Re: A4 Electronic locking diffs...

[ti@bazooka.amb.org (Ti Kan)]

Dan Masi writes:
> For those who were asking, the owner's manual for the A4
> explains that the locking diffs operate only at speeds
> under 25mph, and work by applying brakes to the wheel
> opposite that which is spinning.
> 
> I'm not sure how happy I am about this... seems to
> me that EDL (Electronic Differential Lock) implies
> something else, i.e. a differential that locks.  The
> end result is the same, I guess, except that this system
> only works up to 25mph; above that, and you've got
> open diffs front and back.

The name "EDL" is somewhat of a misnomer, since it does not
actually "lock" the differential.  However, it does supplant
the differential lock function that existed in previous Audis.

Basically, EDL is traction control.  Like the diff locks,
EDL is really meant to be a device to help you get unstuck in
slippery conditions.  Others have found uses for diff lock
in higher-speed applications, but the primary motivation for
locking the diff is to avoid getting stuck.  With EDL, this
is accomplished not by locking the differential, but by braking
the slipping wheel.  EDL uses the wheel speed sensors in the
ABS system to detect wheelspin (in fact, the ABS and EDL functions
are integrated into the same electronic control unit).

The fourth generation quattro models retain the TORSEN as the
center differential, which works like a normal, open differential
except under slippery conditions, where it directs torque to the
side with *more* traction.  The TORSEN handles the speed difference
between the front and rear axles, while standard differentials are
employed in the front and rear axles, each with EDL assist.

FWD Audis are now also available with EDL, which operates only on the
front axle.

Yes, it is true that EDL is only active up to 25mph, but this is
no worse than on second generation quattro models where there
is a TORSEN in the center and manual locking diff in the rear. The
rear diff would automatically unlock about 25mph on these cars and
there is no traction assist on the front differential at all.

This fourth generation quattro system with EDL is completely adaptive
and requires no manual locking switches or driver intervention.
It also provides tractive improvements to the front axle which
was previously unavailable on quattros.  Thus this is definitely an
advancement over the older setups.

Forget about the all-wheel-drive vs. traction control arguments.
In effect, the new Audi quattro models have them all.

There was a good article about ABS, EDL and ASR in the April 1995
issue of Volkswagen Audi Car magazine.

P.S.  For those of you who are not familiar with the quattro generations,
      here is a brief summary:

1st generation quattro:
Open front, center and rear differentials, the center and rear
differential are manually lockable.  For ABS compatiiblity (if
so equipped) the diff locks automatically disengage when the diff
locks are engaged.  This is the system used on pretty much on all
quattro models from 1981-1987.

2nd generation quattro:
Open front and rear differentials, TORSEN center differential.  The rear
differential is manually lockable.  For ABS compatibility, the rear
diff (if engaged) would automatically disenage above 25mph.  This is the
system used in quattro models from 1988-1994 or so, except the V8.

3rd generation quattro:
Open front differential, TORSEN rear differential, and an electronically-
controlled lock-up clutch center differential.  This is used in
1990-1993 Audi V8 models with automatic transmission.  I think the
manual trans V8 models use the 2nd generation scheme (not too sure).

4th generation quattro:
Open front and rear differentials.  TORSEN center differential.
EDL assist to all four wheels up to 25mph.  This is used in the
current Audi A4 and A6 cars.

-Ti
96 A4 quattro
84 5000S Turbo
80 4000
-- 
    ///  Ti Kan                Vorsprung durch Technik
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