K and D-Bus Faults: Notes

The failure of communication on the K or D-Bus can be caused by several sources:

Failure of the bus cable.
Failure of one of the control units attached to the bus.
Failure of the voltage supply or ground to individual modules.
Interference in the bus cables.
Bus interrupted
Bus shorted to battery voltage
Bus shorted to ground
Bus line open
Defective plug connections (damaged, corroded, or improperly crimped)

If problems are encountered trying to establish communication with a control module consider first:

Battery charge level of the vehicle. Maintain a battery charger on the vehicle at all times during diagnosis.
Always check that the diagnosis head and connection are OK before working through a test module for lack of communication.

If identification of the vehicle is carried out by the diagnostics without any problems then the D-Bus is OK.

If several control units are not recognized this indicates that a bus link is defective. Continue troubleshooting using the test modules for those particular bus systems.

Short Circuit to B+: Modules that send a message see that the message was not received and that the bus remains high. However, subscribers are unable to decide whether the fault is due to a shorted line or a defect in the communication interface. The module will repeat its message 5 times before discontinuing and faulting. The module will continue to operate as normal minus any commands that could not be delivered by the bus.

Short Circuit to B-: The subscribers do not interpret a low bus line as a fault but just as a bus line deactivation. The Master and Standby controllers do detect the short and enter it as a bus fault. (No communication).

Bus Line Down: The bus line may be open at any of several locations. As long as the Master or Stand-by is still connected, communication can occur with any modules still remaining. The fault situation will be the same as if the disconnected modules were defective themselves.

Checking the bus line is carried out just like any other wiring. Perform continuity tests between the connections of different modules (all modules disconnected) without forgetting to make sure that the bus has not shorted to ground or another wire.

NOTE: It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.

If Voltage level and the wire test are O.K then looking at the communication signal may be useful. In order to get a signal, operate different devices on the I/K-Bus (e.g. MID/MFL) to stimulate conversations.