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Questions on AWD, ABS, and locking the rear differential

Subject: All-Wheel-Drive (AWD) and ABS questions

Reading a Chilton's manual that covers the GM A-body cars from the
mid-to-late 80's, I found a note that the Pontiac 6000 ABS is disabled
whenever the rear differential is locked for AWD.  The note said that
this was necessary because accurate wheel speed sensing was not
possible with the differential locked.  I'd like to understand this,
and would appreciate comparisons with the Audi Quattro.

   o  What is meant by "locking the rear differential"?  Is this
      required for AWD, or is this simply one way of using AWD?
      Is it done manually or automatically?

   o  Why would locking the rear differential make the ABS speed
      sensing mechanism (just a magnetic sensor and gear-like hub
      disk) less accurate?

   o  Do other AWD cars retain ABS functionality when AWD is
      enabled?  For example, is ABS disabled automatically on the
      Audi Quattro when the rear differential is locked, or is
      this requirement specific to the Pontiac design?

   o  Is there such a thing as AWD with an automatic transmission?
      All the Audi Quattro ads I've seen indicate a 5-speed tranny,
      but I think I've seen reference somewhere to automatic.  Other
      than the "enthusiast" aspect of manual shifting, are there any
      AWD automatic cars that work well?

   o  Finally, is AWD a full-time thing, or is it something that one
      manually turns on and off?

   o  Thanks :-)

Ken Bell :: kenbell@panix.com   :: (212) 475-4976 (voice)
======== :: syklb@giss.nasa.gov :: (212) 678-5516 (voice), 678-5552 (fax)