Multiplex Communication Network

Aboard this vehicle are several electronic components which can be controlled by an off-board tester to provide valuable self-diagnostic information. These components are part of an electrical system network, designed to control powertrain and traction control subsystems. The overall electronic network system consists of two micro-processor-based modules which interface through a common communication link:
- Powertrain Control Module (PCM)
- Anti-lock Brake System with Traction Control (ABS/TC) module

This communication link consists of a twisted wire pair (Data Bus Plus and Data Bus Minus) routed throughout the vehicle. Multiplex communication signals are carried over an unshielded twisted pair. No special cabling is needed for the multiplex communication network. The wire pair is arranged in a bus configuration or party line.

Fault Tolerance
The multiplex communication network system will remain operational:
- If one of the bus wires is severed.
- If one of the bus wires is shorted to ground or battery positive voltage (B+).
- If a termination resistor is lost.

This condition is detected and reported to the host by the module's network bus interface circuits. In addition, one or more failed modules will not inhibit communications on the remainder of the network.

Purpose
This system contains information necessary for diagnosis of communication link conditions which may affect normal operation of the vehicle. Customer symptom may be directly related to these conditions. In order to effectively diagnose the communication link between modules, the diagnostic procedures should be followed with each step performed in the order presented.

Module Information
Modules on this multiplex communications network are:

- Powertrain Control Module (PCM)
The Powertrain Control Module (PCM) is located in the engine compartment on the passenger side of the vehicle on the cowl panel. In certain vehicle configurations the powertrain control module may interact with the Anti-lock Brake/Traction Control (ABS/TC) module to determine if it is OK to provide engine control operation. The powertrain control module and PATS systems communicate with each other on Circuit 914 (T/O) and Circuit 915 (PK/LB). A failure of these circuits may result in a no start condition. In order to properly service the vehicle it is necessary to perform a network communications check. Rotunda New Generation Star (NGS) Tester 007-00500 or equivalent automatically performs this test. In the event New Generation Star Tester fails to communicate with the powertrain control module, refer to Pinpoint Test A and Pinpoint Test B. Pinpoint Tests

- Anti-Lock Brake System/Traction Control (ABS/TC) Module
The Anti-lock Brake System/Traction Control (ABS/TC) module is located under the battery tray in the lower LH side of the engine compartment. The anti-lock brake system/traction control module and powertrain control module share information across the multiplex communications network on Circuits 914 (T/O) and 915 (PK/LB). The anti-lock brake system/traction control module provides diagnostic services from the New Generation Star (NGS) Tester through Circuits 914 (T/O) and 915 (PK/LB). A failure to communicate with the anti-lock brake system/traction control module will result in a failure in the data link diagnostic test performed by the NGS Tester. In the event the data link diagnostic tests fail, refer to no module communications diagnostics.

Off-board tester communication with these modules is carried out through the Data Link Connector (DLC), which is located under the instrument panel to the right of the steering column. For more details refer to Harness and Component Layout. Harness and Components