Principles and Operation

The vehicle has two module communication 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 communications network, which is a single wire network (Circuit 70 [LB/WH]). Both networks are connected to 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 SCP communications 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 voltage, or if some but not all termination resistors are lost.

The ISO 9141 communications network does not permit intermodule communication. When the diagnostic tool communicates to modules on the ISO 9141 communication network, the diagnostic tool must ask for all information; the modules cannot initiate communications.

The ISO 9141 communications network will not function if the wire is shorted to ground or battery voltage. Also, if one of the modules on the ISO 9141 communications 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. Refer to Symptom Charts for diagnosis and testing of the PCM. Symptom Related Diagnostic Procedures

The Restraint Control Module (RCM) is on the ISO 9141 network. The RCM controls the deployment of the air bags based on sensor input. Refer to Air Bag Systems.

The Electronic Automatic Temperature Control (EATC) module is on the SCP communication network. The EATC module controls automatic climate functions that maintain the vehicle interior temperature at a constant setting. Refer to Heating and Air Conditioning.

The Generic Electronic Module (GEM) is on the ISO 9141 communication network. The GEM controls a variety of systems:
^ windshield wipers
^ courtesy lamps
^ 4-wheel shift on the fly
^ warning chimes
^ one-touch down power windows

Refer to Multifunction Electronic Module (General Module).

The Driver Seat Module (DSM) is on the ISO 9141 communication network. The DSM controls the positioning and programming of the driver seat, pedals and both outside mirrors. Refer to Seats.

The central security module (if equipped) is on the ISO 9141 communication network. The central security module controls the keyless entry functions for the vehicle. Refer to Locks.

The 4-Wheel Anti-Lock Brake (4WABS) control module (if equipped) is on the ISO 9141 communication network. The 4WABS control module controls the brake pressure to the four wheels to keep the vehicle under control while braking.

The following describes the rear- and four-wheel air suspension modules:
The air suspension control module is on the ISO 9141 communication network and controls the air compressor motor (through a solid state relay) and all system solenoids. The module also provides power to front and rear height sensors on four-wheel air suspension systems, or rear height sensors on the rear-wheel suspension systems. The module controls vehicle height adjustments by monitoring two height sensors, vehicle speed, a steering sensor, acceleration input, the door ajar signal, transfer case signals and the brake switch. The module also conducts all fail-safe and diagnostic strategies and contains self-test and communication software for testing the vehicle and module.

The control module is mounted in the passenger compartment inside the instrument panel above the radio and temperature controls.

The air suspension control module monitors and controls the system through a 32-pin two-way connector. It is keyed so that the control module cannot be plugged into an incorrect harness. There are two sides of the harness connection to the module. Each is uniquely colored and keyed to prevent reversal of connections.

The instrument cluster is on the SCP communication network. The instrument cluster is also called a Hybrid Electronic Cluster (HEC). It is an analog face cluster with a digital odometer. The instrument cluster displays a variety of information. refer to Instrument Panel, Gauges and Warning Indicators.

The parking aid module is connected to the ISO 9141 communication module. The module controls sensors in the rear bumper that detect close objects when the vehicle is in reverse. Refer to Parking Assist System.