Audi Workshop Service and Repair Manuals

Notes:

• ◆ Data is exchanged between individual control units by means of a bus system.
• ◆ "Bus" is used to describe a system that transports and distributes data.
• ◆ The wires between the control units that are used to transfer the data are known as signal wires.
• ◆ These signal wires are used for serial transmission of data, i.e. data is sent to each of the connected control units in turn.

Testing the bus system

The test in principle is unaffected by the number of control units/systems connected to the databus. As an example the following test is conducted on three control units connected to the bus system.

The fault table includes instructions to test the data exchange between engine control unit, gearbox control unit and ABS control unit.

• ‒ Check that the vehicle is fitted with the correct engine control unit, gearbox control unit and ABS control unit (part number) and that coding of control units is correct. (Display the control unit identifications for this purpose). If the correct control units are installed:

• ‒ Interrogate the fault memory of the gearbox control unit.

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

• ‒ Interrogate the fault memory of the ABS control unit.

=> Running Gear FWD and 4WD; Repair group 01; Self-diagnosis; Electrical tests

If a fault is displayed relating to "Data bus..." or "...communication":

• ‒ Switch ignition off.

• ‒ → Connect V.A.G 1598/31 test box to wiring harness leading to engine control unit; the engine control unit should not be connected =>Page 24-22.
• ‒ Check the signal wires between pin 60 and pin 58 of the engine control unit and the gearbox control unit (if fitted) and the ABS control unit for short or open circuit using the current flow diagram.

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

• ‒ If the wiring is OK, try installing a new engine/gearbox/ABS control unit in turn.

If the communication between three or more control units takes place via a "two-line bus system", the following systematic procedure for fault-finding will be useful:

• ‒ Evaluate the faults stored in the control units.

This helps to localise line faults.

Example 1:

The faults stored in the fault memories indicate that the control unit 1 does not communicate with control units 2 and 3.

• ‒ Switch ignition off.
• ‒ Disconnect the control units connected to each other via the bus lines and investigate whether there is a short circuit between the bus lines.

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

• ‒ If no fault can be detected in the bus lines exchange control unit 1.

Example 2:

The faults stored in the fault memories indicate that the control unit 2 does not communicate with control units 1 and 3.

• ‒ Switch ignition off.
• ‒ Disconnect the control units connected to each other via the bus lines and investigate whether there is a short circuit between the bus lines.

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

• ‒ If no fault can be detected in the bus lines exchange control unit 2.

Example 3:

The faults stored in the fault memories indicate that sending or receiving operations are not possible in any of the control units.

• ‒ Switch ignition off.
• ‒ → Disconnect the control units that 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 cause of fault "Data bus drive defective" cannot be found in bus lines check whether one of the control units is responsible for the fault.

All the control units that communicate via the CAN data bus are still disconnected. The ignition is switched off.

• ‒ Connect one of the control units.
• ‒ Connect fault reader V.A.G 1551. Switch the ignition on and erase the fault memory of the control unit that has just been connected Terminate fault readout by selecting 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 fault "Signal wire defect" is not read out, connect the next control unit and repeat the procedure.