Hey guys I have a 1985 Mercedes 300sd that I just bought, engine sounds perfect and has very good power with 285k miles. I'm having an issue were the car makes a horrible clunking sound when its downshifting to 1st gear, and also when it shifts from 2 to 3rd clunks but not as bad. At first I thought it was the transmission but It seems to me like the clunking is coming from the rear. Also when i'm driving on the freeway there is a vibration at high speeds starting at 50mph, even my steering wheel shakes pretty badly. Another funny thing (aside) is that when I hit 15-20mph there is some sort of funky wobble on the car...but maybe thats just alignment issues? Anyways if someone can point me in the right direction on what might be the cause of this clunking....I'll be eternally grateful !!

Check out the drive-shaft flex-discs, and the centre bearing..

You REALLY dont want one of those to fail, It'll wreck your whole day!

The steering wheel wobble at 50 sounds to be wheel balance issues....

Clunking could be loose bolts on flex-discs, play in the diff or CV joints, loosness in rear suspension members, say--bushes, could be many things...
--Might be that the trans. is shifting a little harshly due to Vac. issues...

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

These sound to me like several different issues. I would check the flex discs as mentioned above. The down shift clunk is usually an issue of not enough vaccum to the tranny- so look for a leak in the lines or measure what your pump is putting out. There are several good threads on how to do this- including adjusting the modulator pressure. I have had the same issue when my vac pump was dieing.
The vibration is most likely a wheel balance issue.

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81 300D Turbo 190K sold
83 300sd 319K best $500 I ever spent-daily driverw/ 2 tank WVO set up

If your vac setup is correct on your tranny, look very closely at the axle half shafts. I worked on my TD for months trying to get my vac system to remove the clunck when it shifted with no luck. It turned out that a chinese replacement axle was junk and causing the problem even though the boots on the cv looked good.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

Don't know temperatures where you are but downshift clunks are often caused by vacuum leaks. You can check to see if those leaks are in the climate control system by pushing in the far right button of the climate control to turn it off. If the clunking goes away, the leak is in the climate control system somewhere.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I talked to a friend of mine today who is "mechanic oriented" he said it sounds to him like it could be the flex discs or center support bearing. Its a really loud metal clunk when I slow down to 10-5 mph. I ordered the flex discs and center support bearing already online, does anyone know how much I can expect to pay for this repair so I won't get ripped off? I live in California by the way.

The clunk is caused by the vacuum modulator and its adjustment. If it is bad it is a $50 part and takes about 30 mins to replace.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I see this thread is old (but so am I) - my clunk is seasonal. During the week or so of cold weather (below about 50°) here in Florida, my 300SD clunks so hard on slowing and downshifting to 1st, that I have to check the rearview mirror to make sure I haven't been rearended. I see the roots of this thread point to either flexdisk (rear suspension issues) or vacuum regulator. Which of these might best fit the scenario of clunking only when the ambient temp (not the engine temp) is cold? Thanks

Look at the line to your modulator. You need vac for a smooth downshift. Maybe the vac line came off at the tranny or under the hood - or possibly the rubber connector under the hood is leaking. This is the easiest thing to check.

Flexdisks are susceptible to ambient temperature changes.