What causes a transmission to clunk when shifting?
 

My car's been clunking when shifting, both up and down, from 1st to 2nd, or from 2nd to 1st. It doesn't do it all the time, but enough that it bothers me.

I'd previously changed the transmission mount, so that is out of the equation. I've looked around for any vac hoses that are bad and replaced whatever seemed bad. The hose connectors all appear to be in good shape.

Any other suggestions?

jeff 1991 300d, 186k

Have you checked driveshaft bearing and flex disks ?

One flex disc replaced 10 years ago
 

I had one flex disc repaired probably 10 years ago, but seems to me there is a front and rear one, is that correct?

I've never checked the driveshaft bearing.

I always think vacuum with those kinds of symptoms. Seems like there are a lot of possibilities. One of the reasons I like the wastegate actuator swap....removes so much of the vacuum system.

^ Agreed, check the amount of vacuum going to the transmission.

Low or no vacuum will cause hard shifts.

Your VCV is probably shot.

It's the thing mounted on the IP that regulates vacuum to the tranny to mimic what a gas engine does with a throttle body vacuum take-off. WHich you don't have in a diesel. So the VCV fakes a gas engine response to throttle position. (Rack position or your right foot pedal).

Your tranny is slamming gears thinking you're flooring the pedal and wanting hard fast shifts when in fact you are pussy footing around.
This misinformation is due to a vacuum leak in the VCV, which is meant to bleed more vac the harder you press the pedal.

Technically low or no vacuum will cause hard downshifts. The tranny is designed to upshift smoothly with zero vacuum; that is why the vac goes to zero immediately at WOT (wide open throttle).

Also check for good operation of the blue UFO.

What about the differential mounts? When mine were bad, I got a hard clunk on those same shifts, even though the trans wasn't shifting that hard. Though you could tell the sound was from the rear. Worth a look maybe.

Rear Subframe bushes dead? Causing a clunk.

So clunk in the rear isn't the differential itself but an issue with a mount or mounts?

My clunk is worse when cold, so that would make sense.

Thank you so very much! :D

You did make sure the ATF was at the proper level, right?

Does the w124 have a vacuum regulated tranny? I thought that went out with the w123

You sure? Unless OEM there is a decent chance of failure. URO being the worst, but MTC and Meyle make horrible mounts too. Just something to keep in mind.

Did you get it fixed?