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

Actually, when I heard the suggestion that it was a fault in the vacuum
system's check valve I shot off an immediate reply to the poster saying that
it was an excellent thought, and that they had probably hit the nail on the
head!  That theory explains all of the symptoms perfectly.  While I
personally have never seen a vacuum check valve fail in the open position,
if you imagined that pressure were applied where vacuum was expected, the
boost pressure would blow the vacuum actuator to the opposite position from
what would be expected if vacuum were present (i.e. it would cause the diff
to lock when pressure is applied to the side where vacuum is normally
applied to unlock).  The controller that disables the diff lock has no
feedback as to the position of the actual diff lock, it simply applies the
source of vacuum to one side of the actuator or the other ... and since the
diff lock control switch is off it doesn't matter what the speed is.  The
controller presumes that the check valve is ensuring that only vacuum gets
applied to the actuator in a way that would disable the diff lock.  

The behavior of the lock based on boost level fits the theory exactly, and
the deactivation of the ABS system makes perfect sense as well.  This
circuit is driven by whether or not the diff is actually locked ... not what
the switch/controller is trying to tell the lock mechanism.  When the boost
is applied sufficient to cause the diff actuator to lock, the switch on the
diff housing closes and shuts off the ABS.  

As I stated before in the earlier private message ... good call Jim

Steve Buchholz
San Jose, CA (USA)

> ----------
> "David Bray" <panacea@istar.ca> wrote in panic:
> 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.)
> -------------------------
> A different guess.  If the diff lock actuates under no vacuum, then there
> may be a failure in your vacuum system check valve allowing the pressure
> to
> rise whenever you step on the gas.   The speed limit function (at least on
> my 90q) is driven by a signal from the speedometer circuit and wouldn't
> affect a vacuum leak.  If you can get to the vacuum lines (assuming your
> car
> uses them) you might try a MightyVac or equivalent to keep them under
> vacuum
> and see if it makes a difference to your symptoms.
>                 .... Kirby   (Kirby A. Smith)
>                               2 x 1988 90q
>                           New Hampshire USA