how does diff lock switch work?

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <01BD41D7.06650620@gpowell.acacianet.com> glen powell writes:

> >The order in consumer vehicles is usually back first, then centre.  On the 
> >original 80Q implementation, a pull-out knob had two positions - click one: 
> >rear, click two: centre.
> 
> Not correct. Pull out knob, click one = center only locked, click 2 = center and rear both locked.
> Pull out knob fully in = both center and rear open.
> Same for the rotary version, pos 0 = both open, pos 1 - center only locked, pos 2 = both center
> and rear locked. (regular production USA models)

Did some checking.  WOW - you better know your car when you lock the diffs.
 
The original ur-quattro had Bowden cables.  Either diff could be locked at 
will.  The second implementation (April 1981 to January 1982) was a single-
position knob that locked them both via a single vacuum circuit.  From 1982 
onwards, two-position knobs were used.  There were several designs - some 
twisted, some pulled, and at least one twisted both ways.
 
The mid-series, until the TORSENs came out, seem then to have settled as you 
(and others) have said - centre first, then rear.  It's easy to change this.
 
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