Just completed a valve job and the car has a lot more power now. This has highlighted another issue - a hard "thunK" when shifting, especially into high gear. The result on shift is distinctly different depending on the situation:

1) Under moderate or heavy acceleration the shift is crisp but no big thunk is apparent.
2) Under very light acceleration, or if you are easing out of the throttle as the trans shifts (like in traffic) you get a BIG thunk from the rear area.
3) similar noise when trans is shifting down but thunk is not as apparent.
4) The noise does not seem to eminate from the front of the driveshaft (tail of the trans), it seems to be in the back near the differential.

A few have advised me this is an indication of the need to replace the Flex disk(s). Do you usually need to replace front, rear, or both ?

Another thing that has been suggested is differential mounts. If these are worn, would there be any other characteristics to look for? Car seems fine other than this hard "thunk" on shifting.

__________________
Andy_M
'98 W140 S320 Black/Black
'06 W164 ML500 AMG White/Grey
'03 Porsche 986 Midnight Blue/Grey

Sure sounds like a tell tale indication of the rear flex disk being bad. I would get under the car and take a look at the rear disk, It is likely showing cracks or at least some movement or slack. Sometimes they look ok from the outside but the rubber is weaka nd too flexible allowing the movement that causes the "clunk" noise.
I would just start out doing the rear disk, it is easy and quick. The front disk is a lot more involved and almost requires a hoist, it can be done without, but not fun at all. The rear is usually the culprit and for $60 and a few hours you can find out. If you really want to be sure and you are going to pay an indy shop to do the work anyhow, have both flex disks and the center drive shaft bearing and support replaced, it will eliminate all noise and you will onlyhave to have it at the shop once. While they are under there, replace all the rubber mounts holding the exhaust up, they get neglected and rot. For all of them you are looking at maybe $20 and they can cause vibration noise and rattles when they go bad. Make sure to have paint marks made on the drive shaft sections prior to seperating the sections so that it can be put back the same way and this will eliminate future vibrations, the drive shaft was balanced at the factory and is intended to be mated up in a certain way as to keep the balance, if it is put back too far off from the original position, you will get vibrations and not be happy.
Good luck

__________________
Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red

Get the flex disks checked ASAP, and f they are bad replace them immediately. You do not want these to fail while driving.
Ed

__________________
Ed
1960 220 SE Coupe (Ponton)
1999 E320 Wagon
2007 E350

a good mechanic can tell which one or both are bad, a failed flex disk will let the drive shaft whip around under the car = bad.

__________________
Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red

Christopher, have you replaced a front flex disk yourself?

If so, please post more details on the procedure.

I couldn't get the bolts loose and decided to try again another day. No room!

Thanks!

__________________
2012 E350
2006 Callaway SC560

I have replaced a front flex disk and I never want to do it again. I did it with no lift or hoist and it was a poor idea. Very difficult to get leverage and space to work. Must pull the rear section of the drive shaft and the center bearing and support to really get at it very well. That is why it is the likely choice to do both flex disks and the center bearig and support at the same time. The rear disk is the one most likey to go bad and is easiest to change, not too bad. the front is a different story, not fun, and not easy. If you can stomach it, have a shop do it for you, I dont like paying them either, but it is not a fun job at all and like usual, I was not even satisfied with myself when I was done, it was just a pain. Sorry no better guidance

__________________
Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red

Thank you everyone. That's what I needed to know. Interesting that there are two different opinions as to which disk is the more likely to go bad. I do believe that, in my case, it is most likely the rear one. I will probably get under there and get the rear changed right away, and take a look at the front and the bearing etc.

__________________
Andy_M
'98 W140 S320 Black/Black
'06 W164 ML500 AMG White/Grey
'03 Porsche 986 Midnight Blue/Grey

The front one is smaller and is subjected to a lot more heat. It will fail sooner.

__________________
2012 E350
2006 Callaway SC560

It's also subject to a lot more torque.