Bus Diagnosis: Introduction

In the vehicles of today, components and control units are networked by means of data buses. Data buses are capable of transmitting messages with signals.

The connected control units only read off those messages and signals that are of relevance to their operation.

Most buses are CAN buses (CAN: Controller Area Network). There are several CAN buses with different data transmission rates in each car.

For example, the PT-CAN has a fast data transmission rate, the K-CAN a slower data transmission rate.

A fibre-optic cable is used for navigation and entertainment: the MOST bus (MOST = "Media Oriented System Transport")

There is a separate data wire for diagnosis: the diagnosis wire, also known as the "K-wire".

The following options are available for locating faults in data buses and in control units:

• Test module for CAN bus diagnosis in the BMW diagnosis system: "Bus system analysis" 

  The test module is called up in the DIS (Diagnosis and Information System) as follows: "Function selection" button > Complete vehicle -> Body -> Bus functions -> Bus analysis -> System analysis

• Checking the terminating resistances: 

  Checking the terminating resistances can also be important for bus diagnosis.

• Test module for diagnosis on the MOST buses: "MOST system analysis" 

The test module is called up in the DIS (Diagnosis and Information System) as follows: "Function selection" button > Complete vehicle -> Body -> Bus functions -> MOST functions -> MOST system analysis.

These two test modules and the installation points of the terminating resistances are described in detail below.