Section 361 (Electronic Control Modules (Service Information)): Communication: Description

WARNING: This page is about a different car, the 2005 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.

The Controller Area Network (CAN) is a serial data bus communication network used for interconnecting numerous electronic control modules throughout the vehicle in a two-wire multiplexed system. Within this context the term serial refers to electronic data that is transferred bit by bit, while bus refers to the shared wires through which that data is transferred. Multiplexing is any system that enables the transmission of multiple messages over a single channel or circuit. The communication protocol being used is a non-proprietary, open standard adopted from the Bosch CAN Specification 2.0b and uses an 11-bit message identifier.

There are actually three separate CAN bus systems used in the vehicle. They are designated: the CAN-B, the CAN-C and the Diagnostic CAN-C. The CAN-B and CAN-C systems provide on-board communication between all nodes in the vehicle. The CAN-C is the faster of the two systems providing near real-time communication (500 Kbps), but is less fault tolerant than the CAN-B system. The CAN-C is used exclusively for communications between critical powertrain and chassis nodes. The slower (83.3 Kbps), but more fault tolerant CAN-B system is used for communications between body and interior nodes. The CAN-B fault tolerance comes from its ability to revert to a single wire communication mode if there is a fault in the bus wiring.

The Diagnostic CAN-C bus is also capable of 500 Kbps communication, and is sometimes informally referred to as the CAN-D system to differentiate it from the other high speed CAN-C bus. A central gateway or hub integral to the Front Control Module (FCM) physically and electrically isolates the three CAN buses from each other and coordinates the bi-directional transfer of messages between the three buses. The FCM is located on the Integrated Power Module (IPM), which is located in the engine compartment near the battery. The Diagnostic CAN-C is used exclusively for the transmission of diagnostic information between the FCM/gateway and a diagnostic scan tool connected to the industry-standard 16-way Data Link Connector (DLC) located beneath the instrument panel on the driver side of the vehicle.

Each node is connected in parallel to its CAN-B or CAN-C bus using a two-wire twisted pair. These wires are wrapped around each other to provide shielding from unwanted electromagnetic induction interfering with the relatively low voltage signals being carried through them. The twisted pairs have between 33 and 50 twists per meter. While the CAN bus is operating, one of the bus wires will carry a higher voltage and is referred to as the CAN High or CAN bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN Low or CAN bus (-) wire. Each twisted pair terminates at the FCM/gateway.

The added speed of the CAN data bus is many times faster than previous data bus systems. This added speed facilitates the addition of more electronic control modules or nodes and the incorporation of many new electrical and electronic features in the vehicle. Like prior data bus systems, the CAN data bus minimizes redundant wiring connections; and, at the same time, reduces wire harness complexity, sensor current loads and controller hardware by allowing each sensing device to be connected to only one node. Each node reads, then broadcasts its sensor data over the bus for use by all other nodes requiring that data.