Rob Edwards

So there's a very hilly toll road here in Orange County which is very heavily patrolled (since it's the only place with so little traffic that you could even think about speeding), so I took the '90 on it for the first time today. Since the chippies sit on the downhill slopes , to avoid geting over 70 or so, I left the car in 5th gear and just coasted down the hill, generating some engine braking rather than being in neutral and coasting and braking (you'd be going 100+ easily at the bottom of these 1000 foot hills).

There's quite a pronounced whine from the rear end when coasting downhill.

A) Is this normal?

B)- What's the source of the whine? the ring driving the backsides of the pinion gear teeth?

C) Am I hurting the ring and pinion, or something in the tranny? It can't be safe to coast downhill in neutral, but brake pads are cheaper than LSD's....

Rob Edwards

Update, changed the tranny/diff fluid with 4.25 quarts of Redline 75W-90 NS and have driven another 100-200 miles. Had her out in the hills again yesterday and get the same gear whine (well, maybe slightly less pronounced) from the rear when either coasting downhill or if I suddenly get off the gas at highway speeds. CV boots are old but intact, haven't checked rear wheel bearings, but I have to think this is a diff issue. Even if it's 'normal' and your car does it too, I'd like to hear. Thanks!

Jim bailey - 928 International

That does sound like ring and pinion noise the PSD boxes with the active computer controlled slips seem to work the friction discs rather hard and the worn friction material circulates in the differential which can be hard on the bearings. Chasing a relatively minor whine can be frustrating since the parts will look pretty good and unless you change everything no guarantee that the noise will go away. One customer with GTS did the ring and pinion all bearings and replaced the worn out PSD friction discs.....you do not want to know how $$$$ that was. His made a little noise at 36 mph under light load not 33mph or 40 mph ....36 mph !

Rob Edwards

Jim-

Thanks for the speedy reply- I haven't flushed the PSD and can find no evidence that it has ever been flushed, so that needs to be done anyway. I hope that prior neglect hasn't accelerated the wear of the friction discs too horribly. The PO did change the tranny fluid at least, as it looked pretty clean when I changed it (again) ..

123quattro

I'd vote for R&P. I would suspect the only way to make it go away is to install new gears. Gears can sometimes gall in the transmission, but when that happens that noise is not throttle related.

Rufus Sanders

Mine does the exact same thing, but it has 207K mi. on it... Because of this, and because a new tranny/dif would cost almost as much as I paid for the car. I live with it. I've been planning to change the diff oil too. If it works I'll report back. - Ruf

Cheburator

My 230K miles 1990 GT has chunks missing from the Crown's teeth and whines at 60km and 102km... Do I care - not really. I will wait until I find a cheap G28.55 box to replace it...

The only think I would disagree is that the PSD is hard on the friction disks - I drive my car at the Nurburgring where the PSD is almost permanently on and I don't get any friction material suspended within the tranny oil...

Rob Edwards

Just for kicks, I priced a 28.55 ring and pinion set last night: $3300 for new. If there are chunks out of my ring and pinion, I'll live with it too. There are no other graunchy noises from the rear end, the box shifts fine when warm, and there were not an excessive amount of shavings on the drain plugs last month- Maybe I'll pull the rear cover at some point and have a look at some point. (maybe when it's CV boot time....)

Vilhuer

Part of the high cost is caused by fact that pinion includes long and complicated shaft in it. First thing I would do is to take really hard look if it were possible to modify existing pinion shaft to accept automatic gearbox pinion gear. Automatic diffs should be available fairly cheaply. Trouble is there were no 2.75:1 diff automatics.

borland

I've got a noise similar to what Jim described on a customer's GTS.... 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.

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

the thread...

https://rennlist.com/forums/showthre...ighlight=noise

RyanPerrella

Rob,

I wonder if you pulled one of the drain plugs in he diff if you would see a similiar buildup like you showed us before. The one that looked like a bunch of metallic snot. That may give you an idea of how iquckly whatever that is is wearing out. You may want to get that oil sample analysed and they could tell you what the metal is from, wether its actually a ring and pinion or a bearing falling apart. I would probably drive at least 500 miles before checking the drain plug magnet again. I know it sucks to open the diff again so shortly after replacing the fluid but get an extra plug so you can swap them immediately without loosing half your trans fluid.

The ring and pinion would start to wear if they are too tight right, or perhaps they arent wearing but they are just noisy from being too loose, in which case something else is wearing and changing the tolerances of the thing. I will shut up now as i have no experience with the G28 boxes, I hate getting eroneous replies from people with no practical experience on the subject they are talking about so take my post with a grain of salt.