Diagnosis on CAN-Bus

Diagnosis on CAN-Bus
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Causes
The failure of communication on the CAN-bus (i.e. fault code entries relating to bus communication in the individual control modules may be caused by the following:
- Breaks in line (open circuits) or short-circuits in the communication lines
- Interference voltages in the vehicle electrical system caused for instance by defective ignition coils or ground connections
- Failure of the communication modules in the individual control modules
- Failure of the voltage supply of individual control modules (a battery voltage decreasing gradually when the battery is almost discharged can also lead to fault code entries as not all control modules switch off simultaneously due to the voltages supply being too low).
Terminal resistors
A 120 Ohm terminal resistor is installed in each of two control modules of the CAN network between the communication lines CAN-H and CAN-L. A resistance of 60 Ohm (parallel connection) can therefore be measured between the two communication lines in the control module network. The lines can be checked easily by measuring the resistance with an adapter at one of the control modules. When disconnected the resistances of both control modules can be measured directly. Control modules without terminal resistors normally show a value from 10 kOhm to 50 kOhm. The terminal resistors are accommodated in the ASC/DSC control module and, depending on the type of engine, either in the instrument cluster or in the engine management.
Cause
Failure of a control module or communication module normally triggers a fault code entry in the other control modules connected to the CAN-bus.
Under certain circumstances, failure of a communication module in the control module may paralyze the entire CAN-bus. Faults in communication with all other control modules are entered in all control modules connected to the CAN-bus.