Information Bus: Description and Operation

The vehicle has two module communications networks. The Standard Corporate Protocol (SCP) which is an unshielded twisted pair cable (data bus plus, circuit 914 and data bus minus, circuit 915), and the International Standards Organization (ISO) 9141 network which is a single wire (circuit 70 [/W]). Both networks can be connected to New Generation STAR (NGS) Tester at one connector called the Data Link Connector (DLC). This makes troubleshooting these systems easier by allowing one smart tester to be able to diagnose and control any module on the two networks from one connector. The DLC is located under the instrument panel between the steering column and the radio.

The ISO 9141 communication network does not permit inter-module communication. When NGS Tester communicates to modules on the diagnostic communication network, NGS Tester must ask for all information; the modules cannot initiate communications.

The SCP network will remain operational even with the severing of one of the bus wires. Communications will also continue if one of the bus wires is shorted to chassis ground or battery positive voltage (B+), or if a termination resistor is lost. This faulted condition is detected and reported to the host by the module's network bus interface circuits.

Unlike the SCP network, the ISO 9141 network will not function if the wire is shorted to chassis ground or battery positive voltage (B+) Also, if one of the modules on the ISO 9141 network loses power or shorts internally, communications to that module will fail. There are two modules on the SCP network and four modules on the ISO 9141 network.

The Powertrain Control Module (PCM) is on the multiplex communication network. The PCM controls the engine for better fuel economy, emissions control, and failure mode detection and storage.

The Natural Gas Vehicle Module (NGVM) calculates fuel level based on inputs from the fuel tank pressure sensor and the fuel tank temperature sensor. The NGVM then provides a voltage signal to the instrument cluster to drive the fuel gage. The NGVM also has separate injector drivers to run the injectors at the higher operating pressure of natural gas. The NGVM does not perform any calculations for this function; calibration for the injectors is maintained in the PCM.

The Integrated Air Bag Diagnostic Monitor (IABM) is on the ISO 9141 network. The IABM controls the deployment of the air bags based on sensor input.

The Generic Electronic Module (GEM)/Central Timer Module (CTM) is on the ISO 9141 network. The GEM is equipped on vehicles with 4-wheel drive or power windows. The CTM is equipped on vehicles with 2-wheel drive or vehicles without power windows. The GEM controls a variety of systems:
^ Windshield wipers.
^ Courtesy lamps.
^ 4-wheel shift on the fly.
^ One touch down power windows.

The Remote Anti-theft Personality (RAP) module is on the ISO 9141 network. The RAP module controls the anti-theft and keyless remote entry of the vehicle. For more information, REFER to the Remote Anti-theft Personality system.

The 4-Wheel Anti-lock Brake (4WABS) control module is on the ISO 9141 network. The 4WABS control module controls the brake pressure to the four wheels to keep the vehicle under control while braking. For more information, REFER to 4-Wheel Anti-lock Brake system.