Principles of 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 [TN/OG] and data bus minus, Circuit 915 [PK/LB]), and the International Standards Organization (ISO) 9141 network which is a single wire (Circuit 70 [LB/WH]). The New Generation STAR (NGS) Tester can connect to both networks through the Data Link Connector (DLC). This makes diagnosis and testing of 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 can be found under the instrument panel between the steering column and the radio.

The ISO 9141 communication network does not permit inter-module communication. When the NGS Tester communicates to modules on the ISO 9141 communication network, the 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 ground or battery positive voltage (B+) or if some, but not all, termination resistors are lost.

Unlike the SCP communication network, the ISO 9141 communication 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.

The Powertrain Control Module (PCM) is on the SCP 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 gauge. 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 Restraint Control Module (RCM) is on the ISO 9141 network. The RCM controls the deployment of the air bags based on sensor input.

The Generic Electronic Module (GEM) and the Central Timer Module (CTM) are on the ISO 9141 network. Only one GEM or one CTM will be on a vehicle. The GEM is equipped on vehicles with 4-wheel drive or vehicles with power windows. The CTM is equipped on vehicles with 2-wheel drive and vehicles without power windows. The GEM controls additional functions that include:
- Speed dependent windshield wipers.
- 4-wheel shift on the fly.
- One touch down power window.

The Remote Anti-Theft Personality (RAP) module is on the ISO 9141 communication network. The RAP module controls keyless entry for the vehicle.

The 4-wheel anti-lock brake control module is on the ISO 9141 communication network. The 4-wheel anti-lock brake control module controls the brake pressure to the four wheels to keep the vehicle under control while braking.

The instrument cluster is also known as a Hybrid Electronic Cluster (HEC) and is on the SCP communication network. The instrument cluster displays driver information. The instrument cluster incorporates the PATS control module functions.