Circuit Description

WARNING: This page is about a different car, the 2000 Cadillac Catera. However, it is still accessible from the selected car via links, so may be relevant.

The Controller Area Network (CAN) serial data circuit is a high speed serial data bus used to communicate information between the Electronic Brake and Traction Control Module (EBTCM), the Engine Control Module (ECM) and the Transmission Control Module (TCM). When the ignition switch is turned to the ON position, the modules begin to transmit data between each other. The following data is transmitted and received between the ECM and TCM:

Brake Switch signal from the ECM to the TCM
Throttle Position Angle from the ECM to the TCM
Engine Speed signal from the ECM to the TCM
Engine Coolant Temperature from the ECM to the TCM
Engine Torque signal from the ECM to the TCM
MIL Request signal from the TCM to the ECM

The following data is transmitted and received between the ECM and EBTCM:

Requested Torque from the EBTCM to the ECM
Delivered Torque from the ECM to the EBTCM
Engine Speed (RPM) from the ECM to the EBTCM

The addressing scheme employed with CAN assigns a label to every message, with each message receiving a unique "identifier". The identifier classifies the content of the message (such as engine speed). Each module processes only those messages whose identifiers are stored in the module's acceptance list. This is CAN's form of message filtering

The identifier labels both the data content and the priority of the message being sent. Each module can begin transmitting its most important data as soon as the bus is unoccupied. When more than one module starts to transmit simultaneously, the message with the highest priority is assigned first access. A module responds to failure to gain access by automatically switching to receive mode, the module then repeats the transmission attempt as soon as the bus is free again