make things worse.
Depending upon the style/shape of the rear sway bar, there could be interference with the girdle's bearing caps preload screws.

JohnnyBs98WS6Rag

iTrader: (1)

UPDATE: After a very busy July & August, I finally got time to put this back together. I skipped disconnecting the driveshaft and just put the cover back on (w/ new gasket), filled it up w/ some posi-additive and got it back on the road. Unsurprisingly the noise is still there as before. I'm just going to drive it, and if it takes a crap on me, time for a S60. Not going to waste more time / $$ on this weak rear.

JohnnyBs98WS6Rag

iTrader: (1)

Follow-Up: On Monday night, took the wife out for a top-down cruise. Wound out 3rd gear (95 MPH) and made a few other WOT pulls through 1st and 2nd. After that, it seems that the noise from the rear is more pronounced, I now hear the driveshaft-speed noise both when accelerating (softly so I can hear it over the engine) and decelerating. Is it possible that the torque arm has yielded or cracked and is allowing the pinion angle to change more (when accel/decel) and causing the U-joint to make more noise now? It seems that when I'm maintaining speed (no accel/decel), I can't hear the driveshaft-speed noise at all. Any advice is appreciated.

JohnnyBs98WS6Rag

iTrader: (1)

Another follow-up: I disconnected the driveshaft last night to check the pinion for any end-play. I could not detect ANY end-play in the pinion (I could feel rotational backlash in the gears, but no lateral motion of the pinion). So while the pinion preload may have relaxed, there still seems to be preload present (I wasn't about to pull the axle shafts and diff out to measure rotational torque to confirm this though).

Again, I'm back to the factory torque arm question: Can the factory torque arm crack and weaken to the point that the rear axle is allowed to rotate more and vary the pinion angle more than it should be allowed?

JohnnyBs98WS6Rag

iTrader: (1)

Finally got it back out on the road and the noise seems to be getting worse. I can now hear it coming up though the shifter! Loudest when accelerating, still loud when decelerating, quiet when just cruising at speed (neither accelerating or decelerating). Its almost to the point that I'm hesitant to drive it. Will probably end up spending my stimulus check on a S60.

JBimmolate

My guess would be a cracked pinion gear tooth. they can crack abut not break off and it will clack when it comes around usually worse off throttle. If this is the case the crack will get worse on eventually the tooth/teeth will break off.

Miles

JohnnyBs98WS6Rag

iTrader: (1)

BINGO, we have a WINNER! This past weekend I removed the rear end from my car in prep for swapping in a new S60. When I loosened the cover to drain it a small piece of gear came out, so I knew things had gotten worse since this noise had started a year ago and subsequently gotten worse. Last night I pulled the cover off to remove the axles (so I can send in the brake backing plates and reluctor rings in to be put on my new rear) and BOY HOWDY, an entire pinion tooth about 2.5" long was stuck to the cover magnet, along with a bunch of smaller chips that came from the inside of the ring gear teeth that probably got whacked-off when the pinion gear broke. So my ring and pinion are trashed. The diff still looked OK, didn't see any damage to the spider or side gears. Now to get those C-clips out to pull the axles............

Bottom line: What probably started as a fractured pinion tooth creating the noise sync'd to the driveshaft eventually resulted in that tooth breaking off and taking the ring gear with it.

Here's a pic of the broken pinion tooth:


Here's a pic of the chipped ring gear teeth:

bsf

iTrader: (7)

What company?

JohnnyBs98WS6Rag

iTrader: (1)

MWC of course. Gotta support our sponsors!

Here's my car on the lift waiting on the S60:

bsf

iTrader: (7)

I look forward to reading updates from you. I still have the stock 10 bolt behind my M6. I think a S60 would be a good choice for me if I were to replace it.