RE: Quattro Info? (& problem with S4's Rear Diff Lock)

["David Bray" <panacea@istar.ca>]

> Ti Kan writes:
> That's an Audi S4 quattro, 5-cylinder 20-valve turbo, 227hp.  Wundercar!
> It was available in the U.S. from model years 92 through 94,
Yep, also available to us Canucks in Canada's deep north - Ottawa ;)

I have a 1993 S4 and it is AMAAAZING ... especially with the mods a local
speed shop did for me:
  custom modified, unrestricted, fresh air intake (complete with *blow-off*
valve)
  re-programmed engine computer (allows boost up to 3 bars)
  new turbo wastegate spring (needed to take the extra boost)
  custom stainless steel flow-thru exhaust
  new springs all around (lowered about 1.25")

They did 6 or 7 others before mine, and claims that it now puts out about
330hp ... even though it hasn't been dyno'd, I believe them because it
certainly behaves like it!!  Anyone from Quebec City to London Ontario with
an S4 or S6, interested in PERFORMANCE, let me know and I can put you in
touch. (just e-mail me)

> and then it was renamed the Audi S6 for the 95 model year, along
> with some minor changes to the bumper and grille trims.
FYI, they also changed the solid sunroof to a glass moonroof, removed the
rear window pull-up shade, and lost the wood shift knob & other accents :(

OK, so every silver lining has a cloud ... the car's been near-perfect for
about 18 months, but NOW .... I have a PROBLEM!  (Scouring the Internet for
a possible answer is how I found this group a few days ago.)

Just in case anyone missed it, the problem is as follows:
Unless I accelerate EXTREMELY gently, the rear differential lock comes on
BY ITSELF!  Even though it is designed to automatically disengage at speed
(25 mph ?), it doesn't.  It's certain that the diff. IS in fact locked
because I can feel the *skipping* tires when turning.  (Of course, the ABS
also becomes disabled.)

Sometimes when going over bumps or turning - particularly to the left, it
will disengage, but this is not consistent.  Replacing the computer module
had no effect, and a wiring check by the dealer has turned up no faults
either. (That doesn't mean there isn't one - just that they FOUND NOTHING.)
The dealer seems to have little idea at this point even though, all told,
I've been without the car for over a month.  So, I got the car back,
persuaded an ace mechanic friend of mine to come out of retirement to help
me tackle the problem.

Thankyou to someone from this group ...
Michael Sheridan Williams (ICQ# 11740998), wrote:
> Someone had a sort of similar problem on a V8.  ...
> ...  The problem with the V8 was the wheel speed
> sensors, from the ABS.  GIve them a check and you will probably find your
> problem, well, maybe at least.....
OK, I had problems with rusty sensors before causing ABS brake *flutter*,
but how do I go about checking these sensors?  (A visual check of sensors
and related wiring didn't turn up anything.)

Thanks also to BUZZ from alt.autos.audi ...
buzz <gogo@mailexite.com> wrote in message
<35f1e9ba.166893296@news.videotron.ca>...
> I know there is a hall sensor in the Differential, it chechk the speed
> of the car to turn it off at over 50Km/h. It could be defective...
I don't have a spare sensor to try this, but here's more info.

If I engage it manually, it's situation NORMAL ... the orange light on the
switch illuminates, the green light on the differential graphic (on the
centre console) illuminates and the orange *ABS Disabled* warning lights on
the dash.  Then at about 25mph, the lock disengages as it should, and all
these lights go out.

However, when the diff lock chooses to engage on its own, the orange light
on the switch does NOT illuminate, although the other two lights DO go ON.
(So, the switch itself is likely not being activated spuriously.)  If I back
off the accelerator immediately, it usually goes off again on its own within
a second or two.  But, if I keep accelerating, it can take a while to
disengage on its own.  THIS HAS HAPPENED EVEN AT SPEEDS OF 100km OR MORE!!

<<< QUESTIONS TO ALL ... >>>
1.  I've ordered the Audi shop manuals from the dealer, but does anyone have
access to the wiring and technical info while I'm waiting?

2.  Also, is there some QUICK & SAFE way to disable the diff lock (without
disabling my ABS nor Quattro drive) while this problem is unresolved ??  ...
at least so I can drive the car without ripping apart the tires and possibly
the drivetrain.

3.  It HAS been suggested that physical torquing of the engine and
drivetrain components under acceleration is possibly causing a wiring short
somewhere.  This sounds great, but does anyone know where I might start
looking?

4.  Other than the console switch, what (if anything), can legitimately
cause the diff to lock?

5.  Is there some way in which the ABS system disables itself and *feeds
back* so as to cause the diff to lock also?  (That is, the mechanism that
causes the ABS to be disabled on diff lock ... can some weird feedback cause
it to work the other way?)

6.  I'm VERY desperate ... has anyone else heard of similar problems or have
any other ideas?

TIA to all who might be able to provide some info, insight, ... heck ...
anything!!

David Bray
Ottawa, ONTARIO
ICQ# 2896073
panacea@istar.ca