W126 - Swimmy rear-end on Hwy driving

[ck42]

Looking for opinions on what may be causing this OR if this 'behavior' is considered normal for my 126.

During hwy driving, I find myself having to make frequent corrections (slight left or right) to drive straight AFTER making a previous correction. Did I lose you yet?

The things is, after I make a correction, it feels like the rear of the car trails the correction by about 1/4 second. This delayed rearend behavior then causes the correction I made to be too much....in which case I have to steer the opposite direction to 'correct' my correction.....as you might imagine, this behavior could then continue indefinitely....each correction followed by another correction.

So....I've developed a driving style that entails making VERY minor and controlled corrections in order to minimize the delayed rearend response.

The rear shocks are originals but appear to be in good shape (no leaking, no bouncing)
All rubber in the rear is also original.

Does this sound like a shock 'thing' or subframe mounts or something else?

[5banger]

ck42,
From my experience, I would suspect the rear subframe mounts first, but also consider the slop may originate in the front guide rod mounts.

I had all these replaced in my C126s and it improved the ride in all aspects.

edit:
For those who struggled, as did I, with the terminology:
The "bearing bracket bushings" serve as the front subframe mounts (2 per side) and if you also hear knocking noises, the "guide rod mounts" may need replacement. All three rubber pieces (per side) assemble into the bearing bracket (aka brake strut mount).

Cheers, Andrew Seidel.

[ck42]

Andrew,

Guide rod mounts replaced in the last year.

Chris
'85 300SD

[WANT '71 280SEL]

Do you ever hear any noises coming from the rear? Perhaps you need rear sway-bar bushings or sway-bar links? You should check the condition of those. Does the car seem to roll too much into turns? I understand that it is a big car and will roll, but does it seem excessive?
Thanks
David

[ck42]

Rear swaybar links were replaced about 2 yrs ago. Not sure of the condition of the bushings.

As for body roll, *I* think it's too much but unfortunately, I don't think it's technically considered too much for the car.

oh, I don't hear any noises coming from the backend either.


Chris
'85 300SD

[sixto]

Look at the plates below the subframe bushings. There should be an even gap all around into which you can fit the tip of your finger. If the plate is tilted relative to the bushing cavity or if there is little to no gap, the subframe bushings are worn. Check also the diff mount. There should be a gap between the interior 'meat' of the mount and the lower frame. In fact there should be a bump on the lower frame. If there is no gap or if the bump has worn flat, the diff mount is worn.

To test the rear, find an empty road, get up to speed and jerk the steering wheel quickly back and forth once. Just enough steering input to weave within a freeway lane. If it feels like the car oscillates before going straight again, there's probably something worn in the rear. Don't weave too much or you'll attract the wrong kind of attention.

Here's some info on how to do a visual check of the bushings -

Left side lower then right?

Sixto
95 S420
87 300SDL

[ck42]

sixto:

Great reference thread. thnx!

I went out and checked the gap and evenness in the dark with a flashlight and things look OK. *feels* even all the way around and there's about 1/4" gap which I can just fit the very tip of my finger into.

I'll check the diff mount tomorrow.

Chris
'85 300SD

[sixto]

Quarter inch doesn't seem like much compared to my SDL which has closer to a half inch. I don't know what the gap should be.

Sixto
95 S420
87 300SDL

[ck42]

Well, I guess then that that's the $64K question.

I'll see if I can find out and will post back.


Chris
'85 300SD

[Crustybuck]

My 1984 300 SD has exactly the same problem that you describe in that the rear end seems to steer the car slightly after any steering input. Makes for difficult and tiring steering on a long trip. I'm having the subframe mounts looked at as well as the rear shocks. I believe it's in the mounts. Car has 274k miles on it. Be interested in any solutions you find.

[stevebfl]

Just a thought, as if I'm right one can sure waste some time and bucks.

The way you describe the symptom is often caused by a tight joint in the front suspension or the steering box itself. Because the steering sticks it always has to be corrected. With a properly loose steering the car generally follows the road and doesn't need constant correction. If the car is hanging the wheels should easily turn by grabbing each side of the tire at 3 and 9 oclock and gently pushing and pulling (push one side pull the other). One should do this slowly and if one stops it shouldn't take much to start again. It shouldn't "stick".

Typical problems occur at the lower ball joints (especially those improperly installed) but tierod ends can do it, idler arms can do it and a tight steering box can do it (one that has been overtightened)

[psfred]

I think front end, too -- the only thing that causes trouble in the rear on these cars is loose subframe mounts or bad control arm bushings, and those give you rather noticable torque steer rather than wander.

Something tight or a sloppy tie rod come to mind, like Steve said. Ball joints in particular will do this if "stiff" -- they will eventually start to squeal, too. Most anything else causes a "floating" feel, where the car randomly follows the road surface texture or crown and steers itself. Very annoying, but not "overcorrection".

Peter

[ck42]

Well....both tie rods and ball joints were replaced less than 25K miles ago.

[stevebfl]

There ya go (bg).

[psfred]

Also check the steering damper for a "hard spot" at the center, they can get bent or otherwise damaged and get very stiff right at the center of travel. This can cause terrible steering vibration, too, when you hit a bump and the damper "bounces" instead of damping.

A bad idler arm usually causes wander, but I'd check it too, just to be sure. New busings are a great deal cheaper than a new steering box....

The "second correction" is because something is stiff and you are turning too far while forcing it over, and the steering box cannot pull it back. This can be a steering box valve problem (loose torsion bar comes to mind for US style boxes, I have no ideal how Benz does theirs!), but is more likely a stiff suspension part.

Peter