AUDI A4 1999 B5 / 1.G Brakes Can Bus Workshop Manual

01-25


CAN-bus

CAN-bus, function


Bus:
The term bus refers to a system that transports
and distributes data.

CAN:
The Controller Area Network is a bus system that
uses two wires. These are called bus wires. Data
is transferred to the connected control modules
serially (one after the other) across the bus
wires.
Data is transferred via the CAN-bus. For
information and test procedure, see page 01
-
27
. The CAN-bus only contains two lines. These
are called bus wires. These transfer all required
data. Two bus wires are used so that
malfunctions in the wires can be recognized.
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01-26
The CAN-bus terminates in a resistor. It prevents
the transmitted data from returning as an echo
from the ends and corrupting data.
The terminal resistances of the CAN-bus have a
value of 120 and are located in the ABS
control module (w/EDL) -J104- and the Engine
Control Module (ECM) -item 3-. This results in an
overall terminal resistance of approx. 60 . The steering angle sensor -G85- transmits its
data to the control module via the CAN-bus. It
does not have a terminal resistance.
As of implementation of control module 8D0 907
389 D/E, the CAN-bus is connected centrally in
the Engine Control Module (ECM) with a
resistance of approximately 66 . All other
control modules connected to the CAN system
(for example the ABS control module -J104-) are
connected with a high ohm resistance of 2.6 k .
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01-27

CAN-bus system, checking


The DTC table indicated you needed to check
the bus:

- Using the relevant wiring diagram, determine
how many control modules are connected to the
CAN-bus.

- Check that the multiple connectors of the control
modules are properly connected (unobstructed).

- Connect the VAG1551 (or VAG1552) scan tool
and press the -0- button twice to select the
"Automatic Check" function 00.

- Before checking the bus wires, make sure that
none of the control modules connected have a
function malfunction.

Note:
In this context, a function malfunction means a
malfunction that doesn't directly affect the bus
system, but that interrupts the functioning of a
system. This could be a malfunctioning sensor,
for example. A malfunctioning sensor would
mean that the sensor signal could no longer be
used for data transfer in the bus system. This
type of malfunction indirectly affects the bus
system. Communication to the control modules
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that use the relevant sensor signal is interrupted.
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Is there is a function malfunction?

- Fix the malfunction before proceeding.

- Note DTCs and erase DTC memory in all control
modules page 01
-15
.

- Press buttons -0- and -6- to select "End Output"
function 06. Press -Q- button to confirm input.

- End Scan Tool (ST) output using the "End
Output" function page 01
-16
.

- Refer to the DTC table in the relevant repair
manual for help in fixing the malfunction.
You have fixed all malfunctions:

- Check bus wires if communication to the control
modules is still malfunctioning.

While checking the bus wires, make a
distinction between the following two
situations:

Two control modules communicate via a "two-
line bus system"

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Three or more control modules communicate
via a "two-line bus system"
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01-29

Two control modules communicate via a
"two-line bus system".

- Switch ignition off.

Electrical Wiring Diagrams, Troubleshooting & Component Locations - Disconnect both control modules.
- Check the bus wires for an open circuit.

Electrical Wiring Diagrams, Troubleshooting & Component Locations - Check whether there is a short circuit between the bus wires.
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01-30


If no malfunctions could be found in the bus wires: - Check the bus wires for a short circuit to B+ or Ground (GND).
- As a test, replace one of the control modules. Select a control module
that will take the least amount of time, and cost the least amount of
money to replace.
- If communication between the control modules across the bus is still
interrupted, replace the second control module.
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01-31

Three or more control modules are
communicating across a "two-line bus
system".

- Analyze the DTCs stored in the memories of the
control modules.

Example 1:

From the DTCs stored in the memories, you have determined that control
module 1 is not communicating with control modules 2 and 3.
Electrical Wiring Diagrams, Troubleshooting & Component Locations Control module
DTCs stored in DTC memories:
1
Missing signal from control module 2Missing signal from control module 3
2 Missing signal from control module 1
3 Missing signal from control module 1
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and
check the bus wires for an open circuit. - Replace control module 1 if no malfunctions can be found in the bus
wires.
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01-32

Example 2:

From the DTCs stored in the memories, you have determined that control
module 2 is not communicating with control modules 1 and 3.
Electrical Wiring Diagrams, Troubleshooting & Component Locations Control module
DTCs stored in DTC memories:
1
Missing signal from control module 2
2 Missing signal from control module 1Missing signal from control module 3
3 Missing signal from control module 2
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and
check the bus wires for an open circuit. - Replace control module 2 if no malfunctions can be found in the bus
wires.
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