W123 Propshaft Alignment Techniques?

[TJ513]

Greetings Gents,
Like many of you, I've replaced all of the rubber components in my driveline just to be greeted by a 30mph vibration from my worn u-joint, now articulating in a new orientation. Vibration ceases past a certain speed, indicating it is not a balance related issue.
So the U-joint "indexes" dead center and has an even notch in the four cardinal directions. This leads me to believe that the driveshaft spent most of its life in rotation perfectly straight.
I shimmed the carrier bearing mount tonight to straighten the driveshaft and to my surprise, it eliminated about 80% of the vibration. The "shims" were the thick washers that are typically on the head side of the screw that fastens mount to body, I simply placed the washer between the body and mount, lowering the driveshaft center approx. 3mm.


So my question is: what is the right way to do this? I saw a pretty nifty post from some gentlemen who used a series of twine strands, weights and then measured the length of the string in between shaft and reference string, but I don't fully understand the intricacies of what he did. I understand that Mercedes has prescribed these methods for this exact problem and I'd sacrifice my first born for that how-to.


Info on Car:
'84 300D Federal
Every bush and mount has been replaced with the proper part.
CSB was aftermarket but looked to be good quality
OEM German flex discs
OEM Trans mount
Everything torqued to spec.
Michelin's
Car was vibration free before driveline teardown, despite the shredded CSB mount.

[Stretch]

Originally Posted by TJ513

Greetings Gents,
Like many of you, I've replaced all of the rubber components in my driveline just to be greeted by a 30mph vibration from my worn u-joint, now articulating in a new orientation. Vibration ceases past a certain speed, indicating it is not a balance related issue.
So the U-joint "indexes" dead center and has an even notch in the four cardinal directions. This leads me to believe that the driveshaft spent most of its life in rotation perfectly straight.
I shimmed the carrier bearing mount tonight to straighten the driveshaft and to my surprise, it eliminated about 80% of the vibration. The "shims" were the thick washers that are typically on the head side of the screw that fastens mount to body, I simply placed the washer between the body and mount, lowering the driveshaft center approx. 3mm.


So my question is: what is the right way to do this? I saw a pretty nifty post from some gentlemen who used a series of twine strands, weights and then measured the length of the string in between shaft and reference string, but I don't fully understand the intricacies of what he did. I understand that Mercedes has prescribed these methods for this exact problem and I'd sacrifice my first born for that how-to.


Info on Car:
'84 300D Federal
Every bush and mount has been replaced with the proper part.
CSB was aftermarket but looked to be good quality
OEM German flex discs
OEM Trans mount
Everything torqued to spec.
Michelin's
Car was vibration free before driveline teardown, despite the shredded CSB mount.

There is a common misunderstanding about these Mercedes drive lines. I think it is because when people see a universal joint (UJ) they immediately they assume that the the UJ is being used in a conventional - car horse sprung - rigid rear axle way.


Most Mercedes passenger vehicles do not use rigid rear axle set ups. They are refined well engineered systems that use different ways of driving wheels.


The W114 / 115 / 116 / 123 / 126 system you have on your vehicle uses constant velocity joints on axles that allow drive to the rear wheels and to allow vertical suspension movement.


The differential in this system does not bounce up and down with the suspension movement of the rear wheels - the UJ does not bend like it would in a conventional rigid rear axle system.


#########


The way this system is meant to work is that there should be a straight line running along the horizontal axis of the drive line with out any deviation in the vertical and lateral directions. So that means there should be a straight line running from the front of the crankshaft pulley - through the crank to the flywheel - to the output shaft of the transmission / gearbox - to the centre propshaft bearing - to the differential input shaft.


The single UJ in the drive line needs to be kept "unbent" (technical term) to stop any self induced vibrations (see https://en.wikipedia.org/wiki/Universal_joint for example)


#######


If in your case you have had to shim to stop / reduce vibration it is most likely that (some of) the resilient mountings need to be replaced.


Check the motor mounts, the gearbox mount, the centre bearing mount, the differential mount and the subframe mounts. All of these parts need to be in good serviceable - non sagging - condition for this system to operate as it is meant to operate.


Whilst you are there check the condition of the flex discs. If there are cracks in the metal fixing holes they are probably about to fail.


#######


Tip for settling the drive line after a rebuild / refitting the propshaft.


1) Assemble the parts in accordance with the FSM but leave the centre propshaft mount loose and the big nut on the propshaft loose


2) Bounce the vehicle on four wheels at each corner


3) Roll the car forwards for about four foot or so and roll it back


4) With out lifting the vehicle tighten the centre bearing mount and then try to tighten the big nut on the propshaft - all with out lifting the vehicle.

[TJ513]

Stretch, thank you very much for your thorough response. I do understand the importance of maintaining the correct geometry throughout the driveline, and that is exactly what I’m trying to figure out. I’ve replaced the engine mounts, transmission mount, both flex discs, the CSB and carrier. After the installation of everything, each mating surface (trans crossmember->mount, CSB carrier->body) received a light coat of grease to prevent binding during alignment. The car was rocked, rolled, and rolled some more, and then put onto ramps. On ramps, I checked to make sure that the hub center to fender distance was the same as it was on the ground. Then I torqued the trans mount, CSB carrier and Driveshaft nut to spec.

So that’s why I’m searching for a method to measure the “straightness” of the driveshaft from the trans to the diff. Hans? Klaus?

[TJ513]

Also, rear bushings and diff mount look to be in very good condition.

Took the car up to 90mph today, and it was as smooth as glass. Vibration exists only after the 1-2 shift, and high load through second gear.

[Stretch]

To the best of my knowledge there isn't an official way of checking the actual straightness of the propshaft. There is, however, a section in the FSM for straightening the chassis (for example after accident damage) - there could be a bit in there about relative heights (sorry not sure - don't have time to check for you!)

High load through second gear vibration might happen in other gears at high load too (just as a responsible driver I guess you wouldn't normally replicate the effect in all gears...) - are you sure it isn't engine related?

The front balancers can have wobble problems (for example)

Just guessing - just wondering

[TJ513]

Strangely enough, 2nd gear is really the only gear with sufficient torque to cause the vibration, but the vibration can be felt on decel with the car in neutral from 30-20.

Update though, and a little progress;
I dropped the car off at my indy today so he could shed some light on the situation. He claims that I'm missing the shim, or bracket, under the trans mount between the cross member. I read about this elusive shim before and was very diligent to *not* lose the magic shim. But there was nothing. No bracket, just a trans mount when I dropped the cross member. My indy shimmed the mount and said the vibration is greatly reduced.

So now what? "Greatly reduced" wasn't what I'm after, its a Mercedes and it should feel like one.

Also, what is the correct orientation of the CSB and carrier? I made sure to put everything back together as it came apart but I'd like some clarification to ensure that I'm not going down the wrong rabbit hole.

[Garagefull]

A u joint will destroy itself running straight. Less than 3 and 1/2 degrees will not force the needle bearings to turn under the caps.

[okyoureabeast]

I thought these shafts were aligned from factory?

If i recall correctly, there are scribe markings on the metal where the shafts connect at the lcenter carrier support.

They look like this

-
<
-

Basically so long as the arrow is on or in the middle of the marks means your drive shaft is aligned correctly.

Note: I know there's an earlier cut off where the marks are there but they didn't use them. Confirm that with the FSM!

[TJ513]

Alas, I have a vibration free Mercedes. Hopefully I can prevent somebody else from banging there head against a wall like I have. Here's what happened:
My Indy was mistaken, not all W123's have a shim under the transmission mount as there are two types of mount; a "heavy duty" version (the larger of the two) and a standard version (with the rubber sleeve in the middle of the mount). A retailer that rhymes with AutoMouseAZ sells both variants. I chose the Febi mount because they're typically a bit better than Meyle and Uro, who make the HD mount. *I did not realize there were two different mount styles, silly me*. So, W123's without the shim between mount and cross member use the HD style and the shimmed cars use the standard Febi mount.
So after I installed the HD mount, I was pleasantly surpised to take my car on a vibration free test drive. I'm glad this conundrum is over, and my Benz is a Benz once again.
Thank you to all who tried to remedy this with me, the PeachParts forum is one of the best out there, you guys are great!

MAKE SURE YOU PURCHASE/INSTALL THE RIGHT TRANSMISSION MOUNT!