Principles of Operation

The 3 module communication networks are:

• High speed controller area network (HS-CAN)
• International Standards Organization 9141 (ISO)
• Standard corporate protocol (SCP)

Information is sent to and from individual control modules that each control specific functions. All 3 networks are connected to the data link connector (DLC). The DLC can be found under the instrument panel between the steering column and the audio unit. The ISO network is for diagnostic use only while the HS-CAN and SCP network allow multiple modules to communicate (transfer data) with each other on a common network.

ISO Network 

The ISO network is a single wire network. The ISO network does not permit intermodule communication. When the scan tool communicates to modules on the ISO network, the scan tool must request all information; the modules cannot initiate communications. The ISO network operates at a maximum data transfer speed of 10.4 Kbps.

The following fault chart describes the specific ISO network failures and their resulting symptom:

SCP Network 

The SCP network uses an unshielded twisted pair cable of data (+) and data (-) circuits. The data (+) circuit is regulated to approximately 0.5 volts and the data (-) circuits to approximately 4.5 volts during neutral or rested network traffic. Voltage on data circuits increases/decreases as bus messages are sent. Multiple bus messages HS-CAN be sent over the network SCP circuits allowing network modules to communicate with each other. The SCP network operates at a maximum data transfer speed of 41.6 Kbps and remains operational at a degraded level if one of the bus wires becomes open or shorted to ground or voltage.

HS-CAN 

The HS-CAN 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 HS-CAN circuits allowing network modules to communicate with each other. The HS-CAN is a high speed network used for the instrument cluster (IC), the PCM, the fire suppression system module (FSSM) and ABS module communication.

The HS-CAN bus (+) and bus (-) circuits must always be terminated. The network termination of the HS-CAN takes place inside a "termination module" by termination resistors rated at 120 ohms, located across the (+) and (-) circuits. The resistors are wired in parallel to the network bus circuits for a total operating resistance of 60 ohms on a properly functioning network.

In the event that one of the 2 network circuits (MS-CAN + or MS-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 HS-CAN operates at a maximum data transfer speed of 500 Kbps and remains operational at a degraded level when certain circuit faults are present. The HS-CAN bus may remain operational when 1 of the 2 termination resistors are not present.

The following fault chart describes the specific HS-CAN failures and their resulting symptom:

The following chart describes all network messages broadcast on the SCP and HS-CANs and the module(s) that receive the message: