Audi Workshop Service and Repair Manuals

Notes:

• ◆ Data is exchanged between individual control units by means of a bus system.
• ◆ "CAN 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.

Checking the bus system

Proceed as follows when fault table stipulates checking of data exchange on bus.

• ‒ Check that multiple connectors for control units are properly seated.

• ‒ Connect the vehicle diagnosis, measurement and information system VAS 5051 or fault reader V.A.G 1551 .
• ‒ Switch on ignition
• ‒ Switch on printer using the Print key
  (Warning lamp in switch illuminates).
• ‒ Press key 1 for "Rapid data transfer".

Note:

Address word 00 is used to start the automatic test sequence i.e. interrogation of fault memories of all vehicle systems with self-diagnosis capability in "Rapid data transfer" mode.

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

Any system faults stored will be displayed in sequence and printed out. The V.A.G 1551 will then transmit the next address word.

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

• ‒ Check to make sure that the vehicle is fitted with the correct engine control units (part number and encoding).

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

Checking a "two-wire bus system"

The communication between three or more control units is carried out via a "two-wire bus system".

• ‒ Evaluate the faults stored in the control units.

This helps to locate wiring faults.

Example 1:

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

• ‒ Switch ignition off.
• ‒ 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 detected in the bus wires, exchange control unit 1.

Example 2:

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

• ‒ Switch ignition off.
• ‒ 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 detected in the bus wires exchange control unit 2.

Example 3:

With the faults stored in the fault memories it can be seen that there is no sending or receiving operation 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" binder

• ‒ → Check bus wires for shorts to positive and earth as well as open circuit.

If the causes of the fault, e.g. "Group convenience data bus defective" cannot be found in bus wires, 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. 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 "Group convenience data bus defective" is now read out, replace the control unit which has just been connected.
• ‒ If the fault "Group convenience data bus defective" is not read out, connect the next control unit and repeat the procedure.