Questions on installing ABS in a 4000Q

[Bernard Littau]

>
> > Here is an observation I made: the ABS in the 90Q (Torsen
> center diff) is a
> > 3-channel system.  But, the ABS in a  80/90 non-Q, as well as a
> 5000Q (non
> > torsen) is a 4-channel system
> >
> > So, here is my question: should I use the ABS system out of a
> 90Q, or should
> > I use the four-channel (four is better than three, right?) out
> of a 90-non-Q?
>
> four may be better, but your *car* has a "3 channel" brake system - one
> line for each front wheel and one that gets split between the rears.  I
> can't attest for that 5kq thing, but 2wd Audis have a "dual diagonal"
> system, ie one line for each wheel, and the LF/RR are driven off one
> section of the master, RF/LR off the other.  Q's, as far I thought I
> knew, were set up with the front running off one section and the rear
> off the other, for some reason.
>
> So... if the 5k*Q* truly has a four channel ABS system, that means it
> must also have four separate brake lines leaving the master.  You could
> run another line to the rear of your 4kq and.... ok, so use the system
> from a 90Q.  You're gonna need the strut housings anyway.
>

The 5ktq was top-of-the-line, so it would surprise me if a 5kq had a more
sophisticated ABS system.

The 5ktq has a three channel ABS system, RF, LF and both rears together.
The Master Cylinder has two channels, one for the fronts, one for the rears.
The hydraulic modulator is what can control the two fronts independently, as
well as the rear.  The single line from the Master Cylinder is split inside
the Hydraulic Modulator to the two front wheels.

The sensors are on all four wheels, so the ABS computer gets input from four
channels, but controls only three channels of modulation of the brake force.
It's the three lines coming from the Hydraulic Modulator that make it
apparent as a three channel system.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq