Communications Network - Principles Of Operation

COMMUNICATIONS NETWORK

NOTE:The smart junction box (SJB) is also known as the generic electronic module (GEM).

Principles of Operation

Vehicle communication utilizes both International Standards Organization (ISO) 9141 and controller area network (CAN) communications. ISO 9141 is used for diagnostic use only, and CAN is a method for transferring data among distributed electronic modules via a serial data bus.
The vehicle is equipped with 2 communication networks:

- ISO 9141
- High speed (HS) CAN

ISO 9141 Communications Network

The ISO 9141 communications network is a single wire network. The ISO 9141 communications network does not permit intermodule communication. When the IDS or equivalent tester communicates to modules on the ISO 9141 communication network, the IDS or equivalent tester must request all information; the modules cannot initiate communications.
The following fault chart describes the specific ISO 9141 network failures and their resulting symptom:

ISO 9141 Network Communication Fault Chart

ISO 9141 Network Communication Fault Chart:

HS-CAN

The HS-CAN network uses an unshielded twisted pair cable of data (+) and data (-) circuits. The data (+) and the data (-) circuits are each regulated to approximately 2.5 volts during neutral or rested network traffic. As bus messages are sent on the data (+) circuit, voltage is increased by approximately 1.0 volt. Inversely, the data (-) circuit is reduced by approximately 1.0 volt when a bus message is sent. Multiple bus messages can be sent over the network CAN circuits allowing multiple modules to communicate with each other. The HS-CAN is a high speed communication network used for the instrument cluster, the powertrain control module (PCM), the four wheel drive (4WD) control module, the restraints control module (RCM), and the occupant classification sensor module communications and the smart junction box (SJB), and designed for real time information transfer and control. The HS-CAN network will not communicate while certain faults are present, but will operate with diminished performance with other faults present. The HS-CAN bus may remain operational when 1 of the 2 termination resistors are not present.
In the event that one of the 2 network circuits (HS-CAN + or HS-CAN -) becomes open to a module on the network, unreliable network communication to all modules on the network may result. The module to which the network circuit is open may repeatedly send network messages indicating there has been partial data received. This type of message is referred to as a negative-acknowledge (NACK) message. Repeated NACK messages may "load" the network with too much activity causing intermittent no communication to other network modules and/or the scan tool.
The following fault chart describes the specific HS-CAN network failures and their resulting symptom:

HS-CAN Network Communication Fault Chart

HS-CAN Network Communication Fault Chart:

The following chart describes the specific CAN network messages broadcast by each module, and the module(s) that receive the message:

CAN Network Module Communication Message Chart

CAN Network Module Communication Message Chart (Part 1):

CAN Network Module Communication Message Chart (Part 2):