CAN Bus Topology

Introduced with the 1993 740i/iL, the Controller Area Network (CAN) provides a fast communication link between the Engine and Transmission Control Modules serving as both an input and output multiple signal bus. The CAN consists of two wires (CAN High and CAN Low) with a ground shield. In it's the early configuration, the CAN bus is a simple end user bus system.







With the introduction of the 1995 750iL, the CAN bus was expanded to include both engine control modules, transmission control module, EML IIIs control module and the DSC II control module.

A "star coupler connector" was used to link the individual CAN bus leads from the modules into one common connector strip.







The 740i/iL models do not use the star connector. The single DME, AGS and ASC control modules are linked at a splice point in the wiring harness.

The 1998 model year brought forth the next generation of CAN Bus communicators. The Instrument Cluster and Steering angle sensor of the DSC III system where also linked to the CAN bus to expand the available signalling capabilities.

The Star connector continues to be used for the 75OiL but wiring layout (topology) was changed to improve communication integrity in the event of a break or short in another section of the CAN bus. This was accomplished by using the DME control modules as gateways for the EML and AGS control modules.

Additionally, twisted pair wiring is used for the Instrument Cluster and Steering Angle Sensor which provides EMI protection without a separate ground shield.







The CAN bus of the 1998 740i/iL and 540i has also changed. The entire CAN bus is twisted pair wiring. The EGS is linked to the DME by a dedicated twisted pair CAN bus with a common twisted pair to all other control modules.

The CAN bus must be considered when diagnosing an engine control system. Faults may manifest due to a breakdown in a segment of the CAN bus with another control module causing a fault to display in the engine control module only due to lack of CAN bus data. When diagnosing a specific system, always refer to the ETM for up-to-date configurations.