Schocki

There is one thing on my 928 that I will try to fix this coming winter. Everything works very well on my GTS (Automatic), the car runs great, lots of power, fast and good gas mileage for a V-8.

But there is one thing that can be improved: The whine from the rear axle area when driving in inner cities and very light load. The differential/tranny makes a very distinct "humming" or deep "whining" noise. Starts exactly at 34 and ends at about 42 MPH, above or below this speed --> complete silence. The noise DOES NOT depend on gear or RPM's, just speed and load. If I take my foot of the accellerator when I'm exactly in the speed region --> complete silence!

I drove Nicole around during the 30 years opf 928 gathering in Stuttgart and she was completely stunned how quiet my GTS is from the inside. Yes it is except for the mentioned speed region. Right now I'm just trying to stay away from the "noisy speeds".

The torque tube is about 10 k old (rebuilt unit from 928 International), the torque converter bearings are also 10 k old. Differential oil is a synthetic gear lube in the correct specification (Mobil 1). BTW the whine is also there if the car is cold or warm...

My plan of attack is to remove the rear portion of the exhaust and open the differential cover in the back. I found the posts from borland on how to check the backlash with a micrometer, that shouldn't be to difficult. The WSM states that the axial position of the ring to the pinion is responsible for the quietness of the differential, not the backlash. The backlash values and optimum ring and pinion position are inscribed on the ring...
Adjusting the backlash shouldn't be to difficult with some shims. But is it worth the time and effort if it's out of the inscribed value? Is the only way a complete check of ALL adjustable distances inside the differential?

Any opinions or experiences with this are appreciated...

Jim bailey - 928 International

That is EXACTLY what one of our GTS owners had with his automatic. It was resolved by replacing the ring and pinion... not what you want to hear

Schocki

Yeah, that would be the most expensive solution for this problem. No adjustment or anything helped? How did the wear pattern look on the ring and pinion?

I will increase the volume on the radio before I change the ring and pinion...

Jim bailey - 928 International

After the shop had the transaxle apart for the second time the owner opted to buy a ring and pinion since the labor charges were quickly adding up. There was nothing visually wrong with the R + P which is why they tried to change the bearings and reset the backlash etc. once that failed it was replaced. The car had 120,000 miles or so and was driven in a spirited manner. Everyone felt that the gears would run just fine for a long time but the noise really bothered the owner as he often drive into a small town with a 35 MPH speedlimit and it sounded like an old worn out chevy truck....

928SS

just downshift and bwwaaaaaa - only sissys cruise in "D" at those speeds anyway

dr bob

I think that backlash is directly the result of axial position of the ring gear.

Setting up a differential is an art as well as a science. Generally, the original setup is done using some very good fixtures that are hard to duplicate. If there's no damage to the faces of the gears, it's probably OK.

Pinion depth is the hardest dimension to adjust, and fortunately it only changes when a pinion bearing wears. That bearing wear results in a rumble at all speeds, loud enough to notice long before you start to hear any gear noise. Ring gear position is something that is easier to adjust with the carrier bearings, and is easiest to measure as lash at the rim of the ring gear. A simple dial gauge is the preferred method. If the lash is within spec, then consider checking the contact patter on the gear faces to make sure the gears are meshing correctly. I think the WSM shows patterns, and there are many general repair books that will give you guidance.

Now the part where I reinforce the JB worst-case scenario--

I like a quiet car, as you obviously do too. I like a --really-- quite car. So 15 years or so agao, I bought a Ford, mostly because of changes in US tax law regarding business use of cars. Anyway, the new Explorer was truck noisy when I got it, but a month of casual effoert with everything out of the car, add sound deadening, foam-fill the body cavities, additional insulation, padded headliner, better carpet... It was pretty quiet. But now I could hear the differential noise. Only on a trailing throttle, 46-57 MPH, just a slight whine. Off to the dealer, where they agreed to look at it. A couple days later, it's got a new R&P installed and the noise is gone. Six months and a few thousand miles later, the noise is back with a vengeance. Dealer "can't hear it" this time, since there's no more warranty money in it for them as rework from Ford. I escalate the issue with Ford, and they do it again to avoid owning the car again. Good for about the same time/distance, then needs it again. I take it to a different dealer, where there's a mechanic with a great reputation for such things. I leave it with him for two weeks, another R&P. It's perfect again, and ten years later it was still perfect. The gears that came out looked -perfect- to me, and the mechanic said he replaced them because Ford told him to-- He doesn't get any warranty reimbursement except for replacement of parts it turns out. What was different? Don't know. All three replacements were set up with the same specs and tolerances, as was the original supposedly. I reported back to the mechanic after the first year with a gift for his most excellent work. So, do you need a ring and pinion? Probably. Maybe. Do you need a set-up guru to make it just right? Most definitely!

Jim bailey - 928 International

The owner is a retired truck driver and has great mechanical sympathy. He listens when the machine is talking to him , that is how you drive millions of miles !

Schocki

Good to know! I will try to check and adjust everything first. I will let you guys know what I find.

Avoiding this speeds would be another option....

borland

Here's from one of my previous posts...
.........

I recently used my digital voice recorder to record noises while driving. My car has a 220 Hz growl at 30mph but it goes away above and below that speed. It's quite faint and not that noticable.

I confirmed the pinion to ring gear as the source by using an Excel spreadsheet calculation to determine the gear teeth meshing frequency.

It's not that easy to guess the noise frequency, so I downloaded a shareware tone generator software program that I can adjust the frequency and amplitude. With Windows media player, I can play both the tone generator and the recorded sound track at the same time, adjusting the tone generator frequency to the tone of the gear noise.

If I playback the sound track on the computer speakers, it just sounds like noise. But with my high fidelity headset, I can hear the gear noise and resonance of the exhaust pipes quite clearly.

You could use this technique to isolate a wheel bearing noise. For the calculation, you'd have to know the bearing I.D. roller bearing diameter, tire size, speed, etc.

.............

borland

here's another thread

https://rennlist.com/forums/showthre...ht=ring+pinion

Schocki

Borland,
Thanks for the info. I will let you know what I find, waiting for a new gasket and the O-Rings...
Hopefully I can eliminate the noise by adjusting backlash...
I will also check if the bearings that hold the differential in place inside the housing have more play than they should...

dr bob

Schoki--

I don't have the WSM's with me (imagine that...) so can't quote from the 928 Bible, but... Most differentials are set up with shims around the carrier bearings to add a certain amount of axial preload to the bearings themselves. Set the correct lash/clearance, verify a good contact pattern, then drive preload shims into the assembly on either side. Verify lash/clearance again. Verify bearing cap torque before closing and adding oil. Bottom line-- I don't think you'll feel any bearing clearance, at least I hope you don't. If you do, it's time for new carrier bearings before you go any further with setup.

Maybe the 928 is different?

borland

dr bob,

The 928 differential is different in that the gear set setup has been determined at the factory. Both the ring and pinion are engraved with the optimum settings.

Schoki,

You can set the backlash with the transaxle installed in the car. The pinion placement requires the differential housing removal after removing the transaxle from the car. But prior to removing either side plate, you will need to support the carrier from below. Here's the tool parts I made to do this.



Here's the stackup. The wood is just cut from furring strips. The taped piece is just a piece of paint stirring stick is used to wedge between the stack. I used hot glue to glue the two top pieces.



You need to use a torque wrench on the side plates in order to measure the free play of the carrier bearings. My spreadsheet was very helpful in determining the setup.



I found another way to measure the pinion placement, without the tools shown in the WSM.




Also, on the rear cover, I recommend that you use Halomar, a non hardening sealant, as a gasket dressing.

Schocki

Thank you for the info, great!

Just foud out from my parts supplier in Germany, that the carrier bearing housing O-Rings are not available! Porsche has no idea when they are available again. The only thing they could offer me is a complete maual transmission gasket kit for 117 Euros. What a joke.

Plan is to check the backlash first and also check for any carrier bearing movement. If the bearings move, or better there is some play. They need to be replaced. If there is some minor movement, that would also explain where the noise comes from. Vibration of the differential in the housing at a certain RPM....

DSC928

That was me. Damn disappointing to have to spend thousands of $$ on replacing a ring & pinion (and all the associated shims & bearings) in a car with only 54k on the odometer. If we were talking about a Kia, I would expect such performance, but a $100K Porsche shouldn't wear out at that mileage. Took my respect for the car down a few notches to say the least.

Mine had the exact same symptoms that you are complaining of. And it came on all of a sudden. A very noticable rear end howl at 30-37 mph only in Drive and only when foot in on the accelerator.

Good luck and hold on to your wallet.