Audi Workshop Service and Repair Manuals

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.

Checking bus system

If fault table indicates that data exchange on bus should be checked.

Requirements:

• ● Vehicle diagnostic, testing and information system VAS 5051 connected up and vehicle self-diagnosis selected
• ● Ignition on

Sequence of operations

→ 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
• ‒ Wait until next display appears.

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. VAS 5051 then transmits the next address word.

If a fault relating to "Data bus..." or "...no communication" is displayed:

• ‒ Check whether correct control units have been fitted for vehicle (part number and code).

If correct control units have been fitted:

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

If multi-pin connectors are properly attached:

• ‒ Check CAN bus system.

Testing "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.

• ‒ 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.

• ‒ 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 memory reveal that none of the control units can transmit or receive.

• ‒ Switch off ignition.

• ‒ → Disconnect control units linked by bus wires and examine bus wires for mutual short circuit.

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

• ‒ → Examine bus wires for short to positive/earth.

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

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

Test requirement:

• ● Vehicle diagnostic, testing and information system VAS 5051 connected up and vehicle self-diagnosis selected => Page 01-3

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

• ‒ Connect one of the control units.

→ Display on VAS 5051:

• ‒ Switch on ignition.
• ‒ From list -1- select appropriate vehicle system.
• ‒ Interrogate and erase fault memory of control unit just connected.
• ‒ Touch ◂ key.

• ‒ From list -1- select 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 train data bus defective" is read out, replace control unit just connected.
• ‒ If fault "Drive train data bus defective" is not read out, connect next control unit and repeat sequence.