1985 Mercedes 300SD makes clunking noise when shifting back to 1st

[maurigiann]

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 !!

[Alastair]

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...

[Paulc66t]

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.

[shadetreemechan]

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.

[kerry]

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.

[maurigiann]

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.

[snookwhaler]

I went through this last year. I replaced the flex discs, carrier bearing, trans. mount and rear diff. mount.

In the end it was the axles that were completely worn out.

As mentioned above, be sure to rule out any big vacuum leaks. Loss of vacuum can cause very harsh shifting.

[charmalu]

You can do the replacements of the discs and center bearing yourself. very easy to do.
you will need a 15mm and 17mm open end/box wrench and a gear wrench also helps. might also need a Hex bit, depends on the bolts used with the flex discs.

the transmission end will be a challenge because of the transmission cross mounting plate. remove the 4 17mm bolts and the 19mm nut on the rubber mount to transmission. this will give you the room you need, kind of tight in there.
Place a jack under the transmission first to keep it from dropping.

mark the driveline before removing it. it is 2 pieces with a spline in the center. If it seperates w/o having it marked, your ballance will be off. you won`t have a reference point.

there is a 46mm nut that keep the 2 sections tight, just loosen it with a pair of large channel lock pliers. works most of the time.

you will need a gear puller with 2 or 3 long arms to pull off the carrier bearing.
I used a piece of exhaust tubing the same diameter as the bearing to drive it back on.

there is also a rubber sleeve that fits between the 2 sections that covers the end of the splines and fits up on the large nut.

http://www.allpartsexpress.com/ShopByVehicle.epc?q=1985-MERCEDES--BENZ-300dt--/--L5--998cc--3.0l--Diesel--Fi--Turbo-Axle--Driveshaft&yearid=1985%40%401985&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1193790%40%40300DT++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300DT&catid=Axle+%26+Driveshaft%40%40Axle+%26+Driveshaft&subcatid=Driveshaft+Coupling+Boot@@Driveshaft+Coupling+Boot&mode=PA

where is calif are you? fill out your signiture info etc... so you won`t be asked each time as to car model etc.....

edit: I just reread your post. you have 126 not a 123 what I was thinking while typing. think you might have a tin plate to remove also. but similar stuff.

Charlie

[maurigiann]

Well I took my 300SD to the mechanic today, he took a test drive on it around the block and came back to tell me something inside the transmission (I forgot what part he said) was bad and needed to be replaced, cost? $1500!! I told him I would think about it. I took it to another guy who specializes in diesel cars and has alot of experience on MB diesel's. I ask him to take it for a drive and tell me what could be wrong, he told me it was a waste of time to do that and asked me to bring it tomorrow so he can put it up in the air and check whats really wrong with it. I hope its not the tranny, although im starting to think so since it always takes 3-4 seconds to engage on D or and does so harshly, but shifts fine while driving. Does anyone have any idea how much it would cost to completely rebuilt the transmission?

[pawoSD]

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.

[maurigiann]

I forgot to mention that i checked the flex discs and they seem fine with no cracks on them. Now the clunk is actually very unusual and harsh, I had 12 early-mid 80s MB diesels before and most shifted hard and some clunked but it was a different kind of clunk not the horrible about-to-fall-apart kind of clunk im getting on this one. Another interesting fact I forgot to mention is that it does not clunk if I start to drive and shifts into 2nd only and I come to a stop, but if it shifts 2nd then 3rd and then stop then it does clunk...pretty mysterious eh?

[maurigiann]

How do I adjust that PawSD..sounds interesting

[maurigiann]

I just took the car for a drive right now and put it on 3rd and the clunk is no more when i come to a stop. If i put it back on D then it does clunk.....any ideas?

[pawoSD]

Originally Posted by maurigiann

I just took the car for a drive right now and put it on 3rd and the clunk is no more when i come to a stop. If i put it back on D then it does clunk.....any ideas?

The clunk is the 4th to 3rd downshift.....mine did that too....

Best bet is to replace the modulator and the plastic push-pin inside that it pushes on....then start adjusting it. When I did that (mine actually failed entirely prior to my replacing it) it shifted much better and the clunk was gone.

[JoeSo]

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