W126 Vibration >50MPH - Shocks? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 10-04-2018, 09:59 PM

As noted on my first thread about my 91 350SD, the car has some sideways action over bumps at highway speed. Wallowing or wobbling I guess it's called. And apparently that indicates bad shocks?

As also noted there, the car has a vibration that is barely noticeable at 50, starts to pickup around 55, then gets progressively worse up to around 70-75 MPH. I havent taken it much above that, but it does seem to get a little better after that. It feels like it is from the floorboards, a little bit felt in the seat, and a little bit felt in the throttle; NO vibration or motion in the steering wheel. The shifter has a bad bushing, so it can be a tiny bit squeaky as well. I have not checked the guibo or any mounts, though the engine idles smooth as glass and no other indications of mount concerns. No sag, no lift/drop in and out of gear, etc.

I drove the car for about 600 miles since first buying it, mainly on a 400 mile road trip on 55-65mph roads. It seemed to marginally improve since I first got it, I think adding air above the recommendation, and driving it a bit (flatspotred tires?) helped. The rear tires are late 2017 production date, fronts were 2004 production. So I just tonight replaced the tires with the same model as was on the rear. The rear tires got moved to the front. I did also have them rebalanced when I did the other two.

Now it's as bad as when I first got the car (admittedly Costco likely reduced the tire pressure). Still no appreciable steering wheel vibration or motion, so the two tires I kept I dont think are the culprit. I had been curious if perhaps one had a bad wheel or broken belt, but if it did, Id think that Id feel it in the steering wheel with those now on the front axle.

Other than the sideways motion over bumps on the highway, the suspension seems good. No indication of rear bushing issues. I lifted the front and there's no appreciable play in anything, only a slight tear in one ball joint boot (and clean and dry underneath). No end play other than the standard steering box play.

Ive tried the "bump test" but Im never good at it. It strikes me that sometimes it does take a few shakes to stop when trying it. The wallowing and wobbling on the highway seems to be an indicator though. Would bad shocks also create a vibration?

Any ideas or suggestion?

Junkman · #2 10-04-2018, 10:19 PM

The car is old with old steering parts. You can piece meal it or pull the front end apart and fix the entire system. The PO of my 84SD half fixed things. The front shocks were almost new Bilstein Comforts. I replaced the set with HD because I prefer them. He had changed the brake master but calipers were stuck and pads down to metal. I replaced all.

Car continued to foll the road crown no matter the alignment so I replaced all rubber in the front end. It's much better and I'll do the back when I have a minute.

Shocks can be done independently of anything else. Remove one and examine how much effort is required to compress it. You'll know if it is bad.

If you want my front Comforts, $30 + shipping but make sure that those from an 84 will fit yours.

Diseasel300 · #3 10-04-2018, 10:22 PM

Bad shocks absolutely will create vibration on the road. The "bounce test" is pretty useless on the Benzes. My SL passed the "bounce test" and it's shocks were like pogo sticks - literally NO damping ability at all. My SDL's shocks were about 80% trashed, and it too passed the "bounce test". Replacing with Bilstein HD's was like a night/day transformation in how the car handles and drives. The SDL is still a little squirrely from the rest of the suspension being very tired, but runs and drives straight without vibration. The wallowing and rolling was dramatically reduced too.

Be aware that the driveshaft can cause vibration too, particularly if you feel it more in the seat area or behind you. The carrier bearing may be trashed, the flex disks may be on their way out, or the U-joint may be stiff. I hit the trifecta on the SDL, the FD's were done, the carrier bearing was torched, and the U-joint was notchy and stiff. FD's and bearing didn't solve it, but a rebuilt driveshaft did. My vibration would pick up right around 55 and reach peak intensity around 65 and taper off around 70. The giveaway that it wasn't tires in my case was the frequency of the vibration - was a little too fast to be a tire.

JHZR2 · #4 10-04-2018, 11:02 PM

Front end is tight and tires are worn evenly. Not arguing that front end rubber on a 1991 car will need to be done, but its not necessarily all dire.

The clue I have is the sideways motion over bumps on the highway. That tells me shocks.

I have bought a set of Bilstein HDs. I plan to install them over the next few days.

Wouldnt a bad carrier bearing be noisy? Is there a test for it? Ditto for U-joint? I can see and inspect the flex discs. Driveshaft has come to my mind, just seems odd given the car's age and condition, when I have W123 cars with original drivetrain parts that look brand new....

Diseasel300 · #5 10-04-2018, 11:22 PM

The carrier bearing is held in a rubber surround. When the rubber surround perishes, it allows the bearing to move more than it should. The result is vibration. If the bearing itself fails, you'll get noise, but it's usually the surround that takes a dump.

U joint is tested by dropping the driveshaft and feeling it. It should move in all directions smoothly. If it feels stiff or has a notchiness to it, the bearings are done.

Start with the shocks and move on from there since you've already bought the parts. If the shocks don't fix the vibration issue, you can look into other sources. The HD's give a nice smooth ride in my SDL, I couldn't imagine running anything softer!

Clemson88 · #6 10-05-2018, 06:42 AM

Tires cause vibrations. Bad shocks, ball joints, tierods and other front end parts cause tires to go bad but you're stabbing in the dark to try and guess what messed the tires up. heck the front end components then expect to pay for a new set of tires after the repairs.

As far as the vibration in the floor I'd check the drive shaft assembly from the transmission to the differential to solve that issue. You may have two issues.

JHZR2 · #7 10-05-2018, 07:08 AM

Quote:

Originally Posted by Clemson88

Tires cause vibrations. Bad shocks, ball joints, tierods and other front end parts cause tires to go bad but you're stabbing in the dark to try and guess what messed the tires up. heck the front end components then expect to pay for a new set of tires after the repairs.

As far as the vibration in the floor I'd check the drive shaft assembly from the transmission to the differential to solve that issue. You may have two issues.

All tires are new, all have been rebalanced, changing them from axle to axle doesn't affect the vibration.

So I'd argue it isn't a tire.

Shocks I'd understand. Drive shaft unfortunately I'd understand as well...

dieselbenz1 · #8 10-05-2018, 07:29 AM

Vibration felt in your seat under power is most likely drive line. Get up to speed feel vibration take foot off pedals if vibration stops that confirms drive line vibration and not tires. Shocks will not be the sole cause of a vibration.

JHZR2 · #9 10-05-2018, 04:11 PM

So I put the car up on ramps, and lifted the rear end up slightly with wheels still on the ground.

I got under just far enough to get photos.

Everything looks great to me.

Long story short the guibos and bearing carrier all look solid. Rubber is perfect. I didn’t lift it and run the car, but the vibration doesn’t start to 50+, and is never run the car on stands to that speed.

Grabbing the driveshaft, there’s no play when trying to rotate or shake it side to side.

Have to wonder if the belt tensioner just creates enough vibration that translates through. If a minute of use at idle gets the shock hot enough to burn me, then I can’t imagine how it gets at highway speeds, 2500-3000 rpm.

TX76513 · #10 10-05-2018, 05:00 PM

Quote:

Originally Posted by dieselbenz1

Vibration felt in your seat under power is most likely drive line. Get up to speed feel vibration take foot off pedals if vibration stops that confirms drive line vibration and not tires. Shocks will not be the sole cause of a vibration.

I chased a similar vibration for months and it ended up being trans. mounts. So I sort of side with don't over look em

JHZR2 · #11 10-05-2018, 05:19 PM

Trans mount is in the picture above. Looks fine to me. ????

dieselbenz1 · #12 10-05-2018, 07:08 PM

Post 9 pic 3 looks like a loose bolt on the flex disk. The last pic shows incorrect installation of the flex disk.

dieselbenz1 · #13 10-05-2018, 07:19 PM

Quote:

Originally Posted by TX76513

I chased a similar vibration for months and it ended up being trans. mounts. So I sort of side with don't over look em

Transmission mount is part of drove line, no?

JHZR2 · #14 10-05-2018, 07:30 PM

Quote:

Originally Posted by dieselbenz1

Post 9 pic 3 looks like a loose bolt on the flex disk. The last pic shows incorrect installation of the flex disk.

I think it’s just different bolts with different numbers of threads.

The incorrect installation is curious. It’s all MB star parts with painted bolts. Looks factory to me - can’t imagine it’s been that way from the start...

But I don’t know enough.

For comparison/FWIW, here’s the original on my 240D:

Every other bolt there is different too...

JHZR2 · #15 10-06-2018, 02:09 PM

Quote:

Originally Posted by dieselbenz1

Vibration felt in your seat under power is most likely drive line. Get up to speed feel vibration take foot off pedals if vibration stops that confirms drive line vibration and not tires. Shocks will not be the sole cause of a vibration.

So I wanted to pull the thread on this a bit.

Cruising at speed and taking feet off pedals (no power state), doesn’t change it. Nor does putting the car in neutral at speed.

I’d kind of not expect it to - the drivetrain is still spinning.

But again, all elements look really good to me, and grabbing and shaking doesn’t indicate any sort of issue. It’s all solid and tight.

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