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 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.

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.

Unlike the SCP communication network, the ISO 9141 communication network will not function if the wire is shorted to chassis ground or voltage. 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. Refer to Computers and Control Systems for diagnosis and testing of 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) is on the ISO 9141 network. The GEM can control such systems as warning chimes, interior and exterior lamps, keyless entry, wipers, and power windows.

The Anti-Lock Brake Control (ABS) module without traction control is on the ISO 9141 communication network. If equipped with traction control, the Anti-Lock Brake/Traction Control Module (ABS/TC) is on the SCP communication network. The anti-lock brake control module controls the brake pressure to the four wheels to keep the vehicle under control while braking.

The Remote Climate Control (RCC) module is on the SCP communication network. The RCC module controls the temperature within the vehicle.