Vehicle Communication

VEHICLE COMMUNICATION
The Programmable Communication Interface or PCI Bus is a single wire multiplexed network capable of supporting binary encoded messages shared between multiple modules. The PCI bus circuit is identified as D25 and is violet with a yellow tracer. Additional tracer colors may be added to the yellow in order to distinguish between different module connections. The modules are wired in parallel. Connections are made in the harness using a splice.

The splice called the Diagnostic Junction Port, serves as the "Hub" of the bus. The Diagnostic Junction Port provides an access point to isolate most of the modules on the bus in order to assist in diagnosing the circuit. The following modules are used on the AN and AN84:
- Airbag Control Module
- Amplifier
- A/C Heater Control
- Central Timer Module
- Controller Antilock Brake
- Compass Mini-Trip Computer
- Electro/Mechanical Instrument Cluster
- Powertrain Control Module
- Radio
- Sentry Key Immobilizer Module
- Transfer Case Control Module
- Transmission Control Module

Each module provides its own bias and termination in order to transmit and receive messages. The bus voltage is at 0 volts when no modules are transmitting and is pulled up to about 7-1/2 volts when modules are transmitting.

The bus messages are transmitted at a rate averaging 10800 bits per second. Since there is only voltage present when the modules transmit and the message length is only about 500 milliseconds, it is ineffective to try and measure the bus activity with a conventional voltmeter. The preferred method is to use the DRB III lab scope. The 12v square wave selection on the 20-volt scale provides a good view of the bus activity. Voltage on the bus should pulse between 0 volts and about 7-1/2 volts. Refer to the following figure for some typical displays.

The PCI Bus failure modes are broken down into two categories. Complete PCI Bus Communication Failure and individual module no response. Causes of a complete PCI Bus Communication Failure include a short to ground or battery on the PCI circuit. Individual module no response can be caused by an open circuit at either the Diagnostic Junction Port or the module, or an open battery or ground circuit to the affected module.

Symptoms of a complete PCI Bus Communication Failure would include but are not limited to:
- All gauges on the EMIC stay at zero
- All telltales on EMIC illuminate
- EMIC backlighting at full intensity
- Dashed lines in the Compass Mini-Trip Computer display
- No response received from any module on the PCI bus (except the PCM)
- No start (if equipped with Sentry Key Immobilizer)

Symptoms of Individual module failure could include any one or more of the above. The difference would be that at least one or more modules would respond to the DRB III.

Diagnosis starts with symptom identification. If a complete PCI Bus Communication Failure is suspected, begin by identifying which modules the vehicle is equipped with and then attempt to get a response from the modules with the DRB III. If any modules are responding, the failure is not related to the total bus, but can be caused by one or more modules PCI circuit or power supply and ground circuits. The DRB III may display "BUS +/- SIGNAL OPEN" or "NO RESPONSE" to indicate a communication problem. These same messages will be displayed if the vehicle is not equipped with that particular module. The CCD error message is a default message used by the DRB III and in no way indicates whether or not the PCI bus is operational. The message is only an indication that a module is either not responding or the vehicle is not equipped.