Audi Workshop Service and Repair Manuals

Notes:

• ◆ The data exchange between the various control units is carried out via a data bus.
• ◆ The term "CAN data bus" refers to a system for transmitting and distributing data.
• ◆ The wires which connect the control units and convey the data between them are called data wires.
• ◆ Data is transmitted via data wires in sequence, i.e. in a specific order to the connected control units (e.g. engine rpm and accelerator pedal position).

Testing bus system

The fault table indicates that the data exchange between the engine control unit, gearbox control unit and ABS/EDL control unit or dash panel insert should be tested.

• ‒ Check that multi-pin connectors for control units are properly seated.

• ‒ Read data block, Display group 125, ignition switched on=>Page01-136.

→ Display on VAS 5051:

• ‒ Check display in display zones 1 ... 3.
• ‒ Specifications:
  Gear. 1 (automatic gearbox only)
  ABS 1
  Combi 1 (= dash panel insert)
• ‒ Press the ◂ button to exit from the function "08 - Read data block".
• ‒ Select diagnosis function "06 - End output".
• ‒ Check that the engine control unit, gearbox control unit and ABS/EDL control unit or dash panel insert are the correct versions for the vehicle (check Part No. and coding).

If a specification is not met:

→ Display on VAS 5051:

• ‒ Select the diagnosis function "00 - Interrogate all fault memories" from the menu -1-.
• ‒ The fault memory of all on-board systems with self-diagnostic capability will be read.

When a control unit answers with its identification the number of stored faults appears on the display or "No fault recognised".

Any system faults that are stored will be displayed one after the other and printed out. VAS 5051 will then transmit the next address word.

If faults have been stored relating to the "Drive train data bus" or to the "CAN data bus":

• ‒ Check that the engine control unit, gearbox control unit and ABS/EDL control unit or dash panel insert are the correct versions for the vehicle (check Part No. and coding).

If the correct control units are installed, test the CAN data bus system.

Testing a "two-wire bus system"

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

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

Note:

This will help to trace a fault in the wiring.

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 binder

• ‒ 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 binder

• ‒ 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 to positive or earth in the bus wires

=> Current flow diagrams, Electrical fault finding and Fitting locations binder

If the cause of the fault "Data bus drive defective" cannot be found in the data bus wires, check whether one of the control units is causing the fault.

At this stage all the control units which communicate via the CAN data bus are disconnected. The ignition is switched off.

• ‒ Connect one of the control units.
• ‒ Connect vehicle diagnostic, testing and information system VAS 5051. Switch on the ignition and erase the fault memory of the control unit which has just been connected. End output from diagnostic system with function "06 - End output".
• ‒ Switch the ignition off and then on again.
• ‒ Leave the ignition switched on for 10 seconds. Then interrogate the fault memory of the control unit that has just been connected, using the fault reader.
• ‒ If the fault "Data bus drive defective" is now indicated, replace the control unit which has just been connected.
• ‒ If the fault "Data bus drive defective" is not indicated, connect the next control unit, and repeat the above procedure.