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 principle depends on the number of control units/systems connected to the data bus.

The fault table instructed you to check the data exchange between the engine control unit and CAN-compatible control units.

• ‒ Start the engine (if the engine does not start, please switch on the ignition).
• ‒ Connect the vehicle diagnostic, testing and information system VAS 5051 or fault reader V.A.G 1551 and select the engine electronics control unit with "address word" 01 .

• ‒ Enter "08" to select the function "Reading measured value block" and confirm entry with Q key.

• ‒ Enter "125" for "display group number 125" and confirm entry with Q key.

• ‒ Check displays in display zones 1 to 4.

• CAN-compatible control units are displayed with the engine control unit:
• ‒ No display (display zone is blank): Engine control unit not CAN-compatible
• ‒ Display 1:CAN-compatible control unit is a data bus user
• ‒ Display 0:CAN-compatible control unit is not a data bus user (e.g. in case of fault)

Note:

Display group 125 indicates the systems which communicate with the engine control unit via the CAN data bus. (The ABS and instrument cluster control unit in the example). If these systems are not installed, the respective display zones in display group 125 remain blank. "Gearbox 1" would be indicated in display zone 1 and "Air conditioner 1" in display zone 4 if these systems communicate with the engine control unit via "CAN".

Note:

Display group 126 indicates the systems which communicate with the engine control unit via the CAN data bus. If these systems are not installed, the respective display zones remain blank.

The following systems are displayed in display group 126, however only if they are available:

Display zone 1:Automatic distance control; display: "Distance 1".

Display zone 2:Steering angle sensor; display: "Steering angle 1"

Display zone 3:Airbag; display: "Airbag 1".

Display zone 4:Vehicle's electrical system; display: "el. CE 1".

• ‒ Press the ⇒key.

• ‒ Press the 0 key twice for the "automatic test sequence" address word and confirm entry with Q key.
• ‒ The fault memory is then interrogated for all systems in the vehicle which are capable of self-diagnosis

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 is displayed relating to "Data bus drive" or "...CAN bus":

• ‒ Check that the engine control unit and other CAN-compatible control units installed are suitable for this vehicle (part no. and code).

If the correct control units are installed:

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

If the multi-pin connectors are firmly seated:

• ‒ Check the CAN bus system.

Checking a "two-line bus system"

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

• ‒ Evaluate the faults stored in the control units.

This helps to localise line faults.

Example 1:

With the faults stored in the fault memories it can be seen 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:

With the faults stored in the fault memories it can be seen 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:

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

• ‒ → 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.