Initial Inspection and Diagnostic Overview

CAN-Bus, function
Bus: "Bus" is a system for transporting and distributing data.
CAN: Controller Area Network (CAN) is a Bus-system that works with two wires called Bus-wires. Via the Bus wires the data are transferred in series, meaning that they are transferred one after the other to the connected control modules. The control modules communicate using the CAN-Bus, meaning that they exchange data with the help of the CAN-Bus.





Layout of the data Bus drive
1. Transmission Control Module (TCM) -J217
2. Bus-driver
3. Engine Control Module (ECM) -J220
4. Bus-driver
5. End resistor





6. Data-Bus drive (CAN-Bus, twisted 2-wire cable)
7. Closing resistor
8. Bus-driver
9. ABS Control Module (J104)

Two-wire Bus system, checking
The DTC table Indicated Bus must be checked
^ Determine by using the applicable wiring diagram how many control modules are communicating on the Bus
^ Press button 0 two times for function "Automatic tests
^ Before checking the Bus wiring, check DTC memory for stored DTC for any of the control modules that are connected to the Bus.
A malfunction will interfere in the communication with other control modules.

If yes:
^ Repair malfunction first.
^ Make note of the malfunction and erase the memory of all control modules; Refer to Check DTC memory and erase DTC memory of the applicable control module.
^ Press buttons 0 and 6 for function "End output' and press Q button to confirm input.
^ Conclude output of VAG 1551 scan tool by function "End output".
^ If communication between control modules is still not OK, check Bus-wiring.

When troubleshooting Bus wiring, there are two possibilities:
^ Two control modules communicate through a 'two-wire Bus system"
^ Three or more control modules communicate through a "two-wire Bus system"

Two control modules communicate through a "two-wire Bus system"
^ Switch off ignition.





^ Disconnect both control modules.
^ Check for open circuit in one of the Bus wires.
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations binder





^ Check for short circuit in Bus wires one against the other.
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations binder





^Check for open or short circuit to plus (B+) or to Ground (GND).

If Bus-wiring is OK:
^ Replace on a trail basis the least expensive control module and the one easiest to get to.
^ If the communication is still not OK, replace the second control module.

Three or more control modules communicate through a "two-wire Bus system"
^ Check DTC memory of the control modules.





Example 1:





Based on DTC's stored in memory of the control modules you can tell that control module 1 has no connection with control modules 2 and 3.
^ Switch off ignition.
^ Disconnect control modules connected by Bus wiring and check each Bus-wire for open circuit
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations binder
^ If Bus-wiring is OK, replace control module 1.





Example 2:





Based on DTC's stored in memory of the control modules you can tell that control module 2 has no connection with control modules 1 and 3.

^ Switch off ignition.
^ Disconnect control modules connected by the Bus wiring and check Bus-wires for open circuit
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations binder
^ If Bus-wiring is OK, replace control module 2.

Example 3:





Based on DTC's stored in the memory of the control modules you can tell that none of the control modules transmit or receive.

^ Switch off ignition.





^ Disconnect Bus-wiring from all control modules and check wiring for short circuit between wires.
Refer to electrical Wiring Diagrams, Troubleshooting & Component Locations binder





^ Check Bus-wiring for open or short circuit to plus (B+) or to Ground (GND).

^ If Bus wiring are OK, check for short circuit between all control modules and Bus wiring to plus (B+) or Ground (GND).
Disconnect one control module after the other and check whether fault "Hardware faulty" has been corrected.
If yes, replace the disconnected control module. If not, reconnect the control modules again and disconnect the next control module connected to the Bus.