Principles 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 [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 diagnostic tool 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 SCP communication 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 communication network does not permit inter-module 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.

Unlike the SCP, the ISO 9141 communication network will not function if the wire is shorted to ground or battery voltage. Also, if one of the modules on the ISO 9141 communication network loses power or shorts internally, communications to that module will fail.

There is one module linked to the SCP communication network and five modules linked to the ISO 9141 communication network.

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

The Generic Electronic Module (GEM) is on the ISO 9141 communication network. The GEM controls a variety of systems:
^ windshield wipers
^ courtesy lamps
^ warning chimes
^ interior lamps
^ exterior lamps
^ power windows

For additional GEM information, refer to Multifunction Electronic Control Module (General Module).

The central security module is on the ISO 9141 communication network. The central security module controls the keyless remote entry of the vehicle.
The Anti-lock Brake Control (ABS) module is on the ISO 9141 communication network. The ABS module controls the brake pressure to the four wheels to keep the vehicle under control while braking.

The Four-Wheel Drive (4WD) control module is on the ISO 9141 communications network. The 4WD module controls all four-wheel drive functions including the 4WDLOW and 4WDHIGH indicator lamps.