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Re: Questions on AWD, ABS, and locking the rear differential

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To: quattro
From: kenbell @ panix.com (Ken Bell) @ smtp
Date: 10/13/95 09:33:35 AM PDT
Subject: 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?

AWD is full-time operation, and cannot be disabled.   As a wheel begins to lose 
traction, power is moved from that wheel to a wheel that has more traction.  
AWD also uses limited slip differentials, this means that the wheels do not 
have to turn at the same rate enabling power to be moved to the wheels with the 
most traction.  When the differential is locked, this forces the wheels on that 
diff to turn at the same rate, this is not desirable in everyday driving 
conditions.  When your car makes a turn, the inside wheel may stop moving 
completely and just pivot, if it were to turn at the same rate as the outside 
wheel then there would be wheel-spin on the inside.  Some 4-wheel drive trucks 
have the wheel lock mechanism on the center hub of the wheel.  Audi's have the 
diff lock controlled from the inside of the vehicle.  Audi's also automatically 
disable the locking mechanism at 15-20MPH because it is only recommended to 
lock when you are stuck or are driving very slowly on hazardous ice/snow.

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

This I'm not sure of, but it must be related to the limited-slip diff.  BTW, 
Audi's disable the ABS upon locking the diffs also.

   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?

See above.

   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?

Now, that you mention it, I don't recall having ever seen an automatic 
Quattro.  I don't see any mechanical reason why they couldn't manufacture one 

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

See above.

   o  Thanks :-)

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