clunking noise on a 83 300SD

I recently bought a 83 300SD with 194k, and when the transmission downshifts into 2nd it makes clunk. The transmission is fine , and i have been told its
a bushinf of some kind. Do you know what this could be?
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Maybe a bad flex disk?
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Josh wrote:

Is that part of the transmission?
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Could it be backlash in the differential?
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Gogarty wrote:

It souds to me like it is something in the differential, it comes from the back half of the car, and it is only when downshifting from second to first. Is backlash caused by a bad differential, or just a mount or something?
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According to my mechanic, just wear. Your car, like mine (1981) is no spring chicken. Over time the tolerances on the gears in the differential will change leaving room for them to go Clunk! when not in intimate contact. If that is the problem it's not a big one as those gears are very robust. But I would think you would hear it whenever you go from coasting to driving or vice versa and not just on one shift pattern.
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the differential on my '93 190e has been 'clunking' for some time also. I replace the flex disk that was a bit lose but made little to no difference. For the cost of the repair/replacement in the tranny (no adjustments I believe) I've just been driving it with no problems other than the annoyance of having it go clunk and then taking off like a jet.
cheers, guenter
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Guenter Scholz wrote:

thanks Guenter, it sounds like we have the same problem. My mechanic told me not to worry, so I guess I won't until I drop my differential!!!!
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No. I thought mine was and ordered a "good used one". The one that arrived was way worse than mine. And made no difference. The I figured out the vacuum crap and fixed it. I still have the "bad" diff in there. Does't seem too fussy about backlash.
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It's part of the drive shaft.

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I know exacly what that is. It's the halfshafts going klunk when the transmission downshifts and was given the wrong modulator pressure.
They pretty much all do that. Here's why.
The transmission shifting smoothness is controlled by the engine vacuum. See, the smoother is shifts the more the clutch packs are slipping. Bad. But, under most conditions the tranny can be told to shift fairly hard, like under full acceleration.
Now, on a gas powered car engine vacuum is used; it's fairly straightforward but on a diesel that has no manifold vacuum it's just not there.
So, there's a vacuum pump, and a bunch of vacuum circuitry to shape the profile and modulate the vacuum as a function of throttle opeing. This is then fed to the vacuum madulartor on the trans that controls the smoothness of the shifts.
If any part of this system fails it's like having no vacuum going to the trans vacuum modulator and the result is, if nothing else a major clunk giong into second.
Good news: your tranny will last a very long time like this. Bad news: your half shafts wont.
The part that usually breaks in this circuit is the little fingers inside the black box on top of the valve cover. Though, a this age all that nice plastic that makes up the vacuum control circuitry has pretty much fossilized.
Good news: you can renew and calibrate all this junk yourself. Bad news: you need some fancy gizmos and the vacuum lines are no longer available in the original colors.
Lemme guess, your door locks don't work either. You can tell it's a Mercedes when it klunks at the back end and the clue is the door locks don't work.
If they do work, congratulations our vacuum pump works. But, even if it does, look for oil in the vacuum line, if there is any it's coming from kill device and will take out the pump which will now need to be replaced or rebuilt.
Complicated enough for you yet?
I replaces the kill device, replaced the vacuum pump, tested and replaced all the vacuum lines and calibrated it. And changed the half shafts. It took me 5 years to figure all this shit out.
Here's the vacuum calibration stuff:
http://articles.mbz.org/transmission /
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(Snip)

Hmmmm. My central locking system just failed. My main concern was getting the fuel door open, but that has been taken care of. There does not seem to be anything else wrong. It was a sudden failure -- worked one moment, didn't the next.
1981 300SD
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Ayup. You have vacuum issues.
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