Transmission Control Systems: Testing and Inspection

CAN-Bus

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 to individual control modules. A slowly dropping battery voltage when the battery is almost discharged can also lead to fault code entries as not all control modules switch off simultaneously due to low voltage.

Terminal resistors
A 120 Ohms 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 Ohms (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 kOhms to 50 kOhms.

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.

Troubleshooting
Failure of a control module or communication module normally triggers a fault code entry in the other control modules connected to the bus.

Under certain circumstances, failure of a communication module in a control module can paralyze the entire CAN-bus, i.e. during communication with all other control modules fault codes are entered in all control modules on the bus.

In this case, one control module can be disconnected at a time and the control modules remaining on the bus then detect the disconnected control module (after disconnecting, clear the fault memories and then read the fault memories). If the disconnected control module is the defective one no other CAN fault codes will be entered concerning communication between the control modules remaining on the bus.

If, however, control modules with a terminal resistor are disconnected using this method, the now missing terminal resistance on the bus may also paralyze communication. This test is therefore not suitable in all cases it is, however, very helpful.