Checking data exchange between engine control unit and control units with CAN capability — AUDI

AVF AWX
Notes:

Data are exchanged between the individual control units by means of a bus system.
"CAN bus" is the term used to describe a data transfer and distribution system.
The wires between the control units on which data are transmitted are referred to as signal wires.
Data are transmitted serially on these signal wires to the connected control units (e.g. engine speed, accelerator pedal position).

Checking bus system

If fault table indicates that check should be made on data exchange between control units (engine, gearbox, ABS/EDL) and dash panel insert or air-conditioner operating unit:

Check for proper connection of multi-pin connectors of control units.

Read measured value block, display group 125, ignition on
=> Page 01-106.

Display on VAS 5051:

Check display in zones -2- and -3-.
Specifications:
ABS 1
Dash 1

Touch s key to switch to display group 126.
Check display in zone -1-.
Specification:
AC 1

Terminate function "08 - Reading measured value block" by touching ◂ key.
Touch "06 - End of output".

If a reading does not match specification:

Display on VAS 5051:

From list -1- select diagnosis function "00 - Interrogating fault memory - entire system".
Fault memories of all vehicle systems with self-diagnosis capability are interrogated

If a control unit responds with its identification, the number of stored faults or "No faults detected" appears on the display.

Any stored system faults are consecutively displayed and printed out. VAS 5051 then transmits the next address word.

If a fault relating to "Drive data bus..." or "...CAN bus" is displayed:

Check whether correct components (control unit for engine, gearbox, ABS/EDL and dash panel insert or air-conditioner operating unit) have been fitted for the vehicle concerned (part number and encoding).

Check CAN bus system if correct control units have been fitted.

Testing "two-wire bus system"

Communication between three or more control units by way of a "two-wire bus system"

Assess faults stored in control units.

Note:

Assessment helps to localise wiring fault.

Example 1:

Faults entered in fault memories reveal that control unit 1 has no link with control units 2 and 3.

Control unit · Faults entered in fault memory
1 · - No message from control unit 2 / - No message from control unit 3
2 · - No message from control unit 1
3 · - No message from control unit 1

Switch off ignition.
Disconnect control units linked by bus wires and examine bus wires for open circuit.

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

Replace control unit 1 if no faults are found in bus wires.

Example 2:

Faults entered in fault memories reveal that control unit 2 has no link with control units 1 and 3.

Control unit · Faults entered in fault memory
1 · - No message from control unit 2
2 · - No message from control unit 1 / - No message from control unit 3
3 · - No message from control unit 2

Switch off ignition.
Disconnect control units linked by bus wires and examine bus wires for open circuit.

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

Replace control unit 2 if no faults are found in bus wires.

Example 3:

Faults entered in fault memories reveal that none of the control units can transmit or receive.

Control unit · Faults entered in fault memory
1 · - Drive data bus defective
2 · - Drive data bus defective
3 · - Drive data bus defective

Switch off ignition.
Disconnect control units linked by bus wires and examine bus wires for short to positive and earth.

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

If cause of "Drive data bus defective" fault is not found in bus wires, check whether one of the control units is responsible for the problem.

All control units using CAN bus for communication still disconnected; ignition off.

Connect one of the control units.
Connect up vehicle diagnostic, testing and information system VAS 5051. Switch on ignition and erase fault memory of control unit just connected. Terminate output with diagnosis function "06 - End of output".
Switch ignition off and on again.
Leave ignition switched on for 10 seconds. Then use fault reader to read out fault memory of control unit just connected.
If fault "Drive data bus defective" is read out, replace control unit just connected.
If fault "Drive data bus defective" is not read out, connect next control unit and repeat sequence.