Specifications

Testing Testing Bus Systems

• No messages are discarded due to lack of time.

• Errors are detected by each controller to determine the severity of a problem.

• Modules automatically disconnect if a particular module is malfunctioning.

• Operation continues on one wire if either the CAN High or CAN Low wires break (only true for

CAN B medium speed; not true for CAN C high speed).

CAN data transfer speeds may vary depending on the CAN type.

• CAN type A: 83.3 kb/s

• CAN type B: 125 kb/s

• CAN type C: 500 kb/s

BMW uses the high speed CAN C for Powertrain control, but the scan tool may update slowly.

Before the data is transferred to a scan tool, it goes through a gateway module which acts as a

translator so different buses can talk to each other. The gateway, changes the fast CAN high

speed data to a slower speed D Bus (serial communication) which is what the scan tool reads.

NOTE:

i CAN bus faults may turn on the ASC/DSC light.

CAN Bus Operational Check

Use the following procedure to check the operation of a CAN bus.

z To check CAN Bus operation:

1. Check the instrument gauges for correct operation.

2. Check that the transmission range selector is functioning.

If these controls operate, the CAN bus is up and running.

CAN Bus Components

The CAN data bus is comprised of the following components:

• A controller receives the transfer data from the microcomputer integrated in the control unit.

The CAN controller processes this data and relays it to the CAN transceiver. Likewise, the

CAN controller receives data from the CAN transceiver, processes it and relays it to the

microcomputer integrated in the control unit. The data bus does not have a designated

receiver. Data is sent over the data bus and is generally received and evaluated by all

subscribers.

• A transceiver is a transmitter and receiver in one. It converts the data which the CAN

controller supplies into electrical signals and sends this data over the data bus lines. Likewise,

it receives data and converts this data for the CAN controller.

• Two data bus terminals are resistors that prevent data sent from being reflected at the ends

and returning as an echo, which would corrupt the data.

• Two data bus lines are bidirectional and transfer the data. They are referred to as CAN High

and CAN Low.

Apart from the data bus lines, the components are located in the control units. The functions of the

control units are the same as before.