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Discussion Starter #1
This following baffles not only me but also the mechanics at the shop. Hopefully somebody here can help.

My 78 year old mother had borrowed my 2003 e39 M5 and was starting to back out from a parking spot when she thought the car had slipped into neutral. But no gear shifting would restore the connection between the engine and the rear wheels. There were no mechanical sounds, she says.

The car was hauled to the shop. They also did not hear any mechanical noises but concluded that the input shaft on the diff was spinning freely, unconnected to the output shafts.

The diff was taken off the car and oil emptied through a sieve. No metal particles were found.

The back was removed. No foul material was found inside.

Everybody suspected that it was the pinion and ring gear that had been messed up since the car had an oil seal replacement all around some months ago. Note, though, that the input flange and nut were marked and retighten to the same position, not over-torqued, all according to TIS, but something could, of course, have gone wrong anyway.

To everybody's surprise, the pinion and the ring gear are found intact. And no backlash or undue turning resistance can be detected. The problem seems to be in the carrier, maybe with the spider and side gears.

But here comes the really weird part: When turning the output shaft while examining the gears inside, there is a distinct (he said loud) clacking sound, and the diff seems to work again.

This seems really confusing and I though I'd ask around before we start taking things apart.

Any ideas, anyone?

/Michael

PS. Note that I was never personally present during any and all of the above. I am only conveying what I have heard from the shop. There is a always a risk that I am unknowingly misrepresenting some facts.
 

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Discussion Starter #2 (Edited)
The plot thickens. Now I have had a specialist transmission firm take the diff apart, Everything! And no fault found. They say it looks like new, Pinion and ring: perfect condition, no backlash. Planet gears: perfect no wear. Side gears: perfect no wear. Bearings, no measurable wear. LSD plates: as new. Everything works. So how could the the rear wheels be totally disengaged. The drive shaft was spinning freely without the car moving.

I got som more detailed information from the mechanics who took the diff out of the car: They placed the diff on a table and rotated the input shaft. It spun freely without either output flange moving. Then two persons grabbed one output flange each, and started twisting in the opposite direction. Soon there was a loud "spring loaded" bang, and after that everything worked as it should.

As described in the beginning of this post, I have had the whole diff disassembled but the transmission specialist could not see anything wrong. No broken parts. Nothing loose in the housing.

This is a complete mystery.

I think this can only be solved by somebody who has an intricate understanding of how the BMW e39 M5 differential is works. I have not been able to fine good enough resources on the Internet to work this out myself. If somebody could point me in the right direction that would be great.

EDIT: changed typo from assembled to disassembled
 

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You said not long before this they did “ an oil seal replacement all around”. Does this mean they did the drive flange seals(#18)? If they did, putting it back together requires you to “hammer” in the flange(#15) to get the lock ring to seat. If they didn’t do that, it will seem like it’s all good, but they can pop out and you will lose drive. You can read about properly seating the lock ring in the DIY thats here. I’ve read about this failure happening a couple times here.
E39 M5 Axle Flange popping out of differential

945887
 

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Discussion Starter #4
Thanks a million for this tip. I actually considered this after watching a YT clip in which I saw how the flange had to be coerced out with a lever, but I never thought that this C-clip did anything more than hold the flange in place when, e.g., removing the diff and tilting it side to side. But I assume now that the retaining force provided by this clip is enough to make sure the CV joints extend and contract to accommodate the suspension movements. Please confirm or dismiss, if you can.
/Michael
 

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Thanks a million for this tip. I actually considered this after watching a YT clip in which I saw how the flange had to be coerced out with a lever, but I never thought that this C-clip did anything more than hold the flange in place when, e.g., removing the diff and tilting it side to side. But I assume now that the retaining force provided by this clip is enough to make sure the CV joints extend and contract to accommodate the suspension movements. Please confirm or dismiss, if you can.
/Michael
I only know just enough to be dangerous, but I believe that’s correct. The lock ring keeps the output shaft movement from disengaging the splines.
 

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Discussion Starter #6
Okay, here's one more, what's the mechanism that adapts to the variations in distance between the diff and the wheels when the rear suspension is moving up and down? Is that the CV joints extending and contracting or the diff output flange sliding in an out of the diff's side gear axle (not moving out passed the C clip, of course)? Or, put another way, are the diff output flanges static in the axial direction or are they designed to slide axially in and out during normal operation?
 

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Discussion Starter #7
Okay, here's one more, what's the mechanism that adapts to the variations in distance between the diff and the wheels when the rear suspension is moving up and down? Is that the CV joints extending and contracting or the diff output flange sliding in an out of the diff's side gear axle (not moving out passed the C clip, of course)? Or, put another way, are the diff output flanges static in the axial direction or are they designed to slide axially in and out during normal operation?
Answering myself, this time. It appears the flanges on the diff are fixed axially with the C-clip, it's not just a stopper. And the basic function of any CV joint is indeed to accommodate not only the variations in angle, but also the variations in distance, between the wheel and the diff, so this should be the case also for the M5. Obvious to most of you I am sure, but I had to verify.

The mechanic who put it together says he did hammer it into place, but he says himself that one cannot know if he actually succeeded 100%. We will now put the reassembled diff back hoping for better luck this time.
 
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