Failure Of The Bus Cable: Notes

The following faults can occur to the I/K-Bus wiring:

Short Circuit to B+
Short Circuit to B-
Bus line down (open)
Defective plug connections (damaged, corroded, or improperly crimped)

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. 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.

The following are some examples of scope patterns that may be observed when checking the I/K-Bus.

Fig 1: BMW Measuring System Oscilloscope Display

Courtesy of BMW OF NORTH AMERICA, INC.

The example shown above is of a correctly operating K-Bus signal. The high portion of the signal is approximately 12 volts. The signal is active when communication is occurring of the bus.

Fig 2: Identifying Flat Line At 12 Volts

Courtesy of BMW OF NORTH AMERICA, INC.

No communication is taking place. The bus may be temporarily off line or shorted to B+.

Fig 3: Identifying Flat Line At 5 Volts

Courtesy of BMW OF NORTH AMERICA, INC.

No output voltage from the Main (master) or standby controllers. Bus line may be open or control unit may be defective.