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

["Dupree, Jim" <dupree@alldata.com>]

I don't have a diagram here with me but from what I recall about the
rear diff lock controls it sounds like you may have a vacuum check valve
that has failed. The rear diff lock is actuated by a vacuum servo,
vacuum is supplied to one side of the servo to lock the diff and
supplied to the other side to unlock the diff. What I suspect may be
happening is that you are getting engine boost pressure past the check
valve and the boost is being applied to the unlock side of the servo.
Since the servo is designed to work with vacuum not boost, it is
operating opposite of normal and actually engaging the diff lock when it
should be using vacuum to keep it disengaged.
it is a thought anyway.
Good Luck with it.
Jim Dupree


> -----Original Message-----
> From:	David Bray [SMTP:panacea@istar.ca]
> Sent:	Thursday, September 10, 1998 6:42 AM
> To:	Quattro
> Subject:	RE: Quattro Info? (& problem with S4's Rear Diff Lock)
> 
> > 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