Vehicle Communication

VEHICLE COMMUNICATION
The Programmable Communication Interface (PCI) data bus system is a single wire multiplex system used for vehicle communications. Multiplexing is a system that enables the transmission of binary encoded messages shared between multiple modules over a single channel or circuit.

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.

Many of the control modules in a vehicle require information from the same sensing device. Multiplexing reduces wire harness complexity, sensor current loads and controller hardware because each sensing device is connected to only one controller, which reads and distributes the sensor information to the other controllers over the data bus. Also, because each controller on the data bus can access the controller sensor inputs to every other controller on the data bus, more function and feature capabilities are possible.

A multiplex system allows the information flowing between controllers to be monitored using a diagnostic scan tool. This system allows a control module to broadcast message data out onto the bus where all other control modules can read the messages that are being sent. When a module reads a message on the data bus that it requires, it relays that message to its microprocessor. Each module ignores the messages on the data bus that it doesn't recognize.

The following modules are used on the DN:
- Airbag Control Module
- Left Side Impact Airbag Control Module
- Right Side Impact Airbag Control Module
- A/C Heater Control
- Amplifier
- 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

Data exchange between modules is achieved by serial transmission of encoded data over this single wire broadcast network. The PCI data bus messages are carried over the bus in the form of Variable Pulse Width Modulated (VPWM) signals. The PCI data bus speed is an average 10.8 Kilo-bits per second (Kbps).

The voltage network used to transmit and receive messages requires biasing and termination. Each module on the PCI data bus system provides its own biasing and termination in order to transmit and receive messages. Each module (also referred to as a node) terminates the bus through a terminating resistor and a terminating capacitor. The Powertrain Control Module (PCM) is the only dominant node for the PCI data bus system.

The PCI bus uses low and high voltage levels to generate signals. The bus voltage is at zero volts when no modules are transmitting and is pulled up to about 7-1/2 volts when modules are transmitting. The low and high voltage levels are generated by means of variable-pulse width modulation to form signals of varying length.

When a module is transmitting on the bus, it is reading the bus at the same time to ensure message integrity.

Each module is capable of transmitting and receiving data simultaneously.

The PCI data bus can be monitored using the DRBIII scan tool. It is possible for the bus to pass all DRBIII tests and still be faulty if the voltage parameters are all within the specified range and false messages are being sent.





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 DRBIII 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 zero volts and about 7-1/2 volts. It is possible for the bus to pass all DRBIII tests and still be faulty if the voltage parameters are all within the specified range and false messages are being sent. Refer to the 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.