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Re: Dragging brakes

Bob Kunz sez:
> I'm gonna have to look at this one. Whenever ABS kicks in on mine, the
> pedal comes back up in addition to pulsing. There is a load motor type
> noise that I assumed was some sort of pump since if the system is
> pulsing the brakes it has to get the pressure from some place and it's
> not going to come from my foot. I wouldn't think the hydraulic system 
> provides this, but maybe it does. What's the real way these systems
> work? I have the Bosch system in my '86 5000s (first option year for the
> non-turbo cars).

I'll relate my understanding of the Bosch ABS 2 system unsed in my '91
Coupe; it is probably the same as yours.  Most of my info has been
garnered from a combo of the _Bosch Technical Handbook_ (2nd and 3rd
edition, I *HIGHLY* recomend this book!), the Bentley manual, the
Bosch ABS 2 test unit, and general hacking around.

The ABS 2 system is a three channel system (there are three separately
controlled hydraulic systems); Audi employs four sensors (don't know
why -- would work with three).  On Quattro cars, the brake system is a
front/rear split system, with a single channel controlling the rear
brakes and each front caliper on a seperate channel.  Thus, the ABS
unit on a Quattro car has 3 hydraulic lines headed for the calipers,
one of which splits into the two rear calipers.  On a non-Quattor car,
Audi uses a diagonal split, and thus the ABS control unit needs 4
hydraulic lines headed for the calipers.  However, even with a
diagonal split, thwe ABS is still 3 channel; two of the hydraulic
servos are operated in tandem.  Proportioning in either system is
accomplished before the ABS control unit -- thus, the ABS control unit
has 2 input hydraulic lines: in a Quattro, these are the front/back
lines from the proportioning valve; in a non-Quattro, these are the
two diagonals from the master cylinder.

In the following, I will simply describe the operation of the Quattro
unit (3 servos).  The non-Quattro unit is similar, except that there
is an additional servo which is operated in tandem with one of the
other three, in order to provide equavallent braking to both rear

The front channel input is teed into two seperate ABS channels, and
the rear is used as the third channel.  Each ABS channel has a
3-position solenoid controlled valve and a tiny (2 cm^3) pressure
accumulator.  In addition, each channel has a connection to the ABS
hydraulic return pump.

The solenoid valve has three positions: OPEN, HOLD, DUMP.  When OPEN,
brake fluid, and thus pressure, flows dirrectly from the input to the
ABS channel (and thus to the controlled wheel caliper -- in the case
of fronts, only one caliper, in the case of rears, both calipers).  

In the HOLD position, the input fuid is prevented from flowing to the
channel output -- the input sees a closed valve, and the output sees
no increase or decrease in pressure.  (Actually, this is not quite
true -- in actuality, there is a very tiny passage between the input
side and the output when in the HOLD position; this is represented on
the Bosch diagrams as a -/\/\/\- symbol (resistor), and prevents total
brake system failure upon ABS system worst-case failure.)

In the DUMP position, the valve allows fluid to flow from the OUTPUT
side to the return pump.  The input side continues to see a closed
valve as with the HOLD position (with the caveat mentioned earlier).
The return pump is operated when necessary to return fluid to the
input side of the system, so as to prevent the pedal from sinking to
the floor during prolonged DUMP cycles.

Both the HOLD and DUMP cycles are responsible for the pedal sensations
during ABS operation; in my hacking I have noticed that actual DUMP
operation is fairly rare.  It seems that the return pump is spooled up
as soon as ABS operation is initiated (i.e.: before actually
required); this is the motor noise that you hear.

The computer determination of solenoid position for each of the three
channels is actually quite complex; briefly, the computer relates
absolute wheel deceleration rate, car versus wheel deceleration rate,
and recent history to come up with the positioning.  The cycle is
essentially OPEN (normal braking), then HOLD at wheel deceleration
rate exceeding absolute threshhold and/or relative threshold, then
DUMP if wheel deceleration does not improve, then OPEN or HOLD after
fixed time interval (OPEN or HOLD is chosen is based on rates, etc),
then the cycle repeats.

As both HOLD and DUMP actually continue to supply pedal pressure to
the channel outputs (albeit at greatly reduced values), it would seem
that even with ABS, simply mashing the pedal to the floor with maximum
force is not the quickest way to stop.  It is better to have the ABS
unit avoid the DUMP cycle, and simply maintain a HOLD cycle.

All in all, the Bosch ABS 2 system is a rather elegant piece of
engineering; it is relatively independant of most brake system parts.
Importantly, it does not depend on the hydraulic booster system (or
even any booster system), and seems to be capcble of tollerating
changes in brake capacity.  I originally got into all this in order to
determine if I can massively upgrade the brakes on my car; while I
haven't installed the new brakes yet, it looks like they will work
with the existing ABS system.  For max performance, I may need to
change the "alpha" value in the ABS computer, tho.  (To give you a
sense of the change, I'm going from the existing 2-piston sliding
calipers to 4-piston 14 inch Alcon's in the front, with tires to

So, as you see, all the braking power *DOES* come from your foot (the
way you put it!); the ABS system is just regulating it.  You can think
of it as ABS reduces the pressure you applied when it judges that you
applied too much, then allows you to apply more when the computer
judges that the car could take more braking.  Very elegant.

Note that while the three channel aspect may seem dissapointing at
first (wouldn't four channel be better?), it's better than it seems.
Consider that the rear brakes are relatively ineffective at
controlling yaw, as the static weight distribution is front biased,
and under extreme braking (where ABS operates) it is much more front
biased.  With ABS 2, yaw is controlled via the front brakes (and the
driver!), and I doubt that much better control could be effected
using the rears separately.

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