ABS - System Description
The main components
of the early ABS are outlined below. The revisions made for the 94 model
year are described in detail below
- ABS Control Module -
This resides under the rear seat, on the left side of the car, and is the
main control unit for the ABS system. As such it interfaces with all other
system components. Notated as
J104 on the schematic.
- Electro-Hydraulic Unit
- Located on the left side of the engine compartment (regardless of LHD
or RHD), this is connected to the master cylinder by solid hydraulic pipes.
This unit also houses an electrical pump, the input/output electro-hydraulic
valves and the associated control relays that are used when ABS operation
is needed. The only maintenance that can be performed on the electro-hydraulic
unit is the replacement of these relays. Notated as N55 on the schematic.
- Solenoid Valve Relay -
Resides beneath the cover on the electro-hydraulic unit. This is a standard
40A relay which is activated by the ABS control module and provides the feed
voltage to the input and output electro-hydraulic valves.
Notated as J106 on the schematic.
- Return Flow Pump Relay
- Also resides beneath the cover
on the electro-hydraulic unit. This is a standard 40A relay which is activated
by the ABS control module to provide flow of brake fluid through system
when necessary. As its name
suggests, the purpose of this relay is to provide voltage feed to the return
flow pump when activated by the ABS control module. Notated as J105 on the
- Combination Relay - This
relay resides in position #1 the auxiliary relay carrier. It's purpose is
to provide a means for the dash mounted ABS cancel switch to temporarily
disable the ABS. It is also wired into the rear differential switch to ensure
that ABS is disabled when the rear axle is locked.
Notated as J156 on the schematic.
- Wheel Speed Sensors -
Each wheel has an inductive speed sensor which is triggered by teeth on the
outer joint of each driveshaft. Each speed sensor is placed in the hub carrier
in close proximity to these teeth so as to induce a sawtooth waveform which
the ABS controller uses to measure individual wheel speeds.
Notated as G44/45/46/47 on the schematic.
- Brake Switch - Not only
does this switch operate the brake lights, it informs the ABS controller
when the brake pedal is pressed.
Notated as F on the schematic.
- Longitudinal Accelerometer
- It is unclear if all early ABS systems on the S2 have this component.
More research is needed as I suspect it may have been deleted
before the introduction of the OBD equipped ABS controller became standard
equipment. The purpose of the accelerometer switch is to close an electrical
contact in the event of sudden G-force increase in a forward direction -
such as when braking heavily. Notated as F113 on the schematic.
ABS - System Description : MY94-95 Revisions
At some point
in time, Audi decided that the risk of someone blaming them for causing an
accident by allowing the driver to disable a safety feature such as ABS was
too much of a risk to continue with. For the 1994 model year, a number of
revisions were made to the ABS on the 80/90 series. The most significant
change was the deletion of the ABS cancel switch. In relation to this change,
there was no need for the combination relay. Finally the longitudional accelerometer
switch was also deleted.
The most significant enhancement to the later ABS was the inclusion of
OBD capability. The ABS controller can communicate with the VAG1551 scan
tool (or emulator) to report any stored diagnostic trouble codes (DTCs) and
provide a group of data channels which can also be used for diagnostic purposes.
The OBD capability does NOT support blink code outputs - only high speed
comms with the scantool.
The ABS control module and relays reside in the same locations as described
for the earlier system... The later system has a different electrical schematic
which will be linked here as soon as I get it.
Last Updated 7th October 2002