Vehicle Communication

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 yellow with a violet tracer. Additional tracer colors may be added to the violet in order to distinguish between different module connections. The modules are wired in parallel. Connections are made in the harness using splices. One 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 WJ:

- Body Control Module
- Door Modules (Driver & Passenger)
- Airbag Control Module
- Controller Antilock Brake
- Powertrain Control Module
- Radio
- CD Changer
- Transmission Control Module
- Automatic Zone Control Module
- Sentry Key Immobilizer Module
- Memory and/or Heated Seats Module
- Electronic Vehicle Information Center
- Mechanical Instrument Cluster

Each module provides its own bias and termination in order to transmit and receive messages. The bus voltage is at zero volts when no modules are transmitting and is pulled up to about seven and a half volts when modules are transmitting.

Display Chart:

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 DRB III lab scope. The 12 V square wave selection on the 20 Volt scale provides a good view of the bus activity. Voltage on the bus should pulse between zero and about seven and a half volts. Refer to the image for some typical displays.

The PCI Bus failure modes are broken down into two categories. Complete PCI Bus Failure and individual module no response. Causes of Complete PCI Bus 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 Failure would include but are not limited to:
- All gauges on the MIC stay at zero
- All telltales on MIC illuminate
- MIC backlighting at full intensity
- Dashed lines in the EVIC ambient temperature display
- No response received from any module on the PCI bus
- 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.

Application Table:

Diagnosis starts with symptom identification. If Complete PCI Bus 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 + AND - 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. Refer to the application table for a list of standard and optional modules and their respective Diagnostic Junction Port pin assignments.