CAN-Bus, Testing

The DTC table may instruct you to test the bus:
- By referring to the appropriate wiring diagram, check how many control modules are connected to the data bus.
- Check that the multi-pin connectors on the control modules are properly seated.
- Connect VAG 1551 scan tool or VAG 1552 scan tool and press button -0- twice to select the function "Automatic test sequence"
- Before testing the bus wires, make sure that there are no malfunctions in any of the control modules, which are connected to the bus.

Note: A malfunction registered in one of the control modules does not affect the data bus system directly, but can result from a malfunction of some kind in one of the systems. This may be a faulty sensor, for example. The result is that the sensor signal can no longer be processed for transmission via the data bus. A malfunction of this kind has an indirect effect on the data bus system, and will upset the communication with other control modules which require the signal from the affected sensor.

If a malfunction exists
- Repair this malfunction first:
- Make a note of any codes and erase the DTC memories of all control modules.
- Press buttons -0- and -6- for the function "End Output" and confirm entry with the -Q- button.
- End the output on the scan tool with the function "End Output".
- Repair the malfunctions as described in the DTC table in the relevant Workshop information.

When all malfunctions have been repaired:
If communication between the control modules is still not working properly, test the bus wires.

When tracing faults in the bus wires, distinguish between two possible cases:
^ Two control modules communicating via a CAN-Bus
^ Three or more control modules communicating via a CAN-Bus

Two control modules communicating via a CAN-Bus
- Switch off ignition.





- Disconnect both control modules.





- Check for open circuit in one of the bus wires
- Check for short circuit between the two bus wires





- Check for short to positive or short to Ground (GND) in one of the bus wires.

If no fault can be found in the bus wires:
- As a trial measure, replace whichever control module is easier (or less expensive) to change.
- If communication between the control modules via the bus is still not working properly, replace the second control module.

Three or more control modules communicating via a CAN-Bus
- Read out the DTC's stored in the control modules.

Example 1:








From the codes present in the DTC memories, you can see that control module -1- has no connection to control modules -2- or -3-.





- Switch off ignition.
- Disconnect the control modules, which are linked by the bus wires and check whether there is an open circuit in one of the bus wires
- If no fault can be found in the bus wires replace control module 1.

Example 2:





From the codes present in the DTC memories, you can see that control module -2- has no connection to control modules -1- or 3-.





- Switch off ignition.
- Disconnect the control modules, which are linked by the bus wires and check whether there is an open circuit in one of the bus wires
- If no fault can be found in the bus wires replace control module 2.

Example 3:








From the codes registered by the DTC memories, you can see that none of the control modules are able to transmit or receive signals.
- Switch off ignition.
- Disconnect the control modules, which are linked by the bus wires and check for a short circuit between the bus wires





- Check the bus wires for short to positive and short to Ground (GND).
- If the cause of the malfunction "Faulty hardware" cannot be found in the bus wires, check whether one of the control modules is responsible for causing the malfunction.

At this stage all the control modules which communicate via the CAN-Bus are disconnected.
- Connect one of the control modules.
- Connect VAG 1551 scan tool or VAG 1552 scan tool. Switch on the ignition and erase the DTC memory of the control module, which has just been connected. End Output from scan tool with function 06 "End Output".
- Switch the ignition off and then on again.
- Leave the ignition switched on for 10 seconds. Then check the DTC memory of the control module that has just been connected, using the scan tool.
- If the malfunction "Faulty hardware" is now indicated, replace the control module, which has just been connected.
- If the malfunction "Faulty hardware" is not indicated, connect the next control module, and repeat the above procedure.