Pagnobito

Having ruled out most everything else, new mounts both ends, different TT and then TT bearings (super bearings), recon TC and new nose bearings (because they were noisy), flexplate checks, new bolts everything lines up etc I think it only leaves the transmission as the cause of vibration around 2800-3100 rpm in any gear and N or P. I have a front flexplate clamp that stops it migrating and crank endplay is well within spec.
I looked at a couple of posts here and one mentions input shaft or TC but hopefully I ruled out the latter having it looked at by a specialist company that just does TC rebuilds.
My limited knowledge of auto transmissions and going by symptoms means it's a common rotating assembly with enough mass to cause harmonics at a particular RPM and the only parts rotating all the time are the TC and clutch pack on the input shaft, correct?
I ran the engine un-coupled at the front flexplate and it's smooth. The vibration is felt through the seat as that's where I sit but I think it makes the entire car vibrate and without anything in the boot/trunk can be heard sonically.

In which case I'm looking at a rebuild or replacement transmission, experts agree?

Thanks

SeanR

I've got 2 cars that we've done everything but replace the transmissions, one auto, one 5-speed and nothing I've been able to do fixes the vibration. One owner isnt particularly concerned but it drives me nuts.

GregBBRD

I'm not sure if I qualify as an expert, however it is doubtful that your problem is inside the transmission. As you move through the different gears, the different masses inside the transmission change....radically. Since you have the same vibration in every gear and in both park and nuetral, that would be a very strange failure. Not impossible, but very rare.

Chasing vibrations and harmonic issues can be very frustrating. I just spent four entire weeks, chasing a harmonic/vibration/noise in a 1988. Not four weeks on and off...four continuous weeks...that is all I did! I took the transmission in and out eight (8) times, changing pieces, each time, before I finally figured it out. I'm currently writing a thread about this nighrmare, but I'm so far behind on other work, I have not had time to finish this very long story. Give me a few days and I'll see if I can finish this...it's a very "scary" story.

In the meantime, what year is your 928? How many miles? Tell me, in detail, what pieces were inside your torque tube originally and what pieces you re-installed. You had the converter rebuilt or just checked for balance? Do you have any pictures of the front and rear flexplates? If you shift into reverse how long does it take to engage? Does the transmission ever slip? Can you back up a steep hill without reverse slipping? What have you found sitting in the transmission oil pan? When was the filter/fluid last changed?

Details are important!

matt968

I had my entire driveline serviced when I bought the car....(new torque tube super bearings, new super clamp, all new seals and bearings, the transmission was gone through by a Mercedes transmission specialist and the Torque Converter was re-balanced by a specialty shop. After the mechanic put it all back together I had a bad vibration at highway speed (somewhere around 75mph I think). The mechanic pulled the torque converter and replaced it with one out of his personal 928 and that fixed the problem.

GregBBRD

The Mercedes torque converters in our transmissions are particularly robust, with very little to fail. They are also extremely well balanced, right from the factory. Cutting one apart and looking at one, rebalancing one, isn't generally needed and the results can be worse than not touching it, at all.
Your story is not uncommon.
I call that: "Fixing it until it is broke".

Pagnobito

I'd like to read your account, hopefully you can get it finished soon!
In answer to your other questions; It's a Japanese market 1990 with 106K KMS or about 67K miles bought 2012. It's always had the vibration and it also had a noise from underneath but wasn't sure where it was located. Front flexplate checked, end float well within spec, new bolt and slighty over torqued along with a PKlamp added, also checked rear bolt all good there.

In 2014 I had Volvo engine mounts and rebuilt rack fitted among other things such as new HT, fuel lines, intake refresh, plugs, knocks, gaskets, hoses, oil filler but not relevant here. Also had a gearbox and diff oil change, looked fine, no glitter.

2015 was when I got into looking for the vibration and noise (hadn't before as the bearing noise wasn't too bad and the vibration could and can be avoided of kept away from RPM range where it starts). This was when the TC was split, checked and re-balanced, tranny oil again with some new 'rebuilt' cooler lines. The noise was the nose bearings those two on the rear flexplate shaft. The shop that replaced them said it looks like they have been done before which is odd for a 64K mile (at the time) car. That raises questions. Could be flexplate pressure but that looked fine. No improvement so fitted a used GTS TT. Both that and the original had the bearings and damper in the right place and it made no difference so I rebuilt the original with Super bearings to spec without damper which made a slight improvement. I also replaced the PKlamp with a Ritech one. In that year I also replaced the shocks, all tyres and lower ball joints on front.

I lived with it until recently when I did a trip to Germany last September and at higher cruising speed the vibration was just annoying and it varied, sometimes worse that others.
I don't have pictures of the flexplates but my specialist has gone over the car this month and said the Volvo mount on one side had twisted and the safety plates were touching so possible vibration cause. I had a choice over new mounts or the far more expensive gearbox swap option, hence going for Porsche mounts which has improved the problem but only because I think they are doing a better job of masking the vibration. As before avoiding the RPM band and it drives well. he found the Hall was bad which has improved performance a little with a new one.

As for slip, he also drove it a fair bit while he had the car and I'm sure he would have noticed slip, he only does 928s all day every day. It does make a woosh/whine between selecting R and it engaging but I don't reverse uphill. It's the only time I hear that noise.
I'm hopefully going to try a couple other S4 928s near me for comparison to see if I'm trying to fix a problem that's inherent but I think it shouldn't be there and there's a reason those bearings in the nose of the transmission needed replacing after a relatively short amount of miles.

edit as I just remembered, when I took it to my specialist earlier this month as soon as he sat in the car at idle he could tell there was vibration there that wasn't normal. He ran a stall test which was fine but also improved the vibration slightly, he put that down to the engine mounts shifting during the test hence replacing them with Porsche ones.

Pagnobito

That's right about where my vibration starts, or about 2800rpm which can also be cured with new engine mounts apparently, excepting mine. I am tempted to get a used TC and fit it but that's a lot of work or cost if someone else does it and it might not fix the problem and I've no guarantee the used TC is a good one.

Speedtoys

I have this too, but a few things have significantly reduced it:

I found a bad plug wire, wasn't a total misfire, but it was broken at a crimped end, and had an extra gap to jump that was very corroded to get to the spark plug.

I had a GB Super Damper installed, my damper was beginning to fail..and resolved -a- vibration that was at all RPMs..subtle, but you knew when it was gone.

Also looking FW to your post Greg.

Pagnobito

I guess you found the lead problem by testing continuity or resistance? I fitted a new lead set and if I run the engine with the front flexplate disconnected it's really smooth right through to 5K.
Different iterations of torque tubes didn't change it much, just a little with the super bearings.

Constantine

Hi Pagnobito,

Great overview of your vibration troubles and it seems you have covered most bases. That the vibrations were made slightly better with the Super Bearings is what most others have found with secondary drive line vibrations sources other than a bad torque tube from a failed bearing unit or one that moved from its intended location.

Suggest the next time you are with your technician to decouple the front flex plate from the flywheel and move it back unto the drive shaft. Then start the 928 and raise the RPM level to 2800 and see if the vibrations have gone away or changed. This procedure takes away the rear spinning items from the equation to include the torque tube internals, torque converter and transmission internals. If the vibrations are still present, it would more than likely be engine sourced.

HTH,
Constantine

Pagnobito

Hi, thanks for your input. As mentioned previously in my posts with the engine de-coupled it's really smooth.

GregBBRD

Just as a data point, two bearing torque tube or three bearing?

Test drive the car. Very gently rest three of your fingers on the shift lever. See if you can feel the vibration on the shift lever.

Constantine

Apologies, didn't see this had been done already. So then it seems located to something in the transmission assembly.

As you, I find it suspicious the torque converter bearings had been previously changed at such a low mileage.

Pagnobito

Both the original and GTS had 2 bearings and damper as standard on the auto which was replaced with 3 super bearings, installed per instructions.

GregBBRD

Yeah, the entire torque tube 2 bearing/3 bearing/frequency absorber is a mystery, until you talk to the people that know what happened and why. (I'm not going to get into this discussion, here.)

An excellent "primer" on the whole torque tube subject is the extensive story about the development of the torque tube in "Project 928", (http://jenniskens.livedsl.nl/brochur...ct_928Book.pdf) (Read the section on "Transaxle".) which everyone should read....and think about. Try to keep in mind, as you read this story, that there was an entire team working on this part of the car....with multiple "high end" engineers that specialized in frequencies (vibrations), trying to solve the problems. [After all, the car was stuck in development, until (and if) the torque tube problem could be solved.]

You might also note that Porsche has never offered a rebuilt torque tube. As it turns out, because of what is required to build the torque tube, there is a really good reason, for this!