Driveshaft Repair
 

I have 280K miles on my wife's 1981 300SD with the original flex disks on it. I replaced the flex disks with Lemforder disks as well as the center bearing and hanger. I also replaced the two centering sleeves. I carefully match-marked the two shaft halves so that they went back together with the same orientation.

Everything seems fine except I now have a slight (barely perceptable) vibration in the 65-75 MPH range. It almost feels like I have too much air pressure in the tires. Could this be due to replacing the old flex disks with some new, stiffer flex disks?

Possibly - but did you adjust the transmission mount? (I think this needs to be done on a W126 as it does on a W123)

Stretch
Could you please explain "but did you adjust the transmission mount?".
Thanks

Paul

On every model MB the transmission mount bracket is done loose and tightened under the transmissions own weight to ensure correct alignment of the drivetrain, same goes for the gland nut on the driveshaft too. Its tightened after you roll the car back and forth 6 or 7 times in order for the driveshaft to take it natural sitting length and position.

That nut is done up pretty tight and is supposed to have CV joint grease applied to the splines under it - same grease is filled into the end sleeves.

I'm wondering if the flex disks were not dynamically balanced properly. Or if the bolts/nuts were not exactly the same weight. The engine/flywheel are balanced as a unit, the transmission is balanced internally, and the driveshaft sections are balanced as a unit. That leaves the flex disks. If they're not balanced properly that would cause your vibration.

I just checked the 126 MB factory manual and it makes no mention of rolling the car back and forth before tightening the clamping nut. That said, I checked my 123 MB manual and it recommends rolling back and forth. It's worth a try.

One more thing. The flex disks have no markings as to which way they go. The manual only specifies a special orientation for the flex disks used with 5-speed manual transmissions. Based on the lack of information, I would assume that the flex disks for the automatic transmission are symmetrical and it doesn't matter which way they are installed.

Any more ideas?

Thanks.

They are symmetrical, however if they left the factory not balanced properly, or if the bolts/nuts were not weight matched, you would have an out of balance situation. They're supposed to be balanced, but QC is not always perfect. I have no idea how one would check, but with the kind of quality issues that seem prevalent with aftermarket parts today, I wouldn't be surprised if these kinds of situations become typical.

this type of issue is HIGHLY unlikely with Lemforder parts...

however, they often come with both long and short length bolts... it's possible they got mixed up in the install...

it's also possible the discs were installed incorrectly, there are protrusions on every other hole, and they must line up with the flanges. and they must SEAT properly prior to tightening the bolts.

the flex disc have markings of either "vorne" as in front or the soft downshift type discs in the later W124 have the marking of "diese seite zur gelenkwe" - meaning face this side to drive shaft.

When I first changed mine I put it on backwards and it caused only a slight harsh downshift - but the thrust action on the soft cushions of the disc could cause it to tear up shortly so I reversed it. If you ever reuse the old bolts/nuts - make sure to use blue threadlock on them.

The discs have raised and flush bosses which should be aligned correctly - the raised bosses always go into the tripods while the flush are only to recieve a washer and nut or bolt head.

The balancing technique mentioned in the W124 manual is very lengthy and calls for first mounting the shaft at different positons at the gearbox and then at differential with a test drive between each.

this means that a total of 6 undercar sessions and 6 drives, if that fails then weighted nuts are added to the rear and tested again.

If all else fails a heavy rubberized block damper is required on the differential.

All this is preceeded by first doing a driveshaft alignment - which may call for shimming the bearing carrier so the shaft runs straight as possible.

The upper is preceeded by the chapter of aligning the driveshaft to the correct splines (cone and line marking).

Its basically written backwards..

I did seat the flanges carefully while tightening with the protrusions lining up in the flanges. All the bolts were the same length so I assume they are all identical. I did let the transmission seat itself before tightening the transmission mount (I learned that the hard way the last time I replaced the transmission).

I have done the Dshaft on another W124 (BIL) and the discs on his car had the MB star along with MB star marked centre carrier.

a replacement of new bushings, new bearing, new carrier and new discs all ordered via good ol' Phil (RIP) and everything was smooth again.

I did notice that the alignment marks were about 5 teeth off on the D-shaft sections - I realigned it properly and used CV grease with a dash of Honda 77 moly paste on the splines and also the end bushings - the old grease was completely dry. Fixed it right up.

You can try to shim the carrier to make the shaft run a bit more in a straight line.

I went back today and removed and reinstalled the driveshaft. I had installed the washers on the flex disks all on one side rather than alternating as shown in the MB factory manual (and on my 1982 300SD). I used a torque wrench for all fasteners. I also shifted the splined joint by one tooth. I had match-marked the two halves, but it could have been interpreted in two different positions. I also rolled the car back and forth before I tightened the clamp nut. BTW, all of the bolts were the same length.

She's good to 85 MPH now.

Thanks for your suggestions.

Oh dear that must be one of my most ambiguous posts to date!

The point I was trying to make (well it was in my head but not communicated to anyone else) seems to have been made in this thread already. You need to jiggle everything about with the weight on the wheels to make sure the drive line is in the best position possible before you tighten it all up.

Stretch, I knew exactly what you meant. You need the all components of the driveline and related componets to be as aligned as possible without any particular component under stress (transmission and differential).

I did so much yesterday that I can't even say definitively what cured the vibration. It appears to be a case of carefully re-assembling the driveshaft and optimizing the driveshaft installation.

My wife is happy and that 's what counts.