I have a 1984 300sd with 325k in which I am trying to troubleshoot a newish 55-65mph vibration. I'm trying to figure out what the next most logical thing to replace/look at is and I have a few curious symptoms.

So far I have replaced:
Left and right tie rods (passenger side boot was trashed)
Both Guide rod mounts (Drivers side was pretty trashed and I had the clunk, surprisingly although the boot on the passengers side was torn the rubber was still in good shape all around, the LCA bushings looked fresh, so I believe the whole arm was replaced by the PO, LCA bushings on drivers side seemed in really good shape also).
Idler control bushings/bolt (used the idler arm repair kit).
Brake pads, front and back

Checked but did not replace:
Upper control arm boots are intact, and the rubber looks decent (no cracks).
Ball joint boots are intact, and the rubber looks good (no cracks).
There doesn't seem to be any real wheel/hub movement with the car jacked up and me prying/yanking on it. I have not repacked the wheel bearings and this is on my soon to do list.

Other Symptoms:
The vibration only seems present when I run 15 inch wheels on the front (tested before I replaced the guide rod mounts), strangely 16 inch wheels (on the front) have no vibration at any speed. I rotated in different combinations, several different 15 inch wheels (different style rims and tire brands) and although they altered the magnitude of the vibration the speed when it started was the same.

I also have a single clunk coming from the rear at low speed with bumps and sometimes stops, but that may be the exhaust or crap in the trunk. I think when I last looked at them the rear sway bar links were ok, but I will recheck them when I swap the tires (see below).

Other factors I will check soon that may be responsible:
I'm running 15 inch rims on the front (michelin tires), and 16 inch rims on the back (normal profile, but nice and wide m/s) the rear 16 inch rims have 20mm billet hub centric wheel spacers made for this application. I really like this look, as you can't easily tell the size difference by eye, and it puts the nose down a little. I'm going to switch these out as my first step, by replacing them with 15 inch all around.

About the vibration:
Comes up at 55mph, is noticeable but not real bad. Before switching out the rubber parts I had a steering wheel shake as part of the vibration, that has disappeared. I just finished the guide rod mounts up a couple of hours ago and test ran it. The steering/handling while not bad before, is really nice now But I still get the highway speed vibration, and this is on a really nice recently paved lane. I'm not sure now though if the vibration is in the front or back, I slowed the car down almost immediately after I felt it vibrating so I can't give a good sound/feel description. I will try to test run it tomorrow with different rear wheels and give a better description if the shakes come back.

Low probability stuff:
I checked the flex discs when I changed the b2-piston last year, I seem to remember the rubber looked pretty new. I don't remember if I checked the support bearing but since I didn't change it, it probably was fine (guessing?). I'll be checking these again just in case, but this seems unlikely.

I have ALOT of tires/rims so I will try switching them around first to see if that cures the problem. I was kinda bummed that after all the work to replace the guide rod mounts and the tie rods, that I still had this shimmy.

what is the condition of you wheel bearings, your rear axle shafts, and your brake rotors?
do you think it could possibly be that your drive shaft is out of balance?

it is something spinning along with the drive train and is only affected by the overall speed of the car. those are my suggestions for things to look for, let me know what comes up.

__________________
'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.

Before you waste any more time/money on replacing parts (unless of course they need them) try to isolate the vibration to what part of the car it feels like. I will list a few short ideas. Also, I don't know how likely you will be to find one of these but back when I worked for a dealer we had a vibration detector that would pick up the tone of the vibration and display it like an occilator. The wave length of the vibration almost always was able to pin point exactly where the issues was.
Try,
1) when you get to middle range of vibration territory toss the car in neutral. That will eliminate or isolate any engine/trans issues.
2) Do you feel the vib in the seat of your pants or in the front end. Front end means steering components/wheels(no such thing as rims, but wheels do have rims on them!!), seat of the pants means driveline or rear wheel issues.
3) If you have someone who can ride with you, have them weat a mechanics stethesacope and "seek" out areas that are louder. Of course sound travel easily but will always be coming from the loudest area.
4) Last but not least.....run the 16" wheels and forget about it....

Good Luck

That's the single piece of data that's relevant.

The vehicle can't know the difference between 15 inch and 16 inch with the exception of rotational speed..........the vibration will be at a slightly faster speed with the 16" wheels.

Since there is no vibration on the 16" wheels, the problem is with the tire/wheel combination of the 15" wheels. Either the wheels.........or the tires.........have too much runout. If the runout exceeds about .060", the driver will feel vibration...........even with a perfectly balanced wheel and tire assembly. Normally, such a condition can be noticed by a relatively large balancing weight necessary opposite the high point of the runout. When the wheel/tire combo is on the balancing machine, see where the weights are placed. If the two weights are within 45 degrees of each other, take a chalk marker and see if it will just touch the outside of the rotating tire. If it contacts the tire at the point opposite the weights, you've confirmed your problem.

I see no other possibilities.

brian, you are right of course on the runout, but with different 15" wheels, and tires, isn't it unlikely?
also, technically, the 16" wheels would rotate slower, not faster... and only if the tires were larger...
another possibility is the wheel bearings improperly adjusted, or perhaps there is a contact issue with the 15" wheels somewhere...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Since the 16" wheels/tires don't exhibit the problem, it must lie with the 15" wheels/tires.

The 16" wheels rotate slower..........therefore for a given tire speed, the vehicle speed is faster. It's the tire speed that governs the excitation frequency.

ahhh

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

go underneath the rear end of your car and grab one of your real axle shafts and try and make it wiggle (side to side) because I had a rear axle that had good boots on it from visual inspection but once I grabbed and wiggled it I realized that it was going bad and fast. once I swapped out the axle no more vibration or weird clunks at low speeds.

you have told us the condition of your rear sway bar bushings and whatnot but what about the axle?

__________________
'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.

Explain how your theory allows the 16" wheels to run without vibration?

the axles only shake at certain speeds and since they are rotating slower then the axle is rotating slower which would put the shaking at a higher speed when running the slower spinning 16" wheels. I would like to see shaking speeds vs. rim configurations and that might help narrow down where to look.

but the main thing I was getting at is just check the axle shafts and see how much sideways play they have. the difference between my old axle and my new axle is amazing, the old one would move about an inch or so (and made a nasty vibration that started at 45mph and stopped above 55mph) and the new axle only moved a few millimeters, maybe 5 or less.

also checking the runout on your tires is important, my friend changed the rear tire on his R1 for a tire he got for cheap (like 50% off) and it had a .93" runout, slow speeds everything was cool, 55-65mph it shook like crazy and then above 65mph it settled back out. the reason for it only shaking at certain speeds is because it is at the resonant frequency of the tire (the frequency where any tiny shakes are exaggerated so to speak)

__________________
'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.

Agreed..........but, the OP reported no vibration with the 16" wheels.

Care to reconcile that with the axles?

he only reported no vibration with the 16" wheels on the front and beyond that with different wheel size configurations just changed the magnitude of the vibration, not the starting speed of the vibration.

so maybe it is all in his wheel/tire combos, I wonder when they were last balanced and even if they are balanced what their runout is. but on the same token he has found a config to almost completely eliminate the shaking, so maybe that can be used to help locate on which end of the car the shaking is located.

I was just trying to get some info as to the condition of his axles because I know that can cause shaking at strange speeds AND clunking at low speeds (especially reverse)

__________________
'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.

What size tires on the 16", it could be that higher speeds are required to get the same shimmy.

We need to find the location of the vibration first I think.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

I unfortunately did not get a chance to test drive again today or swap tires due to getting home pretty late. I was able to crawl under the passengers side rear with a flashlight (car is parked on the street and I didn't want to be run over) and grab the rear half shaft on that side and give it a good couple of hard pulls which resulted in no real discernible movement. I also took a look at the backside of the rear flex disc which looks pristine. I was wrong about the sway bar links though, the boot on the link looked torn/damaged (on the passenger side) so those will get replaced when I change out the rear wheels (probably Sat.) I'm actually slightly relieved I found something damaged that is at least a reasonable suspect, although I'm not sure that fits all of the evidence. Maybe I will get lucky.

The front 15 inch tires I have on now (p205/65 R15) have only a single small weight as a balance on either tire, are relatively new, and have no weird uneven wear. I may get them rebalanced since it's cheap and will help to rule out one possible problem. The 16 inch tires are size (p225/55 R16).

Shouldn't the speed difference between a 15 inch and a 16 inch wheel be 6-7mph or so max? The current speedo is actually accurate within (1-2mph at 65) by GPS so it can't be that big a difference.

With 16's on the front and rear I did not have a shimmy at any speed driven, even on some relatively crappy roads at >70mph. I do wonder if the larger size/weight and contact patch might act as a mass damper for a minor vibration though? I wonder if this would hide a marginal wheel bearing? I'd rather not put the 16's back on the front if it is just covering up some other issue due to a worn/broken part.

It seems like your shake or shimmy is coming from the front end of your car. I would start looking really close at all rotating components in the front end.

It seems like you have already touched all of the suspension in the front end, which narrows it down to rotating running gear. Do you know when the last time the front wheel bearings were replaced? front brake rotors?

Beyond that I am running out of tire speed rotating devices to suggest checking to try and find this wiggle.

It is interesting that the 16" wheels fix the problem I think that you might be right with it masks the wiggle with its shear size and weight. with that being the case it looks like it is either your 15" wheels or something chassis related.

Keep us posted on any new findings.

__________________
'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.