RE: how does diff lock switch work?

[glen powell <gpowell@acacianet.com>]

>The rationale behind the speed-limited lock on the ALZ, BTW, seems to be that a 
>limited-slip differential needs centrifugal (centripetal?) force to work, and 
>this is not sufficient below 25mph.  Above this speed, the limited-slip 
>function is available and the driver-operated lock is counter-productive.

>-- 
 >Phil Payne
 >UK Audi [ur-]quattro Owners Club

Umm, not sure about this.....

IMFO, limited-slip diffs need some amount of resistance at both wheels in order
to achieve limited-slip effect. In LSD implementations with light preload it may
be required to have some resistance at both wheels to achieve LSD action. With
these implementations you can force LSD action by gently applying the brakes
simultaneously with applying power, this will result in some resistance at both 
axles and will force LSD action. The LSD action results from cones or clutch-packs
that connect the carrier to the axles. The preload applies a set minimum of
force between the axles and carrier via the cones/clutch-packs. When there is
some resistance (traction) at both wheels and power is applied then additional
force is applied to the axle/clutch/carrier assembly and lock-up is achieved. If the
preload is weak and traction is nill then insufficient torque may be applied to the
axle/clutch/carrier system to achieve lock-up. This can happen as the cones/clutch-
packs wear and as the preload spring ages over time and miles combined with low/no
traction - just when you need the LSD action.

That having been said, few (if any?) quattros use traditional LSD. Most are either
100% mechanical locking or are torsen or mix them both - but _never in the same
diff_. They are mutually exclusive. You have either open, traditional LSD, 100% 
mechanical locking or torsen in a single diff but never more than one.

Now, I may be confused by the above "on the ALZ" nomenclature, but I think on the
torsen center with 100% mechanical  locking rear diff cars the automatic rear
unlock feature is to simply automate the unlock for forgetful drivers and to automate
return to ABS enable mode (if locking disables ABS on these cars, I don't know for
sure about this point), as the rear locking diff is sposed to be used only to get out
of very low/no traction/already stuck situations. 

-glen