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  #1  
Old 03-21-2006, 07:42 AM
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Changed Rear Shocks -- Now "Thunk"

I have a 99 ML 320 with 165K miles. Last month I changed the rear shocks. (Thanks to everyone here who contributed postings on this subject.) I feel a moderately firm "thunk" or "thwup" in the right rear whenever I roll over even a small pothole or bump in the road. It's not very loud, and I can't say I actually feel it in the drivers seat. But it is definitely there and it's definitely coming from the right rear. It kind of sounds like either the spare tire or the jack is loose. But I've checked both and both are firmly secured.

I followed all the great advice from this forum to R&R the shocks. This job is not easy, but it's not rocket science either. If you can get to all the fasteners and have the tools to break them loose, you can do this. As far as I can tell, everything can only go back together one way. I torqued as many of the bolts as I could in my original work. (Couldn't reach the top three 13mm nuts with a torque wrench.) Made sure to torque the big lower control arm bolts with the weight on the suspension. I went back and re-checked all the bolts to make sure they are tight. I'm _pretty sure_ I did the job correctly.

I got my shocks from Fastlane. They are Bilstien part number BE52985 per the receipt I have from Phil. These are the shocks without the coil spring installed. I had a local mechanic swap the old springs onto the new shocks for me.

Do you think this is the correct shock for my truck?

How do you know how far down to run the top nut on the shock when you install the coil spring? The mechanic that swapped the springs had trouble removing one of the top nuts. He used an impact gun to install the new nut. At some point, the shock shaft just spins so you can't tighten it any more.

Do you think rear sway bar bushings could be the cause? Mine are original.

I don't recall seeing bump stops in the packaging. Is it possible I left out required bump stops?

Do you think the top three 13mm nuts are not actually tight on one side because, with the vehicle on jack stands, the rear suspension is pulling down as you tighten these nuts. I don't think it is possible to access the nuts with the weight on the rear suspension. Just a thought.

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  #2  
Old 03-21-2006, 09:02 AM
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Your vehicle does need to be grounded when you tighten all the nuts and bolts associated with the shock replacement. You could try setting the rear of the truck on cinder blocks, assuming you don't have access to any type of ramp system, and that will give you enough height to get under and tighten everything. As for the rear sway bar, 2 of each: http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=213951&chapter=ERI5768&appSectionid=2788&groupid=51856&subgroupid=2544&componentid=0&make=67&model=M%20class&year=1999&graphicID=5768500&callout=3&catalogid=2
I did mine along with the shocks and had positive results. I'm not saying that is your problem though; good luck.
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Old 03-22-2006, 07:21 AM
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Quote:
Originally Posted by steve32
Your vehicle does need to be grounded when you tighten all the nuts and bolts associated with the shock replacement. You could try setting the rear of the truck on cinder blocks, assuming you don't have access to any type of ramp system, and that will give you enough height to get under and tighten everything.
Interesting. I knew from discussions on this forum the axle shafts had to be parallel to the ground while tightening the lower control arm bolts. I did that. Didn't realize _all_ the other bolts and nuts had to be tightened with the suspension loaded. I did all the other bolts while the truck was still on the jackstands.

I'm anxious to find out if I can even get my medium size hands under the frame rails to access the three 13mm nuts on top of the shocks with the suspension loaded. My expectation is there is not enough room unless you lift the truck and let gravity pull the suspension apart a bit.

I'm also going to replace the sway bar bushings.

Thanks for the response and link.
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Old 03-22-2006, 09:25 AM
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I wouldn't worry about the three 13 mm nuts on the top of the assembly. As long as they were tight when installed, they will be fine. They are self-locking and with the weight of the vehicle on the coil/shock, stress should be reduced.

FYI...........I took my ML to a friendly TiresPlus and had them torque all suspension pieces to proper specs. They used an alignment rack which was perfect..gets vehicle up off the ground, but still has weight on the tires.
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Old 03-22-2006, 10:07 AM
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Your 'thump' is very likely the upper shock bushes....If it did not do this with old shocks, refit the old upper bushes.....
I had this with a self leveling e320 wagon a few weeks ago....the new upper bushes where too thin to absorb the wheel movement..ie : there was not enough compression of the bushes with the top shock nut fully tightened .....I installed other bushes I had in the shop from other shock jobs that had fat bushes....took some compressing to get the nut to start but no more noise .

This problem is known to affect some other Mercedes too.


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Old 03-22-2006, 02:31 PM
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dkveuro -- Thanks for the suggestion. I don't have the original shocks any longer. Do you know if these bushings are available by themselves? And from what source?
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Old 03-22-2006, 03:09 PM
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you need to find a thicker top bush. Try one of the car shops...If you remove the top bush ...see if possible to shorten the spacer that limits compression of the bushing. The 126 chassis had the same problem with replacement shocks....like I said...i used some I had in the shop that where thicker.

.check with MB parts dept...(?)
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Old 03-22-2006, 04:34 PM
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Quote:
Originally Posted by dkveuro
you need to find a thicker top bush. Try one of the car shops...If you remove the top bush ...see if possible to shorten the spacer that limits compression of the bushing. The 126 chassis had the same problem with replacement shocks....like I said...i used some I had in the shop that where thicker.

.check with MB parts dept...(?)
I don't see why the top bushing would cause this. unless the spring broke and the suspension bottomed out. Why would one wants to recycle the spring after 160k miles of driving. The whole spring/strut assembly is reasonably cheap that I would rather replace the whole damn thing, it is a waste of time to recycle that 160k miles well used spring.
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  #9  
Old 03-22-2006, 05:03 PM
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Quote:
Originally Posted by supradupe
I don't see why the top bushing would cause this....... ...................... The whole spring/strut assembly is reasonably cheap that I would rather replace the whole damn thing.............................
For a start 'supradupe' ...I'm not trying to explain this to you .

For another, I am telling 'sunedog' what has happened with noises like this before...'workshop pattern failures'...now, if you have practical input, let's hear it...if not, your not being helpful.

There have been problems like this with shocks not having the upper bushes under enough compression when assembled and the 'thump' is from the wheel dropping into the dip and pulling on the upper mount bushes, the return stroke is what causes the noise, as the lower top bush now has a little room to move upward to cause the noise. A thicker bush or shortened spacer will compress the bushes more to not allow unloading of either bush........this movement of the shock is due to the compression stroke being heavily damped as opposed to the relative soft stroke of the shock extension.

The upper spring mount on VW front shocks causes a similar problem due to end float in the bearing assembly.....the cure is to remove the spacer and shorten it slightly untill the noise goes away.

An what's more 'supradupe'......I fail to see the reasoning in your post .



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Old 03-22-2006, 08:41 PM
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dkveuro and supradupe --- it's all good. I appreciate both of your inputs. No need to focus on each other. Let's steer this discussion back to my favorite subject ===> me.

I look forward to more constructive dialogue on this subject. I still have a problem to fix.

supradupe -- I didn't replace the coil springs because I didn't have to. They were not broken. I think someone on this forum had to replace one spring and it was $200. I didn't see a need to spend an extra $400 on this job. I didn't realize when I bought the shocks online that shocks with new springs were available from other sources for less money than I paid for these. In retrospect, I sure wish I had bought shocks with springs.

dkeuro -- I need imagery to understand what you are suggesting. The Bilstien's I installed have internal bump stops according to folks on this forum. (I am aware of your sig line as I type this. Sorry -- I don't know for a fact.) I don't remember any bushings (is that what you mean when you say bushes?) in the shock/spring assembly. Here is a shot of a complete shock/spring assembly from another post (credit to user named Bobby):



And here is a schematic of the rear suspension:



Can you tell me where the bushes are?
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Old 03-22-2006, 09:07 PM
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Okay.....the ends of the shock mount to the body at one end and the suspension at the other.....the 'bump stop' is fitted around the shock piston shaft and has no bearing on the noise.

Unless the sway bar link/mount is worn....the noise will emminate from the shock assembly.

With normal body weight on the suspension, the spring will/should never unload from it's seats......this leaves us with the top and bottom mounts....one to the
body the other to the suspension.

If the rubber bushing at one end.....and it is usually the body mount..... wheel motion against the shock damping directly acts on the mounts...in this case, the rubber isolating bushes.....where it mounts to the body there is usually a rubber bush on each side of the mount or body panel....if this assembly is not under sufficent compression, as the mount works it will make a thud or thump.
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Old 03-22-2006, 09:16 PM
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OK. So this bushing is attached to the body and is not part of the shock/spring assembly?

Any way to see this bushing without removing the shock/spring assembly?

Sorry for so many questions. I need to learn a lot more.

Oh -- forgot to mention -- my swaybar bushings are original. 165K miles at this point. Do you think I should replace them first (easy and cheap) before I pull the shock assemblies looking for the bushing?
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Old 03-22-2006, 09:24 PM
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Quote:
Originally Posted by sunedog
OK. So this bushing is attached to the body and is not part of the shock/spring assembly?

Any way to see this bushing without removing the shock/spring assembly?

Sorry for so many questions. I need to learn a lot more.

Oh -- forgot to mention -- my swaybar bushings are original. 165K miles at this point. Do you think I should replace them first (easy and cheap) before I pull the shock assemblies looking for the bushing?
...YES.cheap fix and may silence your noise.


The lower arm mount could be your problem, as it mimics the normal upper shock assembly on other vehicles...the other thing you wrote was the mechanic spun down the nut with an air gun...yes ? The instructions for shocks say : DO NOT USE AIR WRENCH TO TIGHTEN .
Doing so can/will damage the foot valve inside the shock valving assembly.

Would it be possible for someone to bounce the truck while your listen at that wheel for noise ?

FWIW.....Top nuts torque is 20 nm
Large nut on the bottom - 85 nm
Nut connecting rod to torsion - 21 nm
Lower wishbone to rear axle carrier - 135 nm. Torque them with rear axle shafts horizontal.
You should replace all self-locking nuts and bolts.( Or use Blue Loctite. )
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Old 03-22-2006, 10:27 PM
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Quote:
Originally Posted by dkveuro
For a start 'supradupe' ...I'm not trying to explain this to you .

For another, I am telling 'sunedog' what has happened with noises like this before...'workshop pattern failures'...now, if you have practical input, let's hear it...if not, your not being helpful.

An what's more 'supradupe'......I fail to see the reasoning in your post .
.
For a start, I don't need your explanation. If you had read carefully, I did mention that the spring might have been broken given the mileage on the old spring. The post wasn't against your reasoning, what's there to get personal. I merely suggest the original poster that for the high mileage, he should have replaced the whole spring/strut assembly since it's been available for under $100 each. If you own a ML of that vintage, you would have snapped up a pair at that price.
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Old 03-22-2006, 10:50 PM
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Quote:
Originally Posted by supradupe
I merely suggest the original poster that for the high mileage, he should have replaced the whole spring/strut assembly since it's been available for under $100 each. If you own a ML of that vintage, you would have snapped up a pair at that price.
I wish I had known about the great deals on shocks with new springs installed. Unfortunately, I had already purchased my shocks sans springs for over $150 each from Phil when the great $53 shock/spring sale emerged on this site.

I have decided the first course of action is to replace my rear sway bar bushings.

Thanks for everyone's help. If it were possible, I would buy you both a tall dark cold one and the three of us would continue this animated discussion in person.

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