Well, I'm about ready to give up.
Any suggestions would be welcomed at this point.
This is kinda long, but it's been a long project.
I've owned this '91 300 SE since new, and it's in flawlwss condition, with 150k miles.

About 5 years ago, I noticed it just didn't have that locked-down feel on the freeway, definitely wanders around a bit.
In the wind, it's downright scary.
First seven years of its life it DID NOT DO THIS.

The only thing non-stock is the wheels/tires.
I went with the 16" 8-hole from Tirerack (with absolutely proper offset).
I didn't notice anything when I installed them, but a couple years ago, I put the stock 15" back on, and the wander still happened.
The MB dealer had checked the car, could find nothing wrong, and accused the larger/wider wheel. That's when I tried the old ones for awhile. No help.

Below is what I have changed over the past few years - to achieve no improvement:

Front:
checked/adjusted wheel bearings.
3 sets of tires - Conti's, Yokos
numerous alignments.
tie rods, radius rod
idler arm busing kits - two
steering damper - two
shocks- Bilstein
lower ball joints (uppers feel like new)
A-arm busings upper and lower.
guide rod bearings
resealed stering box, and adjusted tension on the bench.
(this box has wear compensator - it feels tight and good).
Checked steering coupler, feels tight.

Rear:
subframe bushings and diff mount
spring pads - 10mm taller to correct slight spring sag.
HAVE NOT REPLACED TRAILING ARM BUSHINGS - they seem okay.

Observations:

Steering and suspension feel and drive like new at low speeds and over bumps (they should, they are new).

Car tracks absolutely straight when I release the wheel.

Very little play in steering - 1/4 inch movement in wheel can change the direction of the car at 50 mph. Feels great.

Alignment checks out good, even the non-adjustable rear is in spec. Tire wear is good.

No damage, ever. We did hit a low curb at 15mph once, but saw no damage to wheel or tire, and alignment stayed good.

So, in summary, the car feels and drives like absolutely new around town. Alignment is good, tires are new, All is great.
But at freeway speeds, it wanders around enough to tire me out, and when the wind blows, it becomes dangerous.
Oh, my $15k PT Cruiser does not do this, so I know I'm not just being picky.

I see posts here about MB just being sensitive to road wander and winds, and many folks seem to accept this.
Well, that's NOT true of my 126 when it was newer.
We lived in West Texas, and drove in high winds at 80mph for 5 years, and this thing was like a rock.
So, something is wrong! What am I missing in all my attempts to fix it?

Tks,
S-Class Not-So-Guru

Try adjusting the tire pressures to ge tfront/rear slip angles matched. First raise rear pressure by 2 psi, then another 2 psi to see if it improves. You sould be runing on the high side of recommendations in any case. BE SURE YOUR TIRE GUAGEis accurate. Higher tire pressures do wonders for handling

Then check rear wheel bearings for any play.

Have you added or removed any body parts like a spoiler or sun roof wind deflector. One thing for certain this wandering is not inherent in these cars.

Short sidewall tires generate less self aligning torque than the OE 65 series tires, so low profile tires can lose some on center steering feel.

Post your alignment numbers - actuals and OE range.

Duke

I do not see rear sway bar links in your list.

Made a world of difference when i was doing my SE suspension.

Cheapest part to replace in entire suspension- like $5, 10 minutes to install.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

Thanks, guys.

I do run 30-32 psi, but that's a good suggestion to vary them around a bit. I'll play with it.

Yes, the low profile tires probably do affect it, but the lowly PT Cruiser has 55-series also. And these wheels look soooo much better than the old-style 15's.

Oh, yeah I did replace the rear sway bar end links and bushings.
Had a senior moment and forgot.
not those front turkeys though - got to remove the whole car to get them.

And the rear wheel bearing, funny you should ask - the right rear has a noticable click when you rock it back and forth.
If it was a front, I'd take about 1/8 turn on the axle nut.
however, one day I actually set up the dial indicator and yanked in and out on it. I never thought you could accurately do that, but the indicator repeated readings perfectly, and unbelievably, the readings were right in the center of the spec - can' remember the spec right now, maybe .05mm.
But the left doesn't click at all - think maybe the right is the guilty party? These look like a beast to replace.

Sorry, can't find the last alignment print out. I do remember rear toe was actually +, but was in spec, barely.

Thanks for all the suggestions.

G