6.9 flex disc

[s class]

A well known issue......

I thought I must add, in relation to the thread on 6.9 valve timing and performance, that I have not yet driven either 6.9 more than 20km per occasion - I seem to spend so much time working on the cars I'm usually too tired to drive them. Well there is more to it than that - in the case of the red AMG, it was an excellent well cared for car, but had a couple of basic issues that precluded driving it, the first of which was the rock hard spheres - the first thing I sorted out last year.

There remains the issue of a horrendous drive line vibration that occurs under hard acceleration - under modest throttle its smooth and happy. Without actually investigating, I'm guessing flex disc, and this has actually made me too nervous to really drive the car.

Its on the menu of things to do, just time, time, time.

 

[TJ 450]

It may be a combination of bad engine mounts, transmission mount, flex discs and centre support, you would notice a vibration all the time though. Seeing as it's only under load, I would be looking closely at the centre support and the flex discs are easy to check... look for cracks, they would open up under load allowing the tailshaft to be a few degrees out, especially on an AMG 6.9!

Tim

 

[WGB]

There is a very good pictorial article on the M-100 site showing the correct procedure for flex disc replacement.

I would expect your problem is likely the Gearbox end flex-disc. I managed to trash one many years ago in a 126 model 300SE in about 60,000km from new so I imagine the 6.9 is much harder on them.

Bill

 

[Styria]

Hi everyone, in my experience, I have found that flex discs, especially if genuine 6.9's, last for years. The one on the rear of my Gleaming Beauty lasted for eleven years before I changed it - and even then, the vibration was only minor - and NOT necessarily under hard acceleration. It was more of a constant harshness when driving at suburban speeds on smooth roads. Regards, Styria

 

[s class]

Thanks styria,

I respect your greater experience. I will get under the car this weekend and have a look around. The nature of the vibration suggests the following (well to me at least) :

a) flex disc problem
b) centre bearing problem
c) gearbox rear mount
d) diff rear mount
e) sideshaft problem

Now, I once had a similar symptom in my 280SE under certain conditions, which eventually turned out to be a completely failed rear shock absorber. I just don't think the equivarent (failed strut) is likely on a 6.9 ?

 

[Styria]

Hi S-Class-the strut is an unlikely candidate in the scheme of things. Don't forget tyres, although I seem to recall that they're in good condition. When I initially fitted my 18" Schnitzers and Nankangs, I experienced vibration problems and one could have sworn that any of the items you have mentioned could have been the cause. Leaving the spacers on the rims and balancing and MARKING the position of spacer in relation to rim cured the vibration .

In your case, go with the items you have listed yourself - start at the front (mount and flex disc), then the rear and leave the centre bearing till last. They seldom give problems. Regards, Styria

 

[s class]

Well I investigated today. Both flex disks and centre bearing appear in perfect order. I now suspect the rear diff mount, but I didn't have the energy to lift the back of the car to get in there (the front was on stands already so inspection of the flex discs was possible)

 

[Styria]

Hi S-Class, I don't know where you find the time to attend to all your projects and various problems. I know you're going to say "you do", but are you still enjoying ownership of the various "dinosaurs" ? Hopefully sooner, rather than later, there has to be light at the end of the tunnel !

Anyway, back to your vibration in the driveline. I guess you will just have to keep checking and/or replacing various components. As far as the flex discs are concerned, I am not so sure that by looking at them, you would be able to determine if they are the cause of vibration. As far as the diff mount is concerned, I've not yet had to replace that during almost fourteen years of ownership - again, they are long lasting units. Tyres, engine mounts ? With the last named, you are likely to experience a 'rumble' under hard acceleration as quite likely the engine down pipes are likely to just touch part of the body. Other than that, not sure what the cause could be. Regards Styria

 

[s class]

I agree that a visual inspection of flex discs may not be adequate to ensure they are vibration free, but in this case the inspection at least served to confirm that they not about to explode on me. Certainly engine mounts are also a likely suspect.

I only suspected the rear diff mount because the symptoms are similar to what my 280SE showed, back about 7 years ago when its diff mount separated internally.

Another thought - the rear end of the AMG is sitting too high - I haven't fiddled with that, that was how I got the car. Perhaps the side shafts are not enjoying the angle they have to work through?

As to dinosaurs, ah well, for me the hobby is really working on the cars, in preferance to driving them.

 

[WGB]

I'm with you S-class - my hobby is the getting the hands dirty part along with the kick out of the problem solving.

The internet gives us a wonderful oppurtunity to share knowledge and experiences after nearly forty years of working in isolation with whatever knowledge you could get from books and talking with people you met.

I enjoy the driving so long as there is an improvement each time I drive - but the kick is in the improvement.

My wife just despairs and accepts.

Bill.

 

[s class]

yes, I think you are spot-on there - the kick is in the improvements.

 

[s class]

Progress :

I got under the rear end and found that the levelling valve linkage rod had been set as short as possible (maximising the ride height). I lengthened the rod enough to lower the car about 25 mm (visually, it is an improvement, but it still looks too high).

The shuddering under throttle has reduced considerably. Hmmm.

 

[Styria]

Hi S-Class and WGB - tinkering, fixing things, constant improvement - all of those apply to our cars, and is commonly known as the involvement of heart and soul. Some months ago, I 'freshened up' the interior of my Astral Silver 6.9 - all the vinyl was recoloured, leather dressing on seats, restored timber in a slightly different colour, attend to small rust issues, and now I am happy and satisfied.

The car looks the part inside, it runs well AND reliably - yeah, I'm satisfied.

Regarding your car's vibration issues, S-Class, I guess you will have to keep on looking. I have my doubts if excessive ride height would cause undue problems. I have driven my "Gleaming Beauty" on the high setting and have not been aware of any vibrations. How old are the tyres ? They're often overlooked as the primary source of vibrations. Other than what I have already written previously, I am fresh out of ideas. Regards Styria