Description & Operation

NOTE: Unless otherwise specified, information in this article applies to both Escort and ZX2 models. Escort may be referred to as Sedan and ZX2 may be referred to as Coupe.

NOTE: Electronic crash sensor module may also be referred to as air bag diagnostic monitor.

Vehicle has 2 module communication networks:

The bus network, also referred to as Standard Corporate Protocol (SCP), which consists of a pair of unshielded twisted wires (positive (+) circuit No. 131 and negative (-) circuit No. 132).
International Standards Organization (ISO) 9141 network which is a single wire (circuit 529).

New Generation Star (NGS) tester or equivalent scan tool can connect to both networks through the Data Link Connector (DLC). This makes diagnosis and testing easier by enabling one smart tester to diagnose and control any module on the two networks from one connector.

ISO 9141 communication network does not permit inter-module communication. When scan tool communicates to modules on diagnostic communication network, scan tool must ask for all information; modules cannot initiate communications. SCP network will remain operational even with severing of one of the bus wires. Communications will also continue if one of the bus wires is shorted to chassis ground or battery positive voltage (B+) or if a termination resistor is lost. Unlike SCP network, ISO 9141 network will not function if wire is shorted to chassis ground or battery positive voltage (B+). Also, if one of the modules on ISO 9141 network loses power or shorts internally, communication to that module will fail and communication to all modules on ISO 9141 network may fail.

One module on bus network:

Powertrain Control Module (PCM), which controls the engine for better fuel economy, emissions control, and failure mode detection and storage.

Three optional modules on ISO 9141 network:

Electronic Crash Sensor (ECS) module, which controls the deployment of air bags based on sensor input.
Remote Anti-Theft Personality (RAP) module, which controls the remote entry, trunk release and panic alarm from the remote entry transmitter.
Anti-Lock Brake Control module, which controls brake pressure to the four wheels to keep vehicle under control while braking.

This article is used to diagnose bus and ISO communication network circuit faults. For module locations, see COMPONENT LOCATIONS .