Description Of Bus Diagnosis

Brief description: 

The data exchange of the control units is across various bus systems with different features. These bus systems are interconnected via so-called gateways. The task of these gateways is to interchange data between the connected bus systems.

The bus structure is shown in the adjacent system wiring diagram.

The individual buses are to assigned to the following functional ranges in the vehicle:

• PT-CAN: control units in the area of drive and chassis and suspension
• K-CAN: control units in the area of body electronics (E65: K-CAN system and K-CAN peripherals)
• F-CAN: control units in the area of the chassis electronics (not E65)
• Local Controller Area Network: peripheral control units in the area of powertrain
• byteflight: airbag control units (E65, E60)
• MOST: control units in the area of audio and communication

If faults occur in this communication framework, fault entries are created in the control units involved. Here, a distinction can normally be made between line faults and logical faults such as missing messages. It should be borne in mind that a fault cause generally causes a number of fault entries in different control units.

The following fault causes can lead to bus faults:

• Short circuit of a bus line
• Open circuit of a bus line
• Fault in a gateway
• Fault in the transmitter or receiver of a control unit

This procedure evaluates the fault entries as a whole. The evaluation of the combination of existing fault entries provides the most probably fault cause. The analysis is restricted to the CAN and byteflight systems (E65, E60).

If there has been an undervoltage situation in the vehicle, bus faults can also (erroneously) be entered. Check whether an undervoltage fault is stored in more than one control unit. If this is the case, there is no further evaluation of the bus faults; the fault cause can be found in the area of the voltage supply.