CAN/Class 2 Communications

WARNING: This page is about a different car, the 2003 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The class 2 serial data circuit 1807 is used to communicate information between modules, for example the body control module and the engine controller.

Contained in each modules memory is a list of serial data messages that are important and should be received by the module. Also contained are the default actions to be taken by the module, if one of the important messages is not received in time. One of those important messages is the state of health (SOH) message. This message is sent by a module, if no other message need to be sent, to let all the other modules on the serial data circuit know the sending module is working correctly.

Each time the ignition is turned ON, all the modules on the class 2 serial data circuit first learn the other modules connected to the class 2 serial data circuit. Modules are able to learn the other modules on the serial data circuit because each of the important messages has a source identifier as part of the message.

This process of learning by a module allows the module to know if it has lost communication with a specific module on the class 2 data circuit.

The ECM, electronic brake traction control module (EBTCM), and the transaxle control module (TCM) communicate with each other over the CAN (controller area network) bus circuit 2501 and circuit 2500 (V6 only). The CAN bus is a communication link similar to class 2.

The BCM acts as a gateway between the other modules connected to the class 2 serial data circuit 1807 and the ECM, EBTCM, and TCM connected to the CAN bus circuit 2501 and circuit 2500.

State of health (SOH) messages are sent from the ECM, EBTCM and TCM to the BCM when the ignition is turned ON and during vehicle operation. These messages tell the BCM the ECM, EBTCM and TCM are working correctly. The BCM has an internal reset counter to monitor the status of the CAN bus. When the counter reaches a defined value and no CAN messages have been received from either the ECM, EBTCM or TCM, the BCM sets DTC U2104 - CAN Bus Reset Counter Overrun.