Bill Ball

Recently I had a used 83 LSD swapped into my previously open diff 89 AUTO tranny by a Mercedes mechanic who was partially rebuilding my tranny. I filled the diff with Redline 75W/90 NS. I drove the car about 100 miles then drained the diff oil so I could change the cover (see other thread about that: https://rennlist.com/forums/928-foru...fferences.html). The oil, clear and light amber when filled, came out dirty gray. The magnetic drain plug had a large wad of metal filings attached to it. I never saw any filings there with my open diff.

Note the LSD chatters/shudders rather noticeably with parking lot maneuvers and slow driveway maneuvers, even with very slight turn of the steering wheel. I decided that was merely evidence the LSD was limiting slip, but someone correct me if that impression is wrong. I gather I could reduce the chatter by adding regular gear oil, but wouldn't that just make the LSD slip more? The LSD was not rebuilt.

Anyway, my main question is what is/are the likely source(s) of these filings and how concerned should I be? Note the ring and pinion are original to my car and at 240K miles have a very even wear pattern that I don't think is fresh. The mechanic said he was careful to get the backlash to spec. I don't hear any noises to indicate it's off spec. There is a new slight whir at 65-75 MPH, but things are quiet otherwise and that whir could be from somewhere else, like wheel bearings, as the drivetrain is pretty quiet at other speeds.

Jim Morton

Photo(s) of the metal flakes /shards

What size range are the metal flakes ?

Dirty gray is likely friction material from the LSD clutch plates.

As suspected, metal flakes... not good... could be a couple of different sources such as the LSD drive tangs / clutch ramp plates... hopefully not gears.

How does the diff test for break torque ?

Thought of exercising the LSD with the back cover off to see if any oddity is heard / observed ?

$0.02

Bill Ball

Hi Jim: They are filings - very fine. No pics, sorry. The diff seems stiff - did not check the torque. I'd be glad to drain again and remove the cover if you want to come over and have a look. Currently I have another 928 here and mine is away, but it should be back next week.

Rob Edwards

Bill, like this? This is my n of 2. The first is from the GTS, the friction plates in the PSD were fine. 2nd is from the '90.

Bill Ball

I recall something that now strikes me as odd.

When I got the LSD from Mark Anderson it was covered in what sure looked and felt like Dexron ATF (cherry red, thin). That didn't register with me at the time. Here's a pic.

Bill Ball

Rob: Yep. An impressive amount for just 100 miles.

Jim Morton

Bill:

Would be happy to take a look.

If you "go it alone", try washing the filings off the mag plug with solvent and laying them out on a clean paper towel for inspection. If they are fine splinter like, this suggests likely from the LSD/carrier. If they have more fractured edges, I would be wary for how the ring gear was positioned relative to the pinion. If you have the cover off, a swipe test of the gear mesh with prussian blue would be advised.

As I think you may be suspecting, if the carrier was bathed in ATF, perhaps the LSD carrier needed to be disassembled and cleaned before re-install.

Bill Ball

Jim: They were very fine, almost powdery/pasty with a lot suspended in the oil that did not fall out. I cleaned up the magnet and reinstalled everything, not giving it much thought until it was too late to do any analysis. I was just surprised that there was so much steel (picked up by the magnet). Let's have a look and do that swipe when I get the car back. THANKS!

IcemanG17

odd..............while the LSD in the Estate is VERY weak.....it came back clean after a year of racing...zero metal and the fluid still looked good.... but it is an 88S4 auto-LSD...but the early LSD's have more friction plates.....so they should be better

Bill Ball

Maybe I have this wrong, but I don't think this is an EARLY LSD with more plates. I believe Carl has the years wrong. I think those were only used through 82 and they will not fit the later diff case without the adapter Carl sells. Anyway, I could be wrong, but the diff PN changed in 83.

Hilton

There was a post some time ago by Greg Brown on adding a wear allowance of 20% to the factory backlash spec when setting up a diff with a worn ring and pinion.


When I had my gearbox apart, before disassembly, I measured a 16% increase in backlash over factory specs - so when fitting the LSD, set backlash to what I'd measured, to allow for the wear.

Its possible what you're seeing is new "break-in" wear on the crown and pinion, due to the backlash being set back to factory specs?

The thread is here

https://rennlist.com/forums/928-foru...n-s4-unit.html

WallyP

Finely-machined aluminum is gray.

GregBBRD

I just did a pretty big write-up about this.....with pictures. None of the limited slips are set-up, from the factory, with more than two friction surfaces (on each side of the spider gears), touching outer lamina.

The "early" limited slips have the ability for there to be four friction surfaces touching outer lamina (on each side of the spider gears), but they are not this way, as delivered, from Porsche.

People that have switched from a "late" limited slip to an "early" limited slip, who have not taken the limited slip apart and "separated" the two friction discs (that are touching each other) are completely wasting their money and time.

Ducman82

https://rennlist.com/forums/928-foru...-question.html

here is the link to the LSD post Greg and i did

Rob Edwards

But non-magnetic. (Chain of long-shot hypotheticals warning Any chance the LSD could have come out of a 5-speed car whose diff oil hadn't been changed in a long time? And there was friction modifier in the diff oil, so every surface was black-gray, so Pichai rinsed the outside LSD off with ATF to clean it? And your fresh gear oil rinsed all the old friction modifier out of the nooks and crannies of the LSD, turning it black/gray?

Just a long-shot WAG...