420SEL shudder opon acceleration?
 

I noticed today and a few weeks ago, when i pull away from a stop, i can feel the car shudder, from the gas pedal. I put my hand on the driveshaft tunnel, it feels fine. After a few minutes, it goes away. What could be causing this? Flex disc, diff pumpkin support, or engine mounts?

Bad transmission mount will cause a drumming vibration as the tranny thumps around under torque. Noticable only under acceleration, can be quite loud.

I'd personally do the engine mounts at the same time, I did on the 220D and the 280 SE.

Peter

After doing some reading, a bad driveshaft center bearing makes a squealing sound. But what does a bad flex disc sound like?

And the shudder happens when i'm taking off the line, and it stops when i get to speed, usually around 20mph or 1000rpm.

I had a similar problem on an '85 500 SEL. The car would "chug" or "shudder" under acceleration - usually just when I started it cold but not always. Also, at idle at traffic lights the engine would sometimes skip, and the oil pressure gauge would fluctuate.

As it turns out, the problem was some bad spark plugs. My mechanic replaced a few and now the car runs fine.

Shuddering
 

Also, check your transmission fluid. I have noticed in my 90 420sel that if my fluid gets low, it will shudder upon pull out from a stop for just a second or two. I know you stated yours does this until about 20 mph, but I would definately check the fluid level. When I fill my back to full, my shuddering goes away. I have a small leak somewhere, but have not looked too hard for it yet.

Many things can cause this feeling. Diff mounts, rear trailing arm bushings, motor mounts, rear transmission mount, flex discs, driveshaft U-joint (requires new driveshaft and this is a very common problem on higher mileage W126s), driveshaft center support bearing, rear axle shafts.

Similar low speed driveline shudder in my 1978 280CE 60K miles turned out to be the centre bearing mount.......but I described it as a 'flapping' which stopped at ~ 20mph

Put the back wheels up on stands and push/pull the drive shaft from the centre bearing to the diff.......I got obvious excessive movement.........I ran it carefully in drive on the rear stands and saw the excessive shaft movement and knocking of the bearing in its mount

Problems with the flex discs should show up with this test (I was expecting the flex discs to be the problem)

Centre bearing & mount replacement is not difficult or expensive..........if your problem is the universal it is a different story as the whole shaft needs replacing

I can help with more details on centre bearing replacement if required

Stummala states it well in the parenthesis.

I second the notion!

same thing
 

I have been fighting a vibration on my 87 420 also, i replaced the trans and motor mounts..front and real disks..center brg and holder, both axles..new tires balanced on the road force balancer..still has the vibration at accel..seems mine comes in at 15 mph and is gone at 20.....i was replacing the above parts anyway but thought i would have gotton to that vibration by now..wonder how you test the diff bushings....guess i am advising you to take it to a tech ..lol..let him do the leg work so you dont just buy parts and install them with no real success..gl

Interesting comments. My first reaction is that we are talking about a load induced single cylinder misfire, but it could be a driveshaft vibration. These are obviously two vastly different senarios.

I tie my reaction to this statement: "And the shudder happens when i'm taking off the line, and it stops when i get to speed"

Taking off from the line would include time when there was no driveshaft rotation so I don't think in those terms. Need a further description.