Audi Workshop Service and Repair Manuals

Checking “two-wire bus system” Test sequence –  Use the corresponding current flow diagram to determine how many control units communicate with one another by way of the bus. –  Connect vehicle diagnostic, testing and information system -VAS 5051B- → Chapter and select function “00 - Automatic test sequence”. When doing this, the ignition must be switched on. –  Before examining the bus wires, make sure that there is no malfunction in any of the control units linked to the bus. Malfunctions interfere with communication with other control units.   Note The term malfunction refers to a fault which does not directly affect the bus system but in some way disrupts the operating sequence of a system. This may be a defective sensor for example. The consequence is that the sensor signal can no longer be conditioned for the bus system data transfer. Such malfunctions have an indirect effect on the bus system. Communication with the other control units requiring the relevant sensor signal is impaired. Has a malfunction occurred? –  This must first be repaired. –  Print out fault list and erase fault memories of all control units. Refer to “Interrogating fault memory” and “Erasing fault memory” of corresponding control unit. –  Touch function “06 - End output”. –  Eliminate the malfunctions on the basis of the fault tables in the relevant workshop manuals. All malfunctions eliminated? –  If there are still problems with control unit communication, the bus wires must be examined.

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

Control unit	Fault stored 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. –  Unplug connectors at control units linked by bus wires and check for open circuit in one of the bus wires → Current flow diagrams, Electrical fault finding and Fitting locations.

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

Control unit	Fault stored 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. –  Unplug connectors at control units linked by bus wires and check for open circuit in one of the bus wires → Current flow diagrams, Electrical fault finding and Fitting locations.

Example 3: The faults stored in the fault memories indicate that none of the control units can transmit or receive.

Control unit	Fault stored in fault memory
-1-	t  Control unit defective
-2-	t  Control unit defective
-3-	t  Control unit defective

–  Switch off ignition. –  Unplug connectors at control units linked by bus wires and check for mutual short circuit in bus wires → Current flow diagrams, Electrical fault finding and Fitting locations.

–  Check bus wires for short to positive and earth. –  If the cause of the fault “Control unit defective” cannot be found in the bus wires, check whether one of the control units is responsible for the fault. l  Connectors unplugged at all control units using the CAN bus for communication l  Ignition off –  Connect one of the control units. –  Connect up the vehicle diagnostic, testing and information system -VAS 5051B- → Chapter. –  Switch on ignition and select the relevant vehicle system. –  Interrogate fault memory of control unit just connected and erase fault memory. –  Terminate function “05 - Erasing fault memory” by touching ← key. –  Touch function “06 - End output”. –  Switch ignition off and on again. –  Leave ignition switched on for 10 seconds. Then read out fault memory of control unit just connected. If fault “Control unit defective” is displayed: –  Replace control unit just connected. If fault “Control unit defective” is not displayed: –  Connect next control unit and repeat procedure.