Vehicle Communication

The Programmable Communication Interface (PCI) Bus circuit is a single-wire multiplexed network capable of supporting binary encoded messages shared between multiple modules and the DLC. The PCI Bus circuit is identified as D25 and is a Violet/Yellow wire. Additional colors may be added to the Yellow tracer in order to distinguish between different module connections. The modules are wired in parallel. To determine which modules this vehicle is equipped with, see WIRING DIAGRAMS  .

Connections are made in the harness using a splice. The splice called the Diagnostic Junction Port, serves as the Hub of the PCI Bus. The Diagnostic Junction Port provides an access point to isolate most of the modules on the PCI Bus in order to assist in diagnosing the circuit. Components that are connected to the PCI Bus circuit are as follows.

• A/C-Heater Control Module
• Air Bag Control Module
• Amplifier
• Central Timer Module
• Compass Mini-Trip Computer
• Controller Anti-Lock Brake
• Electro-Mechanical Instrument Cluster
• Left Side Impact Air Bag Control Module
• Powertrain Control Module
• Radio
• Right Side Impact Air Bag Control Module
• 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 PCI Bus voltage is at zero volts when no modules are transmitting and is pulled up to about 7.5 volts when modules are transmitting. The PCI Bus messages are transmitted at a rate averaging 10,800 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 PCI Bus activity with a conventional voltmeter. The preferred method is to use the DRBIII(R) lab scope. The 12-volt 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 7.5 volts.

The PCI Bus failure modes are broken down into 2 categories, Complete PCI Bus Communication Failure and Individual Module No Response Failure. Causes of a Complete PCI Bus Communication Failure include a short to ground or short to battery (12 volts) on the PCI Bus circuit. Individual Module No Response Failure 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 Instrument Cluster stay at zero.
• All telltales on Instrument Cluster illuminate.
• Instrument Cluster backlighting at full intensity.
• Dashed lines in the CMTC display.
• No response received from any module on the PCI Bus (except the PCM).
• No start (if equipped with SKIM).

Symptoms of Individual Module No Response 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 DRBIII(R).

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