Quote:
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.
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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.
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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)
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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.
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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.