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Home >> Chrysler Truck >> 2001 >> Voyager V6-3.3L VIN R >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Relays and Modules - Computers and Control Systems >> Body Control Module >> Description and Operation >> Part 3 of 5 >> Part 3A

Part 3A

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 white 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 RS:
- Body Control Module
- Front Control Module
- Occupant Restraint Controller
- Left Side Impact Airbag Control Module
- Right Side Impact Airbag Control Module
- Controller Antilock Brake
- Powertrain Control Module
- Radio
- CD Changer
- Transmission Control Module
- Automatic Temperature Control Module
- A/C Heater Control Module (MTC)
- Sentry Key Immobilizer Module
- Memory Seat(Mirror Module)
- Overhead Console
- Mechanical Instrument Cluster
- Left Sliding Door Control Module
- Right Sliding Door Control Module
- Power Liftgate Module

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.




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(R) 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 and about seven and a half volts. 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 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 MIC stay at zero
- All telltales on MIC illuminate
- MIC backlighting at full intensity
- Dashed lines in the overhead console ambient temperature 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 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 and 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 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 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 DRBIII(R) 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.

DOOR AJAR SYSTEM
The door ajar and liftgate ajar states are used as inputs for the Body Control Module (BCM). The BCM uses these inputs to determine exactly what position the doors and liftgate are in. The DRBIII(R) will display the state of the door ajar and the liftgate ajar switches in Inputs/Outputs. It's important to note, that when any door, or the liftgate is closed, the switch state on the DRBIII(R) will show OPEN. When any door, or the liftgate is open the switch state on the DRBIII(R) will show CLOSED. During diagnosis, if a door or the liftgate is closed and the DRBIII(R) displays the switch state as CLOSED, it indicates a shorted ajar circuit. If the door or the liftgate is open and the DRBIII(R) displays the switch state as OPEN, it indicates an open ajar circuit.

EXTERIOR LIGHTING SYSTEM
Headlamp Power
The headlamp switch is a direct input to the BCM. The BCM sends a BUS message to the FCM informing it of a headlamp switch status change. The FCM then turns on power to the headlamps through four "fuseless" circuits. These circuits are electronically controlled and continuously monitored for malfunctions. Power is supplied to each filament in a separate circuit. For vehicles equipped with daytime running Lamps (DRL), the FCM electronically steps down the headlamp voltage to provide the desired illumination.

Headlamp Switch
The Headlamp Switch uses a multiplexed (MUX) circuit to the Body Control Module (BCM). The Headlamp Switch controls the Fog lamp relay, Park lamps and the Low and High headlamps. The BCM then sends a signal through the PCI Bus line to the FCM as to what state the switch has selected. The FCM energizes the high side output drivers to turn ON the desired lamps.

Park Lamp Relay
The park lamp switch is a direct input to the BCM. The BCM sends a BUS message to the FCM informing it to turn ON the park lamp relay. The park lamp relay is then powered through low side control of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions.

Fog Lamp Relay
The fog lamp switch is a direct input to the BCM. The BCM sends a BUS message to the FCM informing it to turn ON the fog lamp relay. The fog lamp relay is then powered through low side control of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions. Fog lamp functionality is not equipped on all vehicles. The FCM "learns" that the vehicle is equipped with fog lamps by reading the BCM BUS message.

Fog Lamps
The BCM controls the operation of the fog lamp relay that turns the fog lamps ON and OFF. The Fog lamps can only be ON when the park and low beams are ON. If the high beams are switched ON then the Fog lamps will be automatically turned OFF.

Exterior Lighting Battery Saver
The BCM monitors the status of; and controls, the Park Lamps, Headlamps and Fog Lamp relays. If any exterior lamps are left ON after the ignition is turned OFF; the BCM Will turn them OFF after 3 minutes.

Auto Headlamps
This feature is available on vehicles equipped with both the Electrocromatic Mirror (ECM) and the Compass/Mini-Trip Computer (CMTC). When the BCM detects a day/night signal from the CMTC, an ECM is present and Auto Headlamp mode is selected.

FRONT CONTROL MODULE
The Front Control Module (FCM) is an electrical control and interface center located in the engine compartment. When it is mated to the Power Distribution Center (PDC), it is referred to as the Intelligent Power Module (IPM). The IPM, with its fuses and relays provides power and signal distribution throughout most of the vehicle. The FCM receives both hard wire and digital electronic inputs from the vehicle electrical system through the PDC. Based on these inputs and the ignition switch position, it provides direct power feeds and relay control to some of the vehicles' most critical electrical systems.

The Front Control Module provides the following features:
Controlled power feeds:
- Front airbag system
- Side airbag system
- Headlamp power
- EATX module power (4 speed only)
- Front washer motor
- Rear washer motor
- Brake shift interlock system

Relay controls:
- Fog lamp relay (when equipped)
- Park lamp relay
- Front wiper on relay
- Front wiper high/low relay
- Accessory relay
- Horn relay
- Front & rear blower relay
- Name brand speakers (NBS) relay
- Electronic back light (EBL) run only relay

Electrical inputs:
- Headlamp battery supplies 1 & 2
- Module battery supply
- Power ground
- Ignition switch RUN or START position status
- Ignition switch START only status
- PCI Bus
- Stop lamp switch
- Horn switch
- Back-up switch
- Wiper park switch
- Washer fluid level switch
- Brake fluid level switch
- Ambient temperature sensor
- Right park lamp outage
- Left park lamp outage
- Battery IOD
- Battery (+) connection detection
- Flash reprogramming voltage

Controlled Power Feeds
Front airbag system
The FCM provides power to the Occupant Restraint Control (ORC) system through two "fuseless" circuits (ORC RUN/START, and ORC RUN only). These circuits are electronically controlled and continuously monitored for malfunctions. Power is supplied while the ignition switch is in the RUN and START positions on pin 48 of the FCM connector, and in the RUN only position on pin 29.

Side airbag system
The FCM provides power to the Side Impact Airbag Control Module (SIACM) system through one "fuseless" circuit. This circuit is electronically controlled and continuously monitored for malfunctions. Power is supplied in the ignition RUN and START positions on pin 28 of the FCM connector.

Headlamp power
The headlamp switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it of a headlamp switch status change. The FCM then turns on power to the headlamps through four "fuseless" circuits. These circuits are electronically controlled and continuously monitored for malfunctions. Power is supplied to each filament in a separate circuit (RH low on pin 6, RH high on pin 4, LH low on pin 3 and LH high on pin 5). For vehicles equipped with Daytime Running Lamps (DRL), the FCM electronically steps down the headlamp voltage to provide the desired illumination.

EATX power
The electronic automatic 4 speed transmission module is powered when the ignition switch is in the UNLOCK, RUN or START positions. This circuit is electronically controlled and continuously monitored for malfunctions. Power is supplied through pin 27 of the FCM connector.

Front washer motor
The front washer switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it of a request to wash. The front washer motor is then powered through low side control inside the FCM. This circuit is electronically controlled and continuously monitored for malfunctions. In addition, the FCM electronically protects the washer motor from system voltages higher than 16 volts by automatically switching off the low side circuit. The low side circuit is connected to pin 45 in the FCM connector.

Rear washer motor
The rear washer switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it of a request to wash. The rear washer motor is then powered through low side control inside the FCM. This circuit is electronically controlled and continuously monitored for malfunctions. In addition, the FCM electronically protects the washer motor from system voltages higher than 16 volts by automatically switching off the low side circuit. The low side circuit is connected to pin 46 in the FCM connector.

Brake shift interlock system
The brake shift interlock solenoid receives power from both high side and low side controls inside the FCM. The high side control is on the same circuit as the EATX module power and the low side control comes through pin 47 of the FCM connector. The solenoid is controlled by the low side driver when the brake pedal is pressed. Both circuits are continuously monitored for malfunctions.

Relay Controls
Fog lamp relay
The fog lamp switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it to turn ON the fog lamp relay. The fog lamp relay is then powered through low side control on pin 33 of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions. Fog lamp functionality is not equipped on all vehicles. The FCM "learns" that the vehicle is equipped with fog lamps by reading the BCM Pd Bus message.

Park lamp relay
The park lamp switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it to turn ON the park lamp relay. The park lamp relay is then powered through low side control on pin 13 of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions.

Front wiper on relay
The front wiper switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it to turn ON the front wiper on relay. The front wiper on relay is then powered through low side control on pin 14 of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions.

Front wiper high/low relay
The front wiper switch is a direct input to the BCM. The BCM sends a PCI Bus message to the FCM informing it to turn ON the front wiper high/ low relay. The relay switches power between the low speed and high speed windings of the wiper motor. The front wiper high/low relay is powered through low side control on pin 34 of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions.

Accessory relay
The accessory relay works in conjunction with the FCM's power accessory delay feature to control the operation of the radio, power windows, washer motors, wiper motors and power outlet. The accessory relay is turned ON through low side control on pin 35 of the FCM. This circuit is electronically controlled and continuously monitored for malfunctions. Depending on the ignition switch position, the accessory relay will remain ON or will time-out and turn OFF. The accessory relay remains ON in the RUN and ACCY positions of the ignition switch. In the UNLK and OFF positions, the relay will remain energized for 45 seconds then turn OFF. During this time-out period, if the driver or passenger doors are opened, the relay will turn OFF immediately while the ignition switch is in the START position, the relay will also drop-out, then resume operation. Accessory relay operation is most noticeable by observing the operation of the radio or blower functions.

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