Audi A3 Workshop Service and Repair Manuals

The fault table may instruct you to test the data bus:

• ‒ By referring to the appropriate current flow diagram, check how many control units communicate via the databus.
• ‒ Press key 0 twice for the function "Automatic test sequence".

• Before testing the bus wires, make sure that there are no faults in any of the control units which are connected to the bus.

• A fault will upset the communication with other control units.

If a fault exists

• Rectify this fault first:

• ‒ Make a note of any faults and erase the fault memories of all control units => Interrogating and erasing fault memory of the relevant control unit.
• ‒ Press keys 0 and 6 for the function "End output" and confirm entry with the Q key.
• ‒ End the output on the fault reader with the function "End output" .
• ‒ Rectify the faults as described in the fault table in the relevant Workshop Manual.

When all faults have been rectified:

• ‒ If communication between the control units is still not working properly, test the bus wires.

When tracing faults in the bus wires, distinguish between two possible cases:

• ◆ Two control units communicating via a "two-wire bus system"
• ◆ Three or more control units communicating via a "two-wire bus system".

Two control units communicating via a "two-wire bus system"

• ‒ Switch off ignition.
• ‒ → Disconnect both control units.
• ‒ Check for open circuit in one of the bus wires

=> Current Flow Diagrams, Electrical fault Finding and Fitting Locations

• ‒ → Check for short circuit between the two bus wires

=> Current Flow Diagrams, Electrical fault Finding and Fitting Locations

• ‒ → Check for short to positive or short to earth in one of the bus wires.

If no fault can be found in the bus wires:

• ‒ As a trial measure, replace whichever control unit is easier (or less expensive) to change.
• ‒ If communication between the control units via the bus is still not working properly, replace the second control unit.

Three or more control units communicating via a "two-wire bus system"

• ‒ Read out the faults stored in the control units.

Example 1:

From the faults present in the fault memories, you can see that control unit 1 has no connection to control units 2 or 3.

• ‒ Switch off ignition.
• ‒ Disconnect the control units which are linked by the bus wires and check whether there is an open circuit in one of the bus wires

=> Current Flow Diagrams, Electrical fault Finding and Fitting Locations

• ‒ If no fault can be found in the bus wires replace control unit 1.

Example 2:

From the faults present in the fault memories, you can see that control unit 2 has no connection to control units 1 or 3.

• ‒ Switch off ignition.
• ‒ Disconnect the control units which are linked by the bus wires and check whether there is an open circuit in one of the bus wires

=> Current Flow Diagrams, Electrical fault Finding and Fitting Locations

• ‒ If no fault can be found in the bus wires replace control unit 2.

Example 3:

From the faults present in the fault memories you can see that none of the control units are able to transmit or receive signals.

• ‒ Switch off ignition.
• ‒ → Disconnect the control units which are linked by the bus wires and check for a short circuit between the bus wires.

=> Current Flow Diagrams, Electrical fault Finding and Fitting Locations

• ‒ → Check the bus wires for short to positive and short to earth.

• ‒ If no fault can be found in the bus wires, check whether one of the control units is responsible for causing a short circuit between the bus wires or a short to positive or earth.

• Disconnect each of the control units which are linked by the bus wires in turn, and check whether this rectifies the fault "Hardware defective".
• If this is the case, replace the control unit which has just been disconnected. If the fault has not been rectified, reconnect the control unit and disconnect the next control unit which is linked by the bus wires.