Gentlemen, new to the X166 group. 2017 GLS450. Recently purchased for my wife. She loved the color...needed rotors and tires. Replaced all rotors and 4 tires. Brakes are now good but been fighting with getting the new tires to run smooth starting above 62mph up to 85mph. Never smooths out. Been back and forth with the tire folks. rebalanced twice then replaced two of the new tires. Front is better, but the back is still not right. She drove for a few months because it was tolerable, and we have been too busy to address. Had the rears rebalanced again today. (1oz on one, 0.5oz on the other) Still does not smooth out. Ordered two new tires and will replace before the weekend. But, I started thinking, do I have bad dampers/shocks? Personally, I do not like the way the vehicle drives. To me, the body is too floaty, and jittery. Too much side to side motion. Too much ringing through the chassis when going over bumps. Just not grounded like all other mercedes I've had.
Given the 5 position center console knob that says Comfort, Sport, Individual, off road, ice, I had assumed this not only controlled ride height, but also damping/stiffness an all corners.
I got underneath the vehicle and it looks like both rear bags have been replaced recently. Good. However, it looks like the rear suspension uses standard shock absorbers. Is that true? They are OE MB. I do not see any electrical lines going to these shock dampers.
Using one of the online VIN decoders it says, "AIRMATIC DUAL CONTROL / AIR SUSPENSION SEMI-ACTIVE"
Does this mean I only have active damping on the fronts? I admit I did not get under the front. I'll dig in deeper, but a few questions to the group.
1. possible damping valves at the top of the shock towers that I am not seeing?
2. Shocks look fine, not leaking, etc. Anyone else have same floaty ride and replaced with stiffer shocks? If so, recommendations please.
3. other ideas as to why the rear moves around so much? (I had an early 2000s dodge minivan as a company vehicle and it bounced alot but did not jitter. This feels very underdamped.
Comments welcome.

 

Sounds to me that you're rear shocks are gone.
I have a 17 GLS450 and don't have that issue.

 

Well, got the rear tires replaced - Same shaking vibration. I did confirm all shocks are OE with original stickers. Also, Spent some time with the manual and the best I can determine is that I have Airomatic springs, but not the ADS on any of the corners. Just straight OE shock absorbers. So, when I change the center console knob to sport/comfort, etc. All it does is change the ride height about a 1/2 inch. Does nothing for the damping. Disappointing.
That said, I am starting to think about other sources of vibration. Perhaps worn shocks, also, perhaps carrier bearing, etc? I had a bad one years ago but would feel it at low speeds and high torque, not at high speeds.
Other ideas?

 

Just an FYI, but others in these forums speak of forced road balancing when regular balancing didn’t seem to give good results.

 

How many miles on it? Worn CV joints in the rear axles can cause vibration. The frequency would be 6 times the wheel speed. The problem will increase under acceleration and decrease (likely disappear altogether) under engine braking. CV joints will fail prematurely if the rubber boots that protect them are compromised.

If you have enough miles to wear the CV joints, you probably need new shocks, too.

You said the front tires run true. Have you tried putting them on the back? That would eliminate (or confirm) them as the problem.

I think it's a Hunter trademark: Road Force balancing.

 

What make/model of tires?

 

Bridgestone Alenza AS Ultra 275/50/20
Vehicle just hit 75K miles. 5K miles on this set of tires from March '25. With them being truly brand new on the back as of yesterday and still having the same issue, I'm thinking beyond the tires now. But, as suggested, I could rotate with the fronts and see what happens. Don't think it is CV joints. Everything is dry and clean underneath. But I will not rule out anything. I just need to get this up on a lift and go over it myself. I will look closely at everything. But I don't think it is in the drive line back to the differential. That would be a higher frequency vibration. This feels like a periodicity of a tire out of balance. so maybe output shafts from the differential bent, etc. If everything checks out, I'll revisit replacing the shocks.
Has anyone ever converted from standard shocks to the ADS ones? I wonder if they are plug and play or if the computer would need to be retuned to see the variable dampening. I'll check to see if there is the wiring harness in place when I have it up in the air.
I think the shocks are just way too soft allowing this to perpetuate. I put my knee on the rear bumper and tried to bounce it a bit. Much softer and slower to dampen out than our other vehicles, but not like it is a 1970 boat from the good old days. Also, It seems to bounce symmetrically with my knee. Not looser on one side or the other. Of course, there is a rear sway bar trying to keep things even, but still...Humm.

 

I don't see how that could happen unless maybe if you tried to jack the car under the half shaft. Is anyone that clueless?
My experience is Mercedes never installs extra wiring. Regardless, the computer would at the very least need to have that function enabled.
Put a new set of Bilsteins on it and either fix the problem, or eliminate the shocks as the problem. If it bounces more than once they are bad.

Have you checked the wheels and the drive flanges for runout? Warped brake disks?

 

Got the vehicle up on ramps. Everything in the rear looks and feels exactly as it should. All bushings intact and good shape. Everything with the suspension is tight. 1/2 shafts from the rear differential move appropriately side to side. There were some goo gobs in these axel shafts that I cleaned, but nothing remarkable and nothing leaking, all boots clean and in good shape. carrier bearing in good shape.
However, Moving forward, looks like someone had reason to use a green paint pen on the back of the transfer case and rear driveshaft output. (see images attached) Looks like someone may have disconnected the drive shaft at the transfer case and, as a minimum, had the rear cover off.
Last time I worked on a transfer case was on a '73 Jeep CJ years ago.
Looking at the white striping on the drive shaft, it does not look perfectly aligned, but can any conclusions be drawn from that? Does the Benz have equally spaced splines where one could reattach and be, say 10 degrees off? Is it somewhat keyed so it can only go on in maybe 90 degree increments? Looks more to me these were marked, "in case" they needed to be seperated with not much care. But wouldn't the appropriate thing to do be to separate at the mounting plate with the torx bolts?
Assuming someone was inside the transfer case, any thoughts as to what could be going on that would cause my vibration? There is no sound or noise coming from it. Is there something inside that could have been replaced incorrectly that is causing the imbalance? I'm kind of clueless at this point. Will need to do research to see if the transfer case is chain driven or gear driven, etc. Anyone know what model Tcase this is to do some general research?
Thanks all.

 

Update. I went for a ride in the 3rd row with my wife to her doctor's appt. The 3rd row is squarely on top of the rear suspension. Perfectly smooth. I then rotated the front tires to the back as suggested earlier just to be sure, and again, perfectly smooth in the way back row. My wife confirmed the shake/vibration was still present when she was driving. It just so happens that if you look under the GLS, the transfer case and transmission mount align with the floorboards close to the front seat attachment points which would explain why this vibration is being felt in the front seats and could be misdiagnosed at rear tire/suspension issues.
At this point I am convinced it is related to the transfer case. Probably early signs of a stretching chain. Everything I see on youtube are examples of where the chain has stretched so much that it is skipping teeth on the sprocket. Mine is running quiet and smoothly below 60 mph. But probably flopping around a bit at high speeds.
As a first step I am going to drain and refill the case. Probably is the original 75K fluid. While I've got the plugs out, I will poke at the chain to see how much deflection there is. My guess is I will need to be replacing that chain. I'll update with what I find.