Mercedes 420 sec rough gear change

[420sec]

Hi guys, I have mercedes 420 sec 1989, I have this car for nearly 2 years, but as I have only started using now more often I found that when auto box is changing gear (doesn't matter up or down) a sort of dead knock or clunk noise is heard coming from the back of the car. The knocking sound is not consistent sometimes (rarely) the gear change is smooth - mostly the knock is mild but sometimes can be quite loud and can send quite big shock wave through the car which I can feel through the drivers seat. Loudest knock happens in 30-50mph zone, as mentioned before doesn't matter if it's donwshift or upshift. I have lifted car up, and recognize few things: there were 2 small cracks on rear flex disc, one from one side and one from another (attached photo),there was oil leak from differential from both sides (attached photo) and it was a bit play in driveshaft. What was done already: changed gearbox oil and filter, checked when car was in running temperature, it's between low and max; tried adjusting gearbox modulator, was no difference, tried checking vacuum lines, found one that was disconnected completely, but when I put it back together made no difference. I've done all those things as I thought it might be problem with gearbox, but now I'm starting to think it might be something with flex disc that needs changing. I am no mechanic and any advice with this matter would be greatly appreciated.

 

[Chazchuzzlewitt]

Well I think you've probably found your problem (or at least one of them) with the cracked flex-disc... I'd get that changed sooner rather than later as I'd not be happy with that crack. The diff mounts probably need changing too, one of my past W124s had a big clunk in the back with gear changes when the diff mounts died. Mounts & flex discs aren't expensive, just the labour...

 

[420sec]

thanks for reply! just few more questions.
Where should I order these parts? I found website, it has flex disc for sale https://www.onlinecarparts.co.uk/ca...ass-coupe-c126/834/10807/universal-joint.html but it's not mercedes
Is rear and front flex disc the same?
Should I order center bearing as well as I'll be underneath the car?
and is there 2 diff mounts?

 

[Chazchuzzlewitt]

I would try your local dealership first, I know the parts guys at mine quite well so usually get them to look up the correct part numbers for your car and get me a price- if it's reasonable I order from there and if silly expensive I get them online- usually ebay trusted sellers or ECP. Aftermarket brands like Lemforder, Febi, Swag, Sachs etc are fine, just stay away from the really cheap parts like Topran/APA (uro)...

I've not used the company in your link but I have used Autodoc from Germany (looks like it might be the same people) and they've been ok.

Of all the Mercs I've owned, even into high (200k+) miles, I've never had to replace a centre bearing.

On the 124 there's 3 diff mounts, but it's the 2 at the back that seem to collapse and make it start clonking. As far as i know it's a different design on the 126 with just one large rubber/metal mount at the rear then it's bolted on to the subframe at the front... so should be easier in theory!

 

[420sec]

Thank you!