I just had my transmission rebuilt (blah). But I still get banging on shifts especially from 3rd to 4th gear. The mechanic said that's not the transmission but probably the differential bearings. Why would they bang when the transmission shifts? If this were a vacuum issue, it'd be in every gear, but it's pretty much just 3rd to 4th.

1991 300d, 258k

I doubt is the bearings in the diff. It would bang on all gears wouldn't it?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

X2, the bearings would make noise if that worn. If there excessive play in the diff either the carrier for the ring gear is worn or the ring and pinion are worn. Diff repair is somewhat specialized and expensive. If the diff is really bad I'd suggest looking for a low mileage used unit.

Are you diff mount bushings good, they frequently wear out. How is the center driveshaft center support and its bearing.

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I don't know anything about the driveshaft bearings. I had the big round rubber flex disc replaced 2 years ago.

Why did you have your transmission rebuilt?

My first goto would be the vacuum. The bowden cable adjust the shift timing and the vac tells the transmission how hard to shift, or so the more knowledgeable member report . Typically, on these cars, most issues with the firmness and softness of a shift is linked to a problem with vacuum leaks.

I don't know what vac is required at your transmission, or if a 91 D uses vac to control transmission shifts. All the recently previous diesels did.

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84 300SD
85 380SE
83 528e
95 318ic

Differential BUSHINGS, not bearings. Yes, when clapped, the diff will bang on upshifts and hard downshifts. Really, really irritating. Age and time kills the rubber and lets the diff bounce around when it shouldn't be.

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Current stable:
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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

I had the transmission rebuilt because the car would not go forward. It would go backwards, but not forward. For them to fix that problem, they had to take the entire transmission apart = total rebuild.

How big of a job is that to fix the bushings?

I used a pull and press sleeve kit like this one on ebay to remove and install the differential subframe bushings on mine. $76 or so -it has come in handy a few times for other jobs.
https://www.ebay.com/itm/116323598621
I had the subframe off the car, so that made it a bit easier. There's special MB-specific tools that are similar that enable removing in situ. Good luck

It isnt that bad a job really.

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My drivers:
1987 190D 2.5Turbo
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1987 300TD
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Loss of vacuum will bang on every shift. I know this from when I had the hose got dislodged while doing other work.

Another cause of banging can be a worn out driveshaft center support, and this is more likely to cause the high gear banging than the diff IMO. Also check your flex disks.

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1993 300D 2.5L Turbo

Yes there are two flex discs that could cause this, and the center bushing and bearing (like others said). And check the vacuum going to regulator, and adjust the t-handle adjustor on regulator on side of transmission.

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1990 300d 2.5 turbo
1995 E300d