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

[steveb@newkla.kla.com (Steven Buchholz)]

> 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?

This implies that you have what is referred to as an "open" differential.  
Open differentials are OK for most situations, but when you come upon 
compromised traction situations (e.g. mud, snow, ice) an open diff will 
tend to allow the wheel with less traction to receive power to the point 
where one wheel will spin and very little power will be sent to the other 
wheels.  Locking the differential guarantees that power is sent to both 
output shafts ... in fact both *have* to turn at the same rate.  Older 
quattros have open, lockable differentials as well, the newer cars have 
Torque-sensing ("Torsen") differentials that allow power to be sent in 
both directions even if one is slipping, and in fact tend to send power 
to the place where there is the most traction.
> 
>    o  Why would locking the rear differential make the ABS speed
>       sensing mechanism (just a magnetic sensor and gear-like hub
>       disk) less accurate?

I'm not an expert on ABS, but I don't think the problem is that the sen-
sors are not functioning properly, it is more likely that the locked 
differential(s) reduce the amount of real traction information that is 
available to the computer.  The computer should still be able to modulate 
braking pressure to the front wheels because the front diff doesn't lock, 
so it must be that there just isn't enough information about impending 
lock to properly control braking.  Or perhaps it is as simple as an assump-
tion that youre in some pretty deep stew if you've got the diffs locked!

The Audis that have locking diffs and ABS will also disable the ABS when 
the diffs are locked.  Locking the diffs gives you a "poor man's" ABS any-
way ... if you imagined that all diffs were locked (which is impossible 
to do with a quattro), then by definition if any wheel were locked-up 
(stopped), then by definition all wheels would be locked.  I believe that 
Audi is unique in that it is possible to manually disable the ABS even when
the diffs are unlocked.  They say that allowing the wheels to lock can 
improve stopping ability in snow.  (That's one thing I like about Audi, 
they figure that the driver is able to decide when it is appropriate to
disable ABS :)
> 
>    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?

Audi now has automatic transmissions available for the quattro models.
The first was the V8 Quattro, where most vehicles are equipped with
slushers.  You wont find a [stock] quattro older than ~1990 with an 
automatic.  Personally, I prefer the control of a MT, but for practical
purposes there is probably very little relationship between the style
of tranny you have and AWD, especially when you consider driving on the 
street.  I have compared notes with a colleague that has an SVX with an 
automatic that he has a lot of fun with ...
> 
>    o  Finally, is AWD a full-time thing, or is it something that one
>       manually turns on and off?

With a quattro you always have AWD.  The only option that you have on
some models is whether or not you want to lock the differential(s).
There are several flavors of AWD which have viscous coupling that is
only active when tires are slipping.  I don't remember the title of
the book, but it is something like The AWD Performance Handbook that 
discusses the different AWD implementations; you might want to read
a copy of that book.  
> 
>    o  Thanks :-)
> 
You're welcome ... hope it helped.

Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)