W124 - cause of light knock from rear suspension?

[Peter C1974]

Gents,

I recently bought a stunning old W124: http://forums.mercedesclub.org.uk/showthread.php?t=47624&page=3

Mechanically, the car is 99% perfect. The defective 1% is down to a light knock, which is coming from the nearside rear wheel. The knock can only be heard when driving over uneven surfaces at speeds less than 40MPH. On smooth roads and at higher speeds the knock cannot be heard.

I have checked the following:

1. Exhaust and mountings - all in good condition, no contact between the exhaust and the underside of the car.

2. Spare wheel is secured. No loose items in the boot.

3. All trim is secured.

4. Shock absorber top mounts, both in the boot and under the car are in good condition and the fixings are tight.

5. As far as I can see, with the wheel removed and the car jacked up, all suspension parts appear to be in good condition.

Including myself, three people took a ride in the boot of the car. Armed with a torch and a good sense of hearing, we all came to the conclusion that the knock must be coming up via the shock absorber from the suspension.

Having gone through previous posts on the forum, is the droplink (anti-roll bar link) likely to be worn?

If I disconnect the droplink, would it be ok to drive the car for a little while (couple of days) until I source a replacement?

Any other ideas?

 

[Peter C1974]

Update:

It's not the droplink. I removed the droplink top bolt, went for a quick drive, no change.

I've looked at the suspension - all seems fine:

I guess it can't be anything inside the shock absorber, as all it contains is a pushrod, a couple of seals and oil. The unit is fine, no leaks.

A friend suggested it might be the spring upper mounting rubber, but I doubt that it.

Any ideas guys?

PS: I know that I need new brake discs.

 

[Peter C1974]

Me again!

Could the knock have something to do with subframe bushes? I understand that the front ones tend to go first.

Here's mine, how does it look to you?

There is a little cracking in the rubber, but bear in mind that the bush is extended as the car is jacked up.

Can these really cause knocking sounds?

 

[roadhog]

I wish I could help but noise is difficult enough to locate if you're actually with the car, let alone via a forum.
I'd say it's possible for that bush to cause the knocking noise but then it could also be any number of other bushes, there are quite a few. Mine looks similar and it's quiet. You could end up rebuilding the entire subframe before you get to the one that's causing it. Not much help I know.
I'm not entirely sure but don't you need a special tool to replace the subframe bushes?

Next time you're under there though, have a close look at the bodywork where the subframe bolts to, looks like some corrosion going on there under the paint/underseal. I'd treat it now rather than let it get worse.

As for the brake discs, I've just fitted new ones to mine. Made by Eicher, £39 from ECP for the discs and pads. got to be happy with that. I didn't need the shoes as the old ones were fine after a light sanding of the friction surface.

 

[alindsay81]

my 34k 2005 w203 has this problem, after leaving it in to get fixed they couldnt find anything wrong, they 'greased' everything in sight but the sound remains. its doing my head in!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[Number_Cruncher]

>>we all came to the conclusion that the knock must be coming up via the shock absorber from the suspension.

Swap your dampers side to side - if your knock then moves to the other side, then you need to fit a pair of dampers. Rattling / knocking dampers are quite common.

 

[Peter C1974]

Swap your dampers side to side - if your knock then moves to the other side, then you need to fit a pair of dampers. Rattling / knocking dampers are quite common.

This has crossed my mind.

To remove a damper, I need to undo the retaining nut from within the boot, remove the lower wishbone cover, remove the lower damper retaining nut and Bob's your father's brother. Am I right?

 

[roadhog]

This has crossed my mind.

To remove a damper, I need to undo the retaining nut from within the boot, remove the lower wishbone cover, remove the lower damper retaining nut and Bob's your father's brother. Am I right?

That's how I did mine.

 

[Peter C1974]

Rattling / knocking dampers are quite common.

Really?

Whilst I am not knocking your advice (pardon the pun), in my 20 years of tinkering with cars I have never heard a rattly / knocking damper.

There can obviously be a first.....!

 

[Peter C1974]

That's how I did mine.

Thank you sir! Dare I say, sounds like an easy job.

 

[roadhog]

It is.

My dampers were leaking and both the plastic covers had become detached from the top. They did rattle around the bottom whilst I was driving. Yours does look like it's still attached though.

Swapping them over isn't a major job but depending on mileage and what you have planned for the car you might consider new ones while you're there?

 

[Peter C1974]

My previous W124 was a 200TE, therefore fitted with self levelling rear suspension, which gave a more comfortable ride. In comparison, the 230E feels a lot more firm at the back.

I'll swap the dampers over and if the knock transfers over to the other side then I've found the problem. Seized damper? I'll buy a set of new dampers. If the knock remains on the left side then I know it's not the damper and I probably won't bother replacing them for the time being.

Whilst I am VERY keen to resolve this ****** knock, I don't want to keep spending £££ on every single possible cause. As mentioned above, I could easily re-bush the whole rear end, but at what cost!

 

[roadhog]

Let us know what you find.

 

[5907]

I think I might have a similar issue with my E280 estate; except I only get knocking / juddering when pulling off. I've yet to find the fault.

 

[teddycatkin]

Loose handbrake shoes? = Knock or rattle from inside the wheel hub.

 

[Peter C1974]

I had a think about this last night.

It can't be the shock absorber or anything to do with the brake shoes. The knock is too solid and too loud for it to be caused by a small and light element. Must be one of the suspension bushes....

After worn droplinks, which worn bush is the next most common cause of knocking?

 

[teddycatkin]

This may also sound obvious --spare wheel or jack loose in boot any loose any things -take out and test run ? If its a really solid thump when braking or accelerating --propshaft doughnut or mounting ---or subframe bushes-- getting curios to know what it is myself now? Do post and let us know when you find it.

 

[S80]

The heat shield above the catalytic converter on my 280E has become detached at one end and flaps about a bit over bumps - sounds pretty loud inside.

Just a thought...

 

[wireman]

Had something like this on my tatty old 250D bus last year, after much seeking, rooting and fiddling it turned out to be the eye bush at the lower end of one rear shock absorber, on inspecting the old unit it did not look too tatty to put back into service but it would clonk on the slightest bit of uneven surface, perfect on nice flat roads and bumpy ones as well, but on those roads with slight variations of surface smoothness (M55) it rattled like a bag of spanners.

If you do fit new shocks fit new bolts, note that the bolts are not standard but fine thread types, make sure that you have them in the kit before you start the job.

 

[Peter C1974]

This may also sound obvious --spare wheel or jack loose in boot any loose any things -take out and test run ? If its a really solid thump when braking or accelerating --propshaft doughnut or mounting ---or subframe bushes-- getting curios to know what it is myself now? Do post and let us know when you find it.

Boot was empty during the roadtests.

The rear end is silent between gear selection from P to D to R, so the diff bushes are ok.

I thought about the subframe bushes, but they look ok and its not that sort of knock - see below.