Audi Workshop Service and Repair Manuals

Checking data wire (CAN bus) between several control units   Note t  The exchange of data between the various control units employs a data bus. t  The term “CAN data bus” refers to a system for transmitting and distributing data. t  The wires which connect the control units and convey the data between them are called data wires. t  Data are transmitted serially along these data wires to the connected control units (e.g. engine speed, accelerator pedal position). Checking bus system If the fault table indicates that the data exchange between the engine control unit and control units with CAN capability should be tested. –  Check that multi-pin connectors for control units are properly seated. –  Connect up the vehicle diagnostic and service information system -VAS 5052- and select the engine electronics control unit. Engine must be idling.

Display on tester: –  From list -1- select diagnostic function “Read measured value block”.

Display on tester: 1 -  Enter display group (highest possible entry = 255)   Note Measured value blocks 125 and 126 indicate the drive train CAN bus users. –  Use keypad -2- to enter “125” for “display group number 125” and confirm.

Display on tester: –  Check displays in zones -1- to -4-. The display shows the control units capable of CAN communication with the engine control unit: t  No display: Control unit without CAN capability t  Display 1: Control unit with CAN capability is data bus user t  Display 0: Control unit with CAN capability is not data bus user –  Perform the same check for display group number 126. –  Terminate “Read measured value block” function.

Display on tester: –  Select diagnosis function “End output” from the menu -1-.

Display on tester: –  From list -1-, select diagnostic function “Interrogate fault memory - all fault menus”. t  Fault memories of all vehicle systems with self-diagnosis capability are interrogated When a control unit answers with its identification the number of stored faults appears on the display or “No fault recognised”. Any stored system faults are displayed consecutively. If a fault relating to “Drive train data bus...” or “...CAN bus” is displayed: –  Check whether vehicle has been fitted with correct engine control unit and other control units with CAN capability (part no. and code). If the correct versions are installed: –  Check that multi-pin connectors for control units are properly seated. If multi-pin connectors are properly attached: –  Check CAN bus system. Communication takes place between three or more control units by way of a “two-wire bus system”. –  Evaluate faults stored in control units.   Note This will help to trace a fault in the wiring. Example 1:

The faults entered in the fault memories indicate that there is no connection between control unit 1 and control units 2 and 3.

Control unit	Faults entered in fault memory:
1	t  No message from control unit 2 t  No message from control unit 3
2	t  No message from control unit 1
3	t  No message from control unit 1

–  Switch off ignition. –  Disconnect the control units linked by the bus wires and check for open circuit in one of the bus wires → Current flow diagrams, Electrical fault finding and Fitting locations. –  Renew control unit 1 if no faults are found in the bus wires. Example 2:

The faults entered in the fault memories indicate that there is no connection between control unit 2 and control units 1 and 3.

Control unit	Faults entered in fault memory:
1	t  No message from control unit 2
2	t  No message from control unit 1 t  No message from control unit 3
3	t  No message from control unit 2

–  Switch off ignition. –  Disconnect the control units linked by the bus wires and check for open circuit in one of the bus wires → Current flow diagrams, Electrical fault finding and Fitting locations. –  Renew control unit 2 if no faults are found in the bus wires. Example 3:

The faults stored in the fault memories indicate that none of the control units can transmit or receive.

Control unit	Faults entered in fault memory:
1	t  Drive train data bus defective
2	t  Drive train data bus defective
3	t  Drive train data bus defective

–  Switch off ignition. –  Disconnect the control units linked by the bus wires and check the bus wires for short to positive and earth → Current flow diagrams, Electrical fault finding and Fitting locations. If the cause of the fault “Drive train data bus defective” cannot be found in the data bus wires, check whether one of the control units is causing the fault.   Note Vehicle diagnosis and service information system -VAS 5052- connected and vehicle self-diagnosis selected At this stage all the control units which communicate via the CAN data bus are disconnected. Ignition is switched off. –  Connect one of the control units. –  Switch on ignition.

Display on tester: –  From list -1- select relevant vehicle system. –  Interrogate and erase fault memory of control unit just connected. –  Select diagnosis function “End output” from the menu -1-. –  Switch ignition off and on again. –  Leave ignition switched on for 10 seconds. Then use fault reader to read out fault memory of control unit just connected. –  If the fault “Drive train data bus defective” is now indicated, replace the control unit which has just been connected. –  If the fault “Drive train data bus defective” is not indicated, connect the next control unit, and repeat the above procedure.