Audi Workshop Service and Repair Manuals

Notes:

• ◆ The exchange of data between the various control units is carried out via a data bus.
• ◆ The term "CANdata 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 aretransmitted via these data wires in serial form (i.e. in a specific sequence) to the connected control units.

Testing bus system

If the fault table instructs you to test the data exchange between the engine control unit, gearbox control unit and ABS control unit:

• ‒ Call up the relevant "Control unit identification" and check the part Nos. to make sure that the vehicle is fitted with the correct engine control unit, gearbox control unit (if fitted) and ABS control unit. Also check that the control units are correctly coded. If the correct control units are installed in the vehicle:
• ‒ Interrogate fault memory of gearbox control unit (if fitted)

=> Automatic gearbox; Repair group 01; Performing self-diagnosis

• ‒ Interrogate fault memory of ABS control unit

=> Running gear, Front-wheel drive and 4WD; Repair group 01; Self-diagnosis, Electrical check

If faults have been stored relating to the "Data bus drive" or to data transmission: "No message from ...."

• ‒ Switch off ignition

• ‒ Connect test box V.A.G 1598/22 to wiring harness for engine control unit=>Page 24-13.
• ‒ Referring to Current flow diagram, test data wires from engine control unit, pin 31 and pin 29 to gearbox control unit (if fitted) and to ABS control unit for open circuit and short circuit

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

• ‒ If wiring is OK, replace the engine control unit, gearbox control unit or ABS control unit as a trial measure.

If there are three or more control units in the vehicle linked via a "two-wire data bus system", the following systematic method is recommended when tracing faults:

• ‒ Read out and compare the faults registered in the control units.

This will help to localise 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
• ‒ Disconnectthe control units which are linked by the bus wires and check whetherthere 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 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 faultreader V.A.G 1551. Switch on the ignition and erase the fault memory of the control unit which has just been connected. End output from fault reader 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.