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240TD rear clunking sound

sceh

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Jul 31, 2015
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273
Car
w123 240TD
I have a 1981 240 diesel estate (240TD). It has 75K miles from new.
Occasionally there is a deep 'thunk' sound from somewhere in the rear. I have checked everything that can move under the car and inside the car but can find nothing. The think can happen at low speeds usually and is not dependent on the road surface. I can drive down my drive out of the garage at it will happen.
Does anyone have any ideas at all what I should check? It is not the fuel tank or the baffles btw either and the suspension seems perfect (self-levelling at the rear)
thanks for ANY advice upload_2019-10-5_14-28-45.gif
 
lovely model of car.
it could be the tyres at a guess if nothing else seems the problem.
is it definitely coming from the back or the front ?
 
The back for sure. If you ride in the back with the seats down it is coming from there but it is very hard to pinpoint and it is not consistent either. I have been under the cae car with crowbars etc and can find noting loose. Infuriating!
 
I have a 1981 240 diesel estate (240TD). It has 75K miles from new.
Occasionally there is a deep 'thunk' sound from somewhere in the rear. I have checked everything that can move under the car and inside the car but can find nothing. The think can happen at low speeds usually and is not dependent on the road surface. I can drive down my drive out of the garage at it will happen.
Does anyone have any ideas at all what I should check? It is not the fuel tank or the baffles btw either and the suspension seems perfect (self-levelling at the rear)
thanks for ANY advice View attachment 89383


Check ATF level, if auto.
 
Snap!
Has it ever had a tow bar fitted?
 
Also, exhaust mounts? Transmission mounts?
 
Mercedes-Benz Model 123 - Disc 2

Lots of rubber bushes to inspect and that differential mount warrants a close look
Mercedes-Benz W123 300TD (1977 - 1985) Technical Article Directory | DIY Maintenance and Repair Projects for MBZ 230, 240D, 280 CE, 300D, 300TD | Pelican Parts
favorite would be where rear subframe mounts to the body
95143d1314194108-my-w123-saloon-sedan-old-accident-repair-bit-rust-repair-thread-w123-subframe-removed-again.jpg


Rear subframe removal and bushing replacement - PeachParts Mercedes-Benz Forum
 
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Also, exhaust mounts? Transmission mounts?
I have checked all these using a crowbar too and the only ones I changed were the wishbone mounts - not because they were bad but because I had the parts and some spare time.
Could it be a broken spring or shocker?
 
I have checked all these using a crowbar too and the only ones I changed were the wishbone mounts - not because they were bad but because I had the parts and some spare time.
Could it be a broken spring or shocker?
A damper failure would typically manifest itself as a thud when going over speed bumps, as the suspension is compressed quickly without efficient damping.

A failed damper should not really give any issues when driving on a level road.

(Failed damper might mean the actual hydraulic mechanism, or just the rubber bushing on top - either will produce a thud when the spring is sharply compressed)
 
.................................
Could it be a broken spring or shocker?

Broken springs are notoriously difficult to spot. Close to the bottom end, maybe the last inch or two, is a favourite place to snap. The spring then drops by less than the thickness of the coil wire into the cup and looks perfectly normal. Get in there with a torch and mirror and look very closely.
 
Check rear driveshaft gaiters for splits in them? they had oil in there originally not grease as the later cars had
and when the gaiter splits the oil leaks out then the CV joint becomes noisy just a thought.
 
This is driving me insane (or more insane..)
I changed the differential mount and bought, but didn't change, the flex disks since they are perfect. The car has no rust underneath since it was under-sealed new and then again ten years later so it is near perfect and certainly has less rust than my six year old Landrover..
The exhaust mounts and buffers are in perfect shape. I changed the roll bar mounts and connection rods. There is no play in anything I can get to with a crow bar.
The clunk happens randomly and not very often. I can leave my garage and go down the drive and I will get a bang and then nothing for a mile. It makes no difference if I hit a manhole cover for example - no noise or at least not every time. The self-levelling works perfectly and there are no leaks and no signs of anything broken on either spring. I changed the subframe mounts too (the big ones just under the back doors). It comes from the rear but is hard to pinpoint from inside the car. I wedged the exhaust in place as a test but it still happens.

ANY ideas (including it being taken over by aliens)?
I guess I could ask my mercedes garage but given its age and the average age of their personnel (38 vs 35), I am sceptical they could find out
 
Is it remotely possible that the half shaft joints at the rear need lubricating or that the diff has a glitch? I am not even sure you can grease the half-shafts - anyone know?
 
I am now starting to think this could be related to the SLS systems. There are no leaks at all and I changed the accumulators about 5000 kms ago. I don't remember having to purge the system or anything since it seems to be a circuit where the oil returns to the tank at the front automatically.
Have I possibly screwed up? Should the system be bled? Is the pressure level in the accumulators preset? I see no top-up valve so I assume so.
 
The system is self-bleeding and to encourage this it needs about 150kg in the boot and driving for 15 mins over varied bumps.
 
I want to do some tests on the SLS since I still have a knocking sound at the rear. If I undo the control rod from the SLS valve to the roll-bar I assume I can then operate the valve and the car should rise and fall? If so, any issues? This will test the valve and the pump. If so too, how much movement, rise and fall, should i expect?
 

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