[s-cars] WTB - Rear Diff

[Tom Green]

The dealer should have a vibration analyzer to isolate the source.
Sensors are placed on the suspect parts and nearby rotating parts
and the intensity points to the source.  It can be done by driving or
on a dyno.  A good performance shop should have this ability as
well.

Or, let Bill drive it on a Chicago visit.  He, or other drivers, driving
the car independently, without you hovering in the passenger seat,
may provide a fresh perspective to the issue.  This approach can be
very helpful for those "maddening" problems that affect your sanity
before you let go of the rope.  :-)

Now, I don't discount the value of swapping parts as a good way
to fix problems.  I have three vehicles in close proximity to swap
parts with, so I use that approach a lot, but I am chasing a year
long vibration issue on an allroad where I don't have the ability or
desire to swap parts.  The vibration analyzer is a great tool for
narrowing down the problem, since the dealers other approach
(CPO warranty) is to change this "faulty" part and then this part,
and so on, and not all this stuff is covered by Audi.  Hmmm, this
may not be the best example since they are still changing parts,
but the idea is sound.  :-)

Tom

> Date: Tue, 20 Nov 2007 07:06:01 -0500
> From: "Eric Renneisen" <racingiron at comcast.net>
> Subject: Re: [s-cars] WTB - Rear Diff
> To: "'bill mahoney'" <airbil at gmail.com>
> Cc: 'Audi S Car List' <s-car-list at audifans.com>
> Message-ID: <001101c82b6d$bb2e1c10$0201a8c0 at renhoek>
> Content-Type: text/plain;	charset="US-ASCII"
>
>> Most obvious best guess is a bent wheel, but "maddening"
>> assumes you have ruled this out.
>
> I started with that thought about a year ago.  I do have at least one
> bent Bolero, but the vibes are present with three other sets of  
> wheels.
> FYI, an S4 looks really strange with urq Ronals!
>
>> My own personal best guess then (and ICB wrong,once was)
>> is some bushing around the differential and not the diff
>> itself. My own similar maddening vibration was bad
>> bushings front of diff in the sub frame cross member
>> where diff attaches in the front.
>
> I did have a bad one of those and replaced them both.  I thought for
> sure this was it (as I've thought about the dozens of other bad  
> parts I
> replaced).  It cured a clunk I had during harsh weight transfer  
> moments,
> but vibes are still there.
>
> Basically, I've got a speed-dependent vibration (40 and 80 mph) that's
> independent of gear selected (even neutral) and power application.   
> I'm
> convinced it's a rotational imbalance somewhere, but it's proven
> impossible to isolate, even when running the driveline up to speed on
> the lift--I can see/feel the car vibrate, but not the source.  I've
> swapped wheels, all driveshafts, two transmissions... not to  
> mention all
> the bushings and mounts.  One final clue is that I noticed the  
> effect is
> somewhat more pronounced with a full tank of fuel.  The diff seems
> normal (no bearing wobble or strange noises) but I'm at the end of my
> rope.  If this doesn't do it, I'm buying another one and breaking this
> one for parts.
>
> Thanks for the thoughts.
>
> Eric R.
> '86 urq
> '93 urS