RE:93 Audi Audi 90, Differential

[RELAYER@aol.com]

Robert said:
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Hi Group, When rear wheel drive is selected the vaccuum
actuator is activated, the ABS off light comes on and the
rear diff indicator light comes on, around 40-50kph first
the selector button light goes out, than a few seconds
later the diff indicator goes out and the drive is
disengaged. Any suggestions would be appreciated. Please
also forward responses to automegalr@aol.com
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Robert,
I may be able to help you with some of your questions that were posted to the
Quattro list by John Larson.  Good news is, it sounds like nothing is wrong
with your car.

First, when you hit the rear diff switch, you are not activating rear wheel
drive, you are just locking the rear diff so the rear wheels will spin at the
same speed no matter what wheel has better traction.  As you may or may not
know, usually differentials make it so the wheel with the least amount of
resistance will get the engine's power, so if one wheel is stuck in snow, and
you hit the gas, the wheel WITHOUT traction will get all the power.  Locking
the rear diff takes care of that.  But of course, if the rear wheels are only
allowed to spin at the same speed, you cannot turn, since the outer wheels
will be trying to turn faster than the inner wheels.  So, remember, do not
turn the car when the lock is engaged unless you are on a slippery surface
that will let the wheels slip some (such as gravel or snow).  Otherwise, you
will be putting a tremendous strain on the drivetrain that can cause some
damage.

Now, this feature is only of value at low speeds, when you are stuck.  So the
car will automatically disengage the rear diff as you gain speed.  I think
that it should disengage at 20 mph, but I don't know what that works out to be
in kph.  When the rear diff is engaged, the anti lock system is turned off,
because having a locked rear diff would confuse the heck out of an ABS
computer.  Once you hit the rear diff button, the antilock is supposed to turn
off, and once you gain speed, the diff automatically disengages, and then the
antilock is turned back on.  So, your car sounds like it is behaving normally.
I learned all of this from when I owned a Coupe Quattro, which uses the same
system.

Hope this helps! (I'm not an engineer, I just play one on TV)

  /\        _I        Christian J. Long (& Breeze Parker-Long)
/    \ I_I I_I I      Orlando, Florida, USA
                       University of Central Florida Alumni 1994
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'98.5 Audi A4 1.8tm    Neuspeed .8 bar, K&N, 17" Ronal R28 P7000ss
'96 BMW 318ti         For sale!
'90 Audi 90              
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Past Audis: '84 CGT, '85CGT, '87.5 CGT, '90 CQ