Multiplex Communication Network

Multiplex Communication Network Components

The multiplex communication network provides:
- the ability for module-to-module communication by sharing required inputs and outputs instead of each component having an input or output wired directly to or from each affected module.
- a common link for communication to an off-board tester through the Data Link Connector (DLC) located under the instrument panel to the right of the steering column.

The micro-processor based subsystems included on this network are:
- powertrain control
- climate control
- anti-lock brakes
- anti-theft system

A fifth subsystem, the Natural Gas Vehicle (NGV) Module:
- is used only on Crown Victoria Natural Gas Vehicles (NGV).
- utilizes a stand-alone processor to control the fuel indication system (fuel indication is not supported by the multiplex communication network).
- uses the multiplex communication network to communicate with the Powertrain Control Module (PCM).

The multiplex communication bus between these subsystems and the Data Link Connector (DLC) consist of:
- an unshielded twisted pair of wires:
- Circuit 914 (T/O) bus(+).
- Circuit 915 (PK/LB) bus(-).

Fault Tolerance
The multiplex communication network will remain operational in the event:
- one circuit is severed.
- one circuit is shorted to ground.
- one circuit is shorted to battery positive voltage (B+).
- a termination resistor is lost within a module.
- one or more modules tail internally.

Any of these conditions will be detected and reported to the off-board tester during diagnostics in the form of a Diagnostic Trouble Code (DTC).

Purpose
The information is designed to diagnose multiplex communication network concerns.

These concerns may:
- affect normal functional operations of the vehicle.
- be directly related to customer concerns.
- be undetectable except during diagnostics.

To properly diagnose a network concern, follow the diagnostic procedure in the order presented.

Module Identification
Modules included on this multiplex communication network include:

Powertrain control module
- located in the engine compartment on the passenger side of the dash panel.
- a failure of the communication network to the Powertrain Control Module (PCM) may result in a no start condition.
- if communication to the powertrain control module fails during diagnosis, refer to Inspection and Verification. Testing and Inspection

Electronic Automatic Temperature Control (EATC) Module
- located in the center of the instrument panel.
- loss of communication to Powertrain Control Module (PCM) may result in degraded automatic temperature control.
- if communication to the electronic automatic temperature control module is lost, refer to Electronic Climate Control for No Communication Testing.

Natural Gas Vehicle (NGV) Module
- natural gas vehicles only.
- located in front of the A/C condenser core behind the radiator grille.
- if communication to the Natural Gas Vehicle (NGV) Module fails, refer to Instrument cluster - Natural Gas Vehicle for No Communication Testing. Instrument Cluster / Carrier

Anti-Lock Brake Control Module
- located in the lower LH side of the engine compartment.
- a failed anti-lock brake module may cause a no communication condition on the diagnostic communication network.
- if communication to the anti-lock brake module fails, refer to Anti-Lock Brake System for No Communication testing.

Passive Anti-Theft System Control (PATS) Module
- enables and disables vehicle from starting.
- located under left hand side of instrument cluster.
- if communication to the PATS module is lost, refer to Anti-Theft and Alarm Systems. Antitheft and Alarm Systems

Inspection and Verification

1. Verify the customer's original concern.
2. Visually inspect for obvious signs of electrical damage:

VISUAL INSPECTION CHART

Electrical
- Fuses
- Wiring harness
- Loose or corroded connections
- Anti-lock brake control module (optional)
- Driver Door Module (DDM) (optional)
- Electronic Automatic Temperature Control (EATC) module (optional)
- Electronic Crash Sensor (ECS) module
- Lighting Control Module (LCM)
- Natural Gas Vehicle Module (NGVM) (optional)
- Passive Anti-Theft System (PATS) module
- Powertrain Control Module (PCM)

3. If the concern remains after the inspection, connect New Generation STAR (NGS) Tester to the Data Link Connector (DLC) located beneath the instrument panel and select the vehicle to be tested from NGS Tester menu. If NGS Tester does not communicate with the vehicle:
- Check that the program card is properly installed.
- Check the connections to the vehicle.
- Check the ignition switch position.

If NGS Tester still does not communicate with the vehicle, go to Pinpoint Test M. M: No Module/Network Communication