W108 280SEL - Clunk from Rear End upon Hard Acceleration - Mercedes-Benz Forum

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post #1 of 24 (permalink) Old 12-26-2015, 11:28 PM Thread Starter
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W108 280SEL - Clunk from Rear End upon Hard Acceleration

Hello everyone,

I have a 1972 280SEL. It has AccuAir air-ride suspension, and sits about 1.5" lower than factory ride height when driving. It has recently started making a rather alarming noise from the rear end on hard acceleration, and occasionally when shifting into Drive.

When I hit the gas hard, I hear one loud "Clunk" from the rear end, as if the differential is hitting the underbody of the car. Same clunk is heard when shifting into drive. It seems to me like it's happening when a sudden amount of torque is delivered to the rear end.

It is not noticeable under light acceleration, or in reverse. This makes me think it could be a differential bushing, or something of that nature. I can't see any evidence of anything actually hitting the underbody, but something has to be making an abrupt motion under there to make that kind of noise.

If anyone can offer ways to test and find the source of the noise, or a diagnosis, I would appreciate it! Thanks in advance for the help!
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post #2 of 24 (permalink) Old 12-27-2015, 12:30 AM
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Originally Posted by envoy_to_the_Stars View Post
Hello everyone,

I have a 1972 280SEL. It has AccuAir air-ride suspension, and sits about 1.5" lower than factory ride height when driving. It has recently started making a rather alarming noise from the rear end on hard acceleration, and occasionally when shifting into Drive.

When I hit the gas hard, I hear one loud "Clunk" from the rear end, as if the differential is hitting the underbody of the car. Same clunk is heard when shifting into drive. It seems to me like it's happening when a sudden amount of torque is delivered to the rear end.

It is not noticeable under light acceleration, or in reverse. This makes me think it could be a differential bushing, or something of that nature. I can't see any evidence of anything actually hitting the underbody, but something has to be making an abrupt motion under there to make that kind of noise.

If anyone can offer ways to test and find the source of the noise, or a diagnosis, I would appreciate it! Thanks in advance for the help!
First off: a warm welcome to the benzworld forum.
At least you put : TX and what car you have, that's a great start.

Now I have seen some of these as of late w108 or W114 cars that are lowered , with the rear wheel so crooked, that one think a catastrophic rear axle failure had occurred.

Well, that is probably what you got.
The axles were not designed for these lowered angles.
And even though Mercedes , even 40 year old (or should I say especially those from that age ?) do NOT take any and all abuse.

Now to make a proper long distance diagnosis of your problem, and avoid a 30+ post, ping-pong rally about what it could be, it would probably be wise for you to make a video of the noises (and when they occur) and what kind of strange angles in the suspension (front and back, I guess) we are talking about.

If you're willing to do that much, I am willing to help.

Otherwise it is just too many different possibilities, added to the usual list .

Hope to hear back.

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post #3 of 24 (permalink) Old 12-27-2015, 01:17 AM Thread Starter
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Originally Posted by 6punkt3 View Post
First off: a warm welcome to the benzworld forum.
At least you put : TX and what car you have, that's a great start.

Now I have seen some of these as of late w108 or W114 cars that are lowered , with the rear wheel so crooked, that one think a catastrophic rear axle failure had occurred.

Well, that is probably what you got.
The axles were not designed for these lowered angles.
And even though Mercedes , even 40 year old (or should I say especially those from that age ?) do NOT take any and all abuse.

Now to make a proper long distance diagnosis of your problem, and avoid a 30+ post, ping-pong rally about what it could be, it would probably be wise for you to make a video of the noises (and when they occur) and what kind of strange angles in the suspension (front and back, I guess) we are talking about.

If you're willing to do that much, I am willing to help.

Otherwise it is just too many different possibilities, added to the usual list .

Hope to hear back.

Hello,

I will be happy to get a video for you tomorrow. The suspension system I have uses ride height sensors in each corner to keep the car at one of 3 ride heights. I do not drive in "Low." I typically use the middle setting for driving, and the 3rd position (stock height) if there's an absurdly large speed bump.

I don't see any strange camber angle in the front wheels, regardless of the position. Same story with the rear left wheel. Rear right shows some camber that I believe may be suspension related, but it puzzles me as to why that isn't replicated on the driver's side.

As I mentioned, I do not drive in the "low" position. The middle driving position is certainly lower than factory, but nothing rubs or touches. It'd be similar to driving with your two most portly friends in the back.
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post #4 of 24 (permalink) Old 12-27-2015, 01:46 AM
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To make a long story short.
I really don't care how and what some back yard tinker has done to your suspension, but if you are not riding at factory specified height, you should expect damage sooner or later.
There was quite a bit of serious engineering that was put into these cars, and I seriously doubt the same level was used to alter your car.

And " nothing rubs or touches " is NO criteria.

These cars are built to withstand (in my opinion) a 1/2 foot deep pot hole and 'bottom out' as designed without damage to suspension or frame.....
If YOU TAKE THAT AWAY, YOU'RE ON YOUR OWN, i THINK....

But let's wait for the video.

Thanks
Regards

Alex
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post #5 of 24 (permalink) Old 12-27-2015, 05:05 AM
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W108 280SEL - Clunk from Rear End upon Hard Acceleration

I suspect the center bearing has gone bad and the sound you hear is the drive shaft is slapping around.
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post #6 of 24 (permalink) Old 12-27-2015, 06:22 AM
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Starting point is always to get under car and start moving things around and looking for movement and worn rubber. Use jack stands. As Fonzi said, check your driveshaft, and the bushing the rear axle hangs from.
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post #7 of 24 (permalink) Old 12-27-2015, 12:55 PM Thread Starter
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Quote:
Originally Posted by 6punkt3 View Post
To make a long story short.
I really don't care how and what some back yard tinker has done to your suspension, but if you are not riding at factory specified height, you should expect damage sooner or later.
There was quite a bit of serious engineering that was put into these cars, and I seriously doubt the same level was used to alter your car.

And " nothing rubs or touches " is NO criteria.

These cars are built to withstand (in my opinion) a 1/2 foot deep pot hole and 'bottom out' as designed without damage to suspension or frame.....
If YOU TAKE THAT AWAY, YOU'RE ON YOUR OWN, i THINK....

But let's wait for the video.

Thanks
Regards

Alex
As an engineer myself, I can certainly appreciate that a team of highly qualified German engineers knows how to make a car better than I do. This car was destined for the scrap yard when I bought it and began restoring it, so I'd like to think that even though air ride isn't an ideal solution for the purists, this car has drawn a lot of attention to old german styling here locally, and even inspired others to buy and restore old W108s that may have been scrapped otherwise. If this was a flawless Condition 1 car, then modifying it would be a sin....but this car did not run and was in dire need of mechanical and cosmetic TLC when I bought it for its value as scrap metal.

On to the video. I apologize for the mediocre quality, the weather has been horrible here and doesn't show any signs of letting up.


The Driving position is about 1.5 inches lower than the factory height. The component in the trunk over the axle protrudes about 1/4 inch in driving position. I am still learning the car, I haven't had a chance to get in there and see what it is.

The clunk may be a bit hard to hear in the video becuase the mic tends to focus on higher pitched sounds - but it is quite audible in the car. The sound is heard in driving position, but not in the highest setting.
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post #8 of 24 (permalink) Old 12-27-2015, 01:38 PM
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So, I thinkyou are experiencing the whole reason these cars don't like to be lowered. I have a couple questions and some observations.

You have air ride....exactly what did they/you do. Just remove springs/shocks and replace with bags? What about the compensator between the right axle tube and the cender dif? Is that there? Is it a compensator or just a spring?

The way the MB swing axle in these cars works is pretty interesting. The Driver axle tube is connected rigidly to the center dif. The right axle tube pivots at the bottom of the dif....hence the name low pivot swing arm.

When you deflate the bags to lower the car, the rear axle tries to raise up into the body, but this can't happen with these cars. The center dif is attached to the body of the car by "the thing" you see rising into your trunk.....no, it is not supose to do this. When you lower your car, you are transfering the stress normally carried by your springs to this center dif carrier. This rubber bushing is shot....and they are not cheap or easy to find.

Your rear end is positioned front to rear by the trailing arms under the axle. It is kept from rotating (torquing) by this "no longer functional" bushing. It is positioned right/left by a pan hard rod from the dif to the right frame.

Two things are happening when you lower your car because of this shot bushing....Your dif is being pushed into the body and it is being pushed to the driver side (by the panhard rod). When you apply torque to the rear end (accelerate or put into drive) you are likely slamming the dif support that goes into this bushing into the surrounding framing since there is no longer anything to take the torque.

Yes, I'm also an engineer
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post #9 of 24 (permalink) Old 12-27-2015, 02:10 PM Thread Starter
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Quote:
Originally Posted by TZ_280SEL View Post
So, I thinkyou are experiencing the whole reason these cars don't like to be lowered. I have a couple questions and some observations.

You have air ride....exactly what did they/you do. Just remove springs/shocks and replace with bags? What about the compensator between the right axle tube and the cender dif? Is that there? Is it a compensator or just a spring?
This particular car does not have the hydraulic compensator - it's just a spring in the middle of the diff.

Quote:
The way the MB swing axle in these cars works is pretty interesting. The Driver axle tube is connected rigidly to the center dif. The right axle tube pivots at the bottom of the dif....hence the name low pivot swing arm.

When you deflate the bags to lower the car, the rear axle tries to raise up into the body, but this can't happen with these cars. The center dif is attached to the body of the car by "the thing" you see rising into your trunk.....no, it is not supose to do this. When you lower your car, you are transfering the stress normally carried by your springs to this center dif carrier. This rubber bushing is shot....and they are not cheap or easy to find.

Your rear end is positioned front to rear by the trailing arms under the axle. It is kept from rotating (torquing) by this "no longer functional" bushing. It is positioned right/left by a pan hard rod from the dif to the right frame.

Two things are happening when you lower your car because of this shot bushing....Your dif is being pushed into the body and it is being pushed to the driver side (by the panhard rod). When you apply torque to the rear end (accelerate or put into drive) you are likely slamming the dif support that goes into this bushing into the surrounding framing since there is no longer anything to take the torque.

Yes, I'm also an engineer
Interesting, that's great to know. Does that bushing have a name or part number that I can research? I'm trying to get a better idea of what it looks like before I start tearing things down. I recall this happening with the stock suspension as well if we sat on the rear bumper, so perhaps it was already on its last leg and the extra stress of air bags just killed it.

All I can find in the diagram that looks like it'll reach up into the body of the car is #130 on the right page:

http://i.imgur.com/Cx8c54u.jpg
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post #10 of 24 (permalink) Old 12-27-2015, 02:28 PM Thread Starter
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Looks like this gentleman may be experiencing similar problems with the center diff mount:

Replacing diff carrier bush in w108 - PeachParts Mercedes ShopForum
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