Clunk sound from the rear end of my 300SD - PeachParts Mercedes-Benz Forum

otto huber · #1 07-10-2016, 09:22 PM

The clunk happens when the car shifts between 1st and 2nd, and I think that it's happened in reverse a couple of times as well. I checked the flex disc and it looked okay. I tried to move the drive shaft, and it had a tiny bit of play and I could hear a pinging noise close to where the carrier bearing is. If it's the carrier bearing, how difficult of a DIY is it? It looks like I'll need the bearing, center support, bushing and boot, anything else? I passed this job off to an indie when I owned a 240D years ago because I didn't have the enormous wrenches needed. Does anyone know what size wrenches are needed? Maybe I can rent them. Thanks

SD Blue · #2 07-10-2016, 11:10 PM

You will also need the centering bushings for the driveshaft ends. Even though your driveshaft is probably marked for indexing the two sections, it's still a good idea to mark everything that will be separated before any disassembly. Including the flex disc yokes, the balancer and both sides of the universal joint.

The FSM has a pretty good description of everything but the u-joint itself. (considered by MB as non-replaceable) However, with careful measuring and pressing, it is possible to re-stake a new u-joint. Here is the section of the FSM:
https://www.startekinfo.com/StarTek/outside/12253/program/chassis_300SD.htm

and a You Tube video that talks about replacing a staked u-joint. There are many sections to the video. (15?)
https://www.youtube.com/watch?v=nJIL1vJDILY

I used a clevis pin (farm implement supply) and 3# hammer to drive the old one out. The most painstaking part is using a prick punch to re-stake the new joint.

For the wrenches, I just used a couple of large adjustables from HF.

Junkman · #3 07-11-2016, 12:27 PM

Transmission and differential mounts have been an issue on other vehicles. I'd check them and consider replacing the flex disk unless it looks really good once removed. Mine looked worse or perhaps I didn't know what to look for until it was in my hand.

vstech · #4 07-12-2016, 02:16 AM

Check your rear shocks... btdt

Clemson88 · #5 07-12-2016, 09:19 PM

To loosen the big nut which locks the drive shaft end together you can use a big pair of channel locks. Just put the transmission in park and turn the nut CCW. Your drive shafts probably are not marked. Mark them as soon as you get the car on jackstands. It seems I read in the FSM that the shafts weren't marked until 1983 on the SD.

Check the DYI and FSM before you start. Here's the Reader's Digest version:

Transmission in park, Loosen the big bolt. You'll probably want someone to change the transmission from part to neutral and back a few times when removing the flex disk bolts. It's easier than holding a wrench on both the bolt heads and the nuts. Get the bolts on both disk while you have help.

Drop the carrier housing and ease it down to rest. With a prybar pry the DS off the front flex disk while supporting the carrier housing with the free hand. Let it rest downward and then pop the rear shaft off the differential housing.

There is probably a vibration damper mounted with the front shaft/flex disk/transmission yoke so photo it or take a good look at how it's mounted.

There is a super good DYI on this website. Unless you know when those flex disk were changed last plan on replacing them. If the flex disk are original equipment you're going to need a 1/2" drive, breaker bar to break those flex disk bolts. I just removed the drive shaft assemblies from my SE yesterday. They were original and I couldn't break the those bolts with a standard craftsman 1/2" drive ratchet or a long 19mm wrench.

You'll need something to mark the shafts, a good breaker bar, an assortment of small sockets/ratchet or box end wrenches and that's about it for tools.

Remember, when you reassemble everything, leave the boot off that big nut and put the car on the ground. Put the transmission in neutral and roll it back and forth about three feet each way. That 'settles,' the drive shafts in their 'natural position of comfort,' or somthing. It is the cure for shaft vibrations. Jack the body up just enough to get at that bolt and tighten it. You can then jack the car up high enough to get a good grip on that big nut's boot.

okto · #6 07-12-2016, 11:51 PM

Diff mount is the most likely culprit for the clunk. Dead flex discs are obvious, and if you're hearing the clunk from the rear it's not the center bearing or U joint—those are right under your elbow when you're sitting in the drivers' seat, you'd know if the noise was coming from there.

It may be a center bearing also, but pinging sounds like an odd noise for a rubber-mounted bearing or a U joint. My loose center bearing makes a shuddery thumping noise, and a U joint usually clicks or clunks when it's dying.

otto huber · #7 07-13-2016, 01:30 AM

I'll start with the diff mount and front flex disc and see what happens. Even though the rubber on the diff mount hasn't collapsed, I'm guessing that it's the original. The car has 260K on it and I've owned it for the last 80K without doing any repairs on the rear end other than brake pads. I might replace the rear shocks to improve ride quality. The rear end gets a lot of work with me hauling my music equipment around frequently, but it doesn't have much of a sag to it.

otto huber · #8 07-15-2016, 01:15 PM

I'm going to install the diff mount this weekend. Do I need to remove the sway bar to get a wrench on the top bolt on the back of the diff or can I lower the diff with a jack several inches after removing the 4 bolts that hold the mount into the frame?

SD Blue · #9 07-15-2016, 01:49 PM

There are several areas for noise from the rear. Here are some that I've experienced and the most noticeable symptoms. Maybe these will help you narrow it down.

Differential mount = rise/fall when changing fwd gear/reverse
Sway bar ends = clacking noise when running over uneven pavement
Trailing arm bushings = shift of weight when rounding a highway interchange
Trailing arm mounts = sagging of the rear
Shock mount = when running over uneven pavement
Wheel bearing = rumbling in gradual highway turns

spark3542 · #10 07-15-2016, 02:46 PM

Quote:

Originally Posted by vstech

Check your rear shocks... btdt

I had the shock-mount clunk too. Bottom mount rotted away, but kept the shock trapped inside. so it's hard to notice.

okto · #11 07-20-2016, 08:14 AM

You don't have to remove the diff from the subframe to replace the mount, but you absolutely need to put the car on jackstands and keep a jack under the diff, as the rear diff mount is the rear attachment point for the subframe and holds the subframe against the downward force of the suspension springs. If you don't, the subframe can deflect downward (possibly violently) and may knock your exhaust off. Ask me how I know.