1996 AUDI A4 Brakes Description Of Systems Workshop Manual

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Description of systems

The individual components have the
following functions:

ABS

Anti-lock Brake System
ABS prevents the wheel from locking up during
hard braking initiated by the driver. Despite high
brake effectiveness, lane stability and steering
capacity are maintained.
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EBD

Electronic Brake Distribution
Specialized software stored in the ABS control
module -J104-, called Electronic Brake
Distribution (EBD), assumes the function of the
brake pressure regulator.
The signal sent to the instrument cluster when
ABS fails is pulsed if the EBD function remains
operational in emergency mode. The instrument
cluster distinguishes between no signal, a pulsed
signal, and a steady signal. The ABS/EDL
indicator lamp and the red symbol for "Brake
system malfunction" will light up accordingly.
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1 - Terminal 15
2 - Signal for controlling the ABS/EDL indicator lamp and the red "brake
system malfunction" symbol.
3 - Voltage supply for the ABS/EDL indicator lamp.
4 - Voltage supply for the red "brake system malfunction" symbol.
A - In interval "A" (with "ignition on") a self-test of the ABS/EDL indicator
lamp is carry outed for two seconds using signal "2" (lamp lights up).
During this interval, the instrument cluster suppresses the red "brake
system malfunction" symbol.
B - Signal "2" is not available in interval "B". The ABS/EDL indicator lamp
and the red "brake system malfunction" symbol do not light.
C - In interval "C", the On Board Diagnostic recognized a DTC, which did
not deactivate the EBD. Signal "2" is pulsed. Only the ABS/EDL indicator
lamp comes on.
D - In interval "D", the On Board Diagnostic recognized a DTC, which did
deactivate the EBD. Signal "2" is steady. Both the ABS/EDL indicator
lamp and the red "brake system malfunction" symbol light up.
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The red "brake system malfunction" symbol
indicates the EBD is deactivated. The vehicle
is only allowed to be driven under restricted
conditions; "Road test vehicle".
For additional information about EBD indicator
lamp control, see: "Overview of warning lamp
functions"; The vehicle version as relates to EBD
can be recognized by checking the note column
under "Check control module version".
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EDL

Electronic Differential Lock
The Electronic Differential Lock (EDL) is a
traction control aid for driving away from a
standstill. An electronically-controlled brake
application at the wheel that is spinning provides
a torque reaction point for the differential. This
enables the wheel with better traction to make
use of the engine power. EDL is effective in both
forward and reverse.
EDL is part of the basic configuration of vehicles
with All Wheel Drive (AWD). Although EDL only
regulates the front wheels on vehicles with Front
Wheel Drive (FWD), EDL regulates all four
wheels on vehicles with All Wheel Drive (AWD).
This makes it possible to eliminate the driver
operated mechanical differential lock.
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Onset of EDL regulation is permitted up to a
speed of 40 km/h. The upper switch-off barrier
for already active EDL is 40 km/h for vehicles
with Front Wheel Drive (FWD), but 80 km/h for
vehicles with All Wheel Drive (AWD).
The hydraulic unit is equipped with suction
dampers. They improve pressure build-up in the
appropriate brake cylinders during EDL
regulation. The functional range of the pressure
relief valve in the hydraulic unit is 170 25 bar. To reduce the risk of brake overheating, EDL
regulation is not permitted above a specific brake
temperature. The temperature limit is defined in
the ABS control module (w/EDL) -J104-. The
control module determines the actual brake
temperature via an algorithm. If the algorithm
determines that the brakes have reached a
temperature below the temperature limit, the EDL
system becomes effective again. Function of the
vehicle brake system and ABS are not affected
by intervention in this system.
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When ignition is switched off, the temperature
calculated for the hot brakes is stored until
ignition is switched on again. When ignition is
switched on, the instrument cluster transmits a
time signal so that the ABS control module
(w/EDL) -J104- can calculate the standing time.
This standing time is required by the temperature
algorithm for calculation of the new brake
temperature. If the indicated standing time
exceeds one hour, the brake temperature is
automatically set to the lowest temperature in the
temperature algorithm.
EDL shut-off for temperature reasons is indicated
in the measuring value block for a period of 20
ignition cycles.
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ASR

Anti-Slip Regulation (ASR)
The ASR option is only available on Front Wheel
Drive (FWD) vehicles. Observe control module
coding for these vehicles. The function of ASR
requires an exchange of signals between the
ABS control module -J104-, the Engine Control
Module (ECM) and the Transmission Control
Module (TCM). The control modules
communicate with each other.

There are two communication systems at work:

- Data is transferred via several single wires.

- Data is transferred via the CAN-bus. The
CAN-bus only contains two lines. These
transfer all required data.

To determine which communication system is
transferring data, use the "Check control module
version" function and check the table "Control
module versions".
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