[A4] ABS Module....

[Scott Simmons]

The quattro IV (generation IV) system uses the ABS to module the brakes 
to regain traction.

I guess Wikipedia can say it better than I:

Starting from 1996 on Audi A4 <http://en.wikipedia.org/wiki/Audi_A4> / 
S4 <http://en.wikipedia.org/wiki/Audi_S4> / RS4 
<http://en.wikipedia.org/wiki/Audi_RS4> (B5 platform), Audi A6 
<http://en.wikipedia.org/wiki/Audi_A6> / S6 
<http://en.wikipedia.org/wiki/Audi_S6> / RS6 
<http://en.wikipedia.org/wiki/Audi_RS6>, Audi A8 
<http://en.wikipedia.org/wiki/Audi_A8> / S8 
<http://en.wikipedia.org/wiki/Audi_S8> with both manual and automatic 
transmissions. Also on VW Passat B5 
<http://en.wikipedia.org/wiki/Volkswagen_Passat>, where it was initially 
referred to as /syncro/, but by the time it reached US soil, it had been 
re-christened 4motion <http://en.wikipedia.org/wiki/4motion>. Also used 
on the Volkswagen Phaeton 
<http://en.wikipedia.org/wiki/Volkswagen_Phaeton> and Volkswagen Group D 
platform <http://en.wikipedia.org/wiki/Volkswagen_Group_D_platform> 
sister vehicles; also the Volkswagen Touareg 
<http://en.wikipedia.org/wiki/Volkswagen_Touareg> where they use 
separate transmissions, PTU <http://en.wikipedia.org/wiki/PTU>'s and 
front axles.

The manually locking rear differential from the earlier generations was 
replaced with a conventional open differential, with "Electronic 
Differential Lock" (EDL) (which, detects wheelspin via ABS road wheel 
speed sensors, and applies brakes to the one spinning wheel, thus 
transferring torque via open differential to the opposite wheel which 
has more traction). EDL works at speeds up to 80 km/h (50 mph) on all 
quattro models (on non-quattro models: up to 40 km/h (25 mph).

System type: Permanent four-wheel drive 
<http://en.wikipedia.org/wiki/Four-wheel_drive>.

Torsen <http://en.wikipedia.org/wiki/Torsen> T-2 centre differential, 
50:50 'default' split, automatically apportioning up to 75% of torque 
transfer to either front or rear axle.

Open rear differential, Electronic Differential Lock (EDL).^[4] 
<http://en.wikipedia.org/wiki/Quattro_%28four_wheel_drive_system%29#cite_note-EDL-3> 


Open front differential, Electronic Differential Lock (EDL).^[4] 
<http://en.wikipedia.org/wiki/Quattro_%28four_wheel_drive_system%29#cite_note-EDL-3> 


How does the system perform: In on-road conditions the car will not be 
able to move if all four wheels lose traction altogether. Torsen effect 
with one wheel losing traction and spinning freely will not happen on 
quattro IV because EDL will apply brakes to the spinning wheel, and 
Torsen differential will transfer torque to another axle. However, in 
severe road conditions (wheels on ice or raised in the air and an 
obstacle restricting the vehicle from moving forward, on a slope) the 
car will not be able to move when one front and one rear wheel lose 
traction. EDL will brake the single spinning wheel on an axle trying to 
transfer torque to the opposite wheel, but because Audi's EDL action is 
too "soft" it is not able to brake the wheel to the full stop. The car 
will end up spinning one front and one rear wheels but not moving forward.



Peter Kirby wrote:

>Can you expand on this?  This is news to me that the 2001 A4 has some
>sort of traction control built into it, outside of the torsen centre
>diff.
>
>Peter
>
>On Wed, Sep 24, 2008 at 12:33 PM, Scott Simmons <indischrot at gmail.com> wrote:
>  
>
>>You also lose your "quattro" ability.  There will no longer be any
>>assistance if one wheel slips, beyond the Torsen center diff.  In a
>>worst case scenario, if one wheel were to completely lose traction, all
>>power would be diverted to that wheel.  Granted, I'm fairly certain that
>>most cases would be that a wheel would lose SOME traction and the Torsen
>>differential would multiply that loss into grip for the other wheels.
>>*shrug*
>>
>>~Scott S.
>>    
>>
>
>  
>