Rear Differential 'Clunk', 1984 non-turbo 300D
 

Hi everyone,

I am sorting out some of the little noises and issues on my '84 non-turbo 300D before I sell it on. One of the things that I think might keep away buyers is a slight but noticiable 'clunk' sound coming from what seems to be the rear differential. I have changed the rear shocks and the swaybar endlinks, they needed to be changed but did not contribute to the sound that I am hearing.

It only seems to happen when I am decelerating and not under any kind of load from the engine, possibly when I am coasting. :confused:

I am doing my best to search the different forums and find someone who has the same issue sound that I have, but I can't seen to place it just right.

Any help is appreciated. Thanks!

- DC Josh

Quote
'It only seems to happen when I am decelerating and not under any kind of load from the engine, possibly when I am coasting'.

My 90 300D W124 (non turbo Uk car) would do the above and the cause was the front flex disc on the propshaft which would clunk as the auto box shifted down gear each time as I rolled to a stop.
Ok not a rear end noise but your description of how and when it occurs is exactly the same as what I had.

Transmission?
 

Are you sure it isn't the transmission 'clunking' as it downshifts? It seems to me you recently had another post about transmission shifting and adjusting the modulator...

On my car, too little vacuum getting to the transmission causes it to shift hard and clunk as it downshifts. I have a California-spec '85 300D, so my vacuum system is somewhat different from yours, but most every post I've read about vacuum-related shifting issues has included at least some discussion of clunky downshifts.

If you haven't already, you should T into the line going to the modulator on the transmission with a vacuum gauge, drive the car, and post the readings you get. Then others much smarter than I in the magic of vacuum can help you determine the best course of action...

Do a search for "axle noise", or even "clunk", and you might come up with similar reports. It will start out with a mild intermittent clunk or click that is most noticable when decelerating. Eventually, it will turn into a mild drone. Then, you will "chase your tail" trying to find out what is wrong with your driveshaft. All the while, the CV joint will eventually flex enough that the boot will start leaking. (This is how my axle scenario happened.)

Raise the rear and check the cv boots. Also, check the "angle" of the joint. If there is any kind of "offset" appearance of the axle, rather than a normal direct bend, then I would suspect a cv joint is severely worn.

sounds to me like a diff mount failure. does the car sit very low in the rear?

It does seem to sit lower than other cars I have seen...
I thought that changing out my shocks would help, but that did not do anything for the ride height or effect the noise.
I will look at the diff mount.

Tell me what you guys think. The part looks to be cheap enough and I can fix it on my own (I think). Does anyone think that it could not hurt to go ahead and replace this mount?

https://fbcdn-sphotos-g-a.akamaihd.n...77810823_n.jpg

The mount is definitely in need of replacement. You can see where the inner segment is striking the rubber peak at the bottom.

whats the process for replacement? easy DIY job?

Support the weight of the differential.

Loosen the two big bolts shown in the photo above.

Loosen the four (13mm?) bolts that hold the mount to the chassis.

When you are sure that all of the bolts will come out with out any hassle - remove bolts - replace mount with new one.

When I bought my mount there was a world wide shortage of them or something and I had to buy a dealer part - that wasn't too cheap!

Use a smidge of locktite on the four bolts that fit the mount to the chassis. The torque for these bolts should be 30Nm for this vehicle.

The torque for the big bolts shown in the picture is 120Nm

BUT before you do this job check to make sure you are fitting the correct mount => because the first version is different and has different torque specs.

See chapter 35 - 050 in FSM

Did you check your axle boots? That mount doesn't look to bad in my eyes....not enough to cause a clunk.....I think your axles are going out....no need to do the mount if the boots are cracked and leaking.....they are right next to that mount....

By the way....something isn't right wth your car....by 1984....all USA Mercedes were turbos....either you have a euro or not the correct title....

The axle boots are fine, not leaking. No sounds coming from them. My car is a German import. Grey market for the win.:cool:

The inner segment is not really touching the peak, it is close, but the photo was taken at a funny angle.

I ordered mine from m-source so I hope they are carrying the correct one!