Persistent vibration, 87 5KQT

[john at westcoastgarage.net]

After a long break, necessitated by family priorities involving my aged 
parents, I'm back on this car.  I thank all who replied last fall when I 
first put this up.  I've looked at the bearings, swapped in a set of 
known good wheels and tires, and looked again at the driveshaft.  Here's 
a further question:  My home made fixture aligned the driveshaft to be 
straight.  The manual doesn't specify an angle, it just says to use the 
factory fixture.  Is the shaft supposed to be straight?  Or, is there a 
specific angle at which to set it?  Another thing.  Prior to the noise, 
I had the center support bearing, the support, and the center U-joint 
replaced at a local driveline shop.  Is it likely I need to take the 
shaft back to them for redo?  Maybe I should drain the rear diff and 
inspect the magnet for debris from a failing bearing.

Recap:  What's the optimum angle for the driveshaft halves, and could 
that be the problem (changing it from just being thrown in to 0 degrees 
made no difference)?  Or, should I send the driveshaft back out? I 
suspect the driveshaft, or its alignment, to be the root of the issue, 
because it all started when I replaced it with the locally reconditioned 
unit (and the 034 sourced center support).

Thanks again for your time!

John


On 10/31/2010 10:45 AM, Louis-Alain Richard wrote:
> To my ear, this is either the propshaft u-joint or the center bearing, or
> maybe the gearbox output shaft bearing. Might as well be the rear diff input
> shaft bearing. Maybe you can hear them with a stethoscope while the car is
> on the lift ?
>
> Louis-Alain
>
>
>

-----Original Message-----
From: John Larson<john at westcoastgarage.net>
Sender:quattro-bounces at audifans.com
Date: Sat, 30 Oct 2010 18:41:13
To:<quattro at audifans.com>
Subject: Persistent vibration, 87 5KQT

I'm about at the end of my rope.  I have a customer's well worn, high
mileage, 5KTQ here that has developed a low pitched humming vibration at
freeway speeds, although the noise persists as low as 30MPH, maybe even
lower.  I've checked the axle shafts for bends, replaced a bent RR
wheel, and aligned the driveshaft with a cool home made fixture.  I
spoke with a chassis and alignment guy this afternoon (he's gonna drive
it Monday) and he asked about the wheel bearings.  I've done probably
500 double row wheel bearings in my career, and I've never had one sound
like this.  They mostly howl or make a low frequency rumble, and usually
(but not always) change tone or volume when side loaded by turning the
car one direction or the other.  This noise comes on as you build speed
on the freeway, and is there whether you're going straight or turning.
The volume, tone, and frequency appear to remain pretty constant.  As
always, any and all constructive input would be sincerely appreciated!

TIA!  John
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