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.

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.

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.

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.

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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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?

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.

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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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):

http://members.cox.net/ryunk/wholeshock.jpg

And here is a schematic of the rear suspension:

http://www.peachparts.com/shopforum/...5&d=1041298270

Can you tell me where the bushes are?

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.

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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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.

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.

Being obtuse again !:silly:


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I think the reason why some people are keeping the original springs are because the Bilstein Rear Shocks don't come with any springs. The way to use them is to use the original shocks or buy a new pair of rear springs which would cost you $200!!! I have experimented with H & R rear springs, but the ride turned out to be really harsh and uncomfortable. One of the rear spring was broken, so I had to order an original shock/spring assembly and took the new spring off to use with my Bilsteins!!!

The best reason is that the Bilsteins Shocks are so much more superior as compared to the OEM Sachs shock absorber. The ride, with the Bilsteins, are so, so much better! They make the ride of my 2000 ML430 feel like my 2005 X5 3.0 :) I am so glad I did not get the cheaper solution of replacing them with the OEM units.

If you can afford Bilsteins, and would not mind re-using the original springs, they are the BEST combination!!! If you are low on budget, then the OEM unit would suffice.

Anyhow, I don't think the rear springs degrade with age, unless they are broken... all they do it suspend the vehicle, they do not loose their compressive strength over time that much... The shocks are the major thing that affect you ride... well, the spring rate is the other important thing, but you can't really change that unless you use a different spring, like the H & R springs.

I think you made an excellent choice of using Bilsteins Shocks, I don't think you will regret it!!!

Sunedog, thanks I will take a raincheck for that. On a second thought, you originally said you heard a "thunk", is it a metallic clunk sound? or a dull "thug"? If the swaybar bushing is bad, usually you will hear it squeek when you go over bumps, not "thunk". Before you go out and buy anything, at least jack up the rear end and check for any loose parts. The swaybar link has 2 rubber bushing sandwiched the swaybar, check to see if your mechanic put them back the way it supposed to. Again, I am no expert, just something to think about before you start replacing parts.

yeah, better than obnoxious. I don't own MBs and pretend to be a VW expert. maybe the VW boys are treating you too rough there. People here are more civilized.

sho1 -- This is exactly my situation. New Bilstien's with original springs. Thanks for the vote of confidence.

supradupe -- The noise it definitely not metal on metal. Dull "thunk" or maybe "thwap" is the best way I can describe it. You can hear/feel it even over very slow maneuvers, such as rolling slowly backwards from my driveway to the street.

I already jacked the truck up and made sure everything was tight. The mechanic didn't do the complete install. He just swapped the old springs onto the new shocks. I did the rest of the installation. I am confident I put the sway bar bushings back correctly. Since a new set of bushings is about $50 and mine have 165K miles on them, I don't mind replacing them as the next step.

Oh -- dkveuro suggested having someone bounce the car while I listen at the wheel for noise. I wll try that first.

Man I owe you guys. Someone needs to set up a paypal type site where you can buy beers over the web.

Update -- Went to pick the truck up after the cat's were replaced and asked for a mechanic to listen to the sound. Actually two mechanics were a tremendous help. We could hear a fwap if I rocked the car (thanks dkveuro) and they were crawling all over the ground trying to isolate it. They finally pulled it into a bay at 10 minutes to closing time and pulled a wheel and inner fender. Two mechanics spent about half an hour with me and didn't write it up. I really appreciate their help.

Turns out the shocks are not properly mounted to the coil springs. I can't say I understand exactly what is wrong until I pull the shock/spring assemblies out this weekend. It is definitely not the top three 13mm nuts holding the shock assembly to the body. Evidently the mechanic that swapped the springs screwed up. The Mercedes mechanics pointed out what was loose when they had the truck on the lift with the inner fender pulled back.

So the plan is to remove the shocks this weekend and determine what needs replaced. The left shock mount is toast. Don't know about the right. I'm hopeful the Bilstien's aren't damaged. Once I have them out, I'll be able to figure out if I just need to replace the shock mounts and properly fit and tighten them or if the Bilstien's are ruined as well. If they are, I'll definitely be looking for the best source for the shock/spring assembly.

Anyone know the current best source for these? (Not Phil for this one. That's where I bought the Bilstien's. I believe he is only offering shocks without springs at this time.)

I have a lot of work in front of me. I'll let you know what I find in a few days.

Glad you were able to figure out the source. You might get some of your money back from the original mechanic if you explain the problem and the trouble you've encountered.

I bet you can find the assemblies if you look hard enough. A google search
should give you plenty of choices. Good luck !!

The upper shock mount appears to be a separate item you can order see #14 in the link.

http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=213951&chapter=ERI5768&appSectionid=2788&groupid=2523&subgroupid= 2534&componentid=17502&make=67&model=M%20class&year=1998&graphicID=5768495&callout=13&catalogid=2

Thanks, supradupe.

Ouch. $53 each. Funny, they call this part a "bearing bearing back."