Done that, yes the noise will decrease to almost none when shifting to netural at that moving speed.

 

Thanks for the comment.
So can I now eliminate the trans as a suspect? If I can, it is short listed from the diff and back to the outer CV Joints.

 

If the CV joint swap does not have the desired effect, I agree with the other writers.
The problem lies inside the differential housing. The problem you are describing is more known in the GTS. The final machining of the R&P in the S4 was done differently. The S4 used the Oerlikon Spiroflex machining process. Which basically means that the R&P was machined as a pair and finally the R&P surface hardened.
The GTS uses the Gleason machining process. The R&P is machined and paired after the hardening process. With this backlash could be reduced even more in the final drive of the differential. With this there is no noticable play in the R&P (GTS around 0.1 mm). Acceleration and decelaration does not produce any jolting in the drivetrain.

It would be very unusual for a S4 to have this problem (sturdier), but it is not impossible. I would continue and check the backlash and also the differential bearings. If the differerential bearings do not provide the required support, there will be a speed at which this noise is produced (vibration). Happens especially when coasting at the mentioned speed.

 

Update: My mechanic had my car ( all wheels) jacked up in the air and run to about 2500 rpm in "D". The speedometer shows nothing, and the PSD warning light are on all the time, are these normal? (shaking when over 2500rpm, the car seems hesitated to run above 2500 rpm, Was it due to the enagement of the PSD??)
There was noise from the differential but not as loud as on the road.(maybe no load) I used a metal bar to touch the differential housing and listened to it, it had noise but not sure if it was a normal working diff noise.

(CV joints are not swapped yet, my mechanic is busy and need to make an appointment next month but he is not buying that this is the cause.)

This is not my car but my noise is similiar to this

Any comments please. Thanks and appreciated.

 

PSD warning light on is normal, hte tires are not rotating at similar speeds. PSD tries to lock the differential and prevent "spinning" of the wheels.

Without any load on the wheels there is only minimal noise. It would be good if you would have a reference 928 to make sure that it is the R&P.

The speedo does not show anything because the signal is taken from the front left ABS sensor. I will try to load my sound file to my dropbox and give you the link later. Not accessible from work right now...
I recorded it with my mobile phone lying flat on the center console armrest...

 

Thanks Schocki. Really appreciated.

So the PSD was trying to lock the diff when the car is up in the air that's why I feel the shaking, the car can never reach the testing speed (40-50km/hr) hence this is not able to simulate the road speed situation. Do you think that I should disconnect the PSD relay first?

 

If you want to run it as an open differential, then yes. The green PSD light will come on as soon as there is a small difference in speed on the two rear wheels. Both wheels should have turned into the same direction, while PSD was activated.
Not like an open differential where they would turn in opposite directions (when both are in the air).

 

Hola Kevin,

here are the recordings.

This one was taken before I added a teflon additive in an attempt to quiet it down (did not help a lot): https://www.dropbox.com/s/kurz48yx5f...NoRVS.WAV?dl=0

This recording was taken one year later after I installed a S4 2.2 R&P from 928 International: https://www.dropbox.com/s/4cpg2ib7pg....2009.WAV?dl=0

Remember I recorded this with my mobile phone lying flat on the center console. Listen to the noise with a headset, it was a lot louder in the car. BTW this happend exactly at 50 km/h (30 MPH), cold or warm. No difference.
With the new R&P the noise is gone. If you experience the noise at higher speeds, I would guess that the differential bearings are not good anymore.

I´m saying this because my 944 S2 developed a humming noise at around 50 MPH (more like the video above).....the differential bearings were shot.

 

HI Schocki,
Thanks for your sound recordings. Yes I am having the exact same noise more or less at 50km/h when accelerating. Are those sounds recorded from your 928GTS? I still can hear that kind of noise in your second sound recording, is that normal diff working noise?

I don't experience that noise in higher speed but I think is good to just replace the differential bearings if the diff is opened for installing the new R&P.
May I ask what exactly did you purchase from 928 international for fixing this problem?

Thanks.

 

Hola Kevin,

the noise in the second recording is very faint and in reality with a little road noise and the radio on it is practically gone
Of you want to replace just the R&P you can get them from Mark Anderson.
Since I do all the work in my car, I only needed the parts, labor is free. But that is exactly the problem it is a labor intense and tricky job. Honestly I would not trust my average mechanic with this. I will try to find my link to my old post and upload the pictures again. If he agrees to do the job, he will most likely have to fabricate special tools to adjust backlash and the correct position of the ring.
My GTS developed this nose and after some research on the topic I decided to use an S4 R&P with an US 2.2 final gear ratio. The GTS comes from the factory with 2.53. With this my GTS is (IMHO) better geared than before. The longer final drive harmonizes prefectly with the bigger engine. There is no noticeable difference in daily driving. Fuel economy went slightly up and I lost about 5 km/h top speed. I'm very happy with the results!

But ! There is also a chance (according to the MB automatic WSM) that there is a problem in the planetary gear drive! So after all this would mean the problem lies in the tranny itself.

In your situation l would rather go a different route. Buy a good used complete tranny with a good final drive and get it completely rebuilt by Greg Brown. I have no idea if he has any new transmissions still in stock? With this your mechanic has just to swap the tranny, that's it and you can still drive your car while you wait for your rebuilt...

Regardless what you do, it will not be cheap....

 

Thank you very much for your advices.
This car really gets me. I am located in far east and the shipping cost of a good rebuilt trans/diff might be more expensive than the unit itself. I am sorry to say but 928 is just too problematic compare with other old cars I have ( I had other classic cars like Mercedes R129, Alfa 75, Golf GTi). I had just spend USD 3000 to replace the TT and TT bearing and now the diff need to be rebuilt ( My S4 only has 80000 km genuine mileage on the clock). I find myself constantly having major issues to fix on this car every month. It leaves me no choice but just have to scrap it, it has a perfect body and interior, what a shame! Unlike a 911, noone is going to buy a 928 with a diff problem here.
Cheers.

 

With 30000 km on the clock (did not know that) it is a very unusual problem. This is a very low mileage car!
Before you give up, I would check the backlash on the R&P (can done installed in the car) and also check the differential bearings.
This is not a typical S4 problem, this happens more often with the GTS. Remember I changed to a S4 unit because it it's sturdier. The different machining process in the GTS R&P is normally the problem , which allows for even tighter tolerances.
I will post some pictures how to adjust and check backlash later...
Have to go to my favorite farmers market now in Spain and get some fresh fruits and veggies

 

Here is my post on how to replace the R&P, I uploaded the pictures again on page two: https://rennlist.com/forums/928-foru...n-s4-unit.html

Attached are the pictures on how to check backlash. You need to fabricate a special tool (like an oversize hook). With this you will lock the pinion into place while moving the ring and cheking the backlash freeplay.

 

Very doubtful that at that mileage, you would have a differential, axle, or wheel bearing issue.

An "defectively made" part would have failed long before this....and unlikely that anything is worn out.

Did you try new tires, yet?

The PSD units can put up a lot of protesting noise, if they had the wrong gear oil. Change it. Use Mobil One 75w-90 LS only....it works great with the PSD friction plates. Note that if this is the issue, it can take 150 miles for the friction plates to get "happy".

Change the transmission fluid and the filter. There's always the chance that someone used the wrong fluid. Makes sure that the correct Dextron fluid gets used. Don't use anything exotic....simply not necessary.

Make sure the transmission fluid is topped off when hot...and neither overfilled or under filled. Virtually impossible to see. The car needs to be driven 10 miles and put back on a lift to check the level. Lazy mechanics hate this...make sure it happens.

Parting out a 928 with this kind of a problem would be like putting your dog to sleep because he has a sticker in one paw! You can do it, but no one is ever going to think you made a good decision!

I sincerely think that this is an easy problem to solve....but you need to do the easy, obvious things, before deciding it's "terminal".

 

Hi Greg, thanks for your encouragement. I am going to need it.
This morning, I had changed the diff fluid with the Valvoline High Performance 75W-90. I am surprised to see that the old fluid is very black and dirty in color. It was changed in a repair shop in last Oct or less than 2000km before (brand and grade unknown but the noise was there already before that change).
I did a test drive and the noise is still there but is a bit reduced. After reading your comment, I will drive it for couple hundreds of KM and see. I will keep my fingers cross.
Maybe I will try to source the Mobil One 75w-90 LS and change it again in few hundred KM later since I have got nothing to lose.

Tyres changed.
Trans fluid changed and level checked Ok.

Cheers.








This diff fluid was only 2000km old/ 4 months





This diff fluid was only 2000km old/ 4 months