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Home >> Jeep >> 2001 >> TJ (CANADA) L6-4.0L >> Repair and Diagnosis >> Powertrain Management >> Relays and Modules - Powertrain Management >> Relays and Modules - Computers and Control Systems >> Body Control Module >> Description and Operation >> Functional Operation >> Part 2 of 2 >> Part 2B

Part 2B

PANEL DIMMING ILLUMINATION
The Instrument Cluster contains 5 (five) illumination bulbs that are hardwired to the cluster. Illumination and dimming is accomplished when the cluster senses voltage at the Headlamp Switch Output from the Park Lamp Feed circuit of the Multi-Function Switch. The cluster then applies voltage to ground on the Panel Lamps Dimmer Signal circuit through the Multi-Function Switch. The cluster interprets the resulting voltage drop through the Multi-Function Switch dimming rheostat to supply a dimmed voltage output to the illumination bulbs and other components on the Panel Lamps Feed circuit. The cluster and component bulbs are permanently grounded so the bulbs illuminate with the Panel Lamps Feed voltage. The cluster illumination bulbs are replaceable.

Panel Dimming Fault
- If the cluster detects a panel dimming resistance greater than 9250 ohms for 5 (five) seconds, the Panel Dimmer Switch is considered open and the cluster will set a DTC.
- During an open circuit condition, the cluster VF display will default to full brightness and general panel dimming will default to full brightness with the headlamps on.

Radio Illumination
- The cluster provides an output to switch a 12-volt signal to control the radio display illumination.
- Radio dimming Will follow the Headlamp switch except when in the Parade mode or Dome Lamp on positions.
- When in the Parade mode or Dome Lamp on positions, the cluster interrupts the switched 12-volt signal to the radio. This allows the radio display to remain bright.

Dimmable Indicators
- The cluster provides a one-step dimming for the Cruise, Upshift, and Part Time indicators.
- When the headlamps are on, these indicators will be dimmed at 60% (sixty percent).
- When the headlamps are off, the indicators will be maximum brightness.

PCI BUS INDICATORS
The following indicators are internally controlled by the Instrument Cluster using information received from PCI Bus messages. These indicators are only operational when the ignition is in the RunStart position.

Air Bag
- Illuminates when cluster receives PCI Bus data request from the Airbag Control Module (ACM).
- The cluster performs an indicator check each time indicator is commanded on or off. If an open lamp or short fault is detected by the cluster, the fault will be sent to the ACM over the PCI Bus.
- Non-replaceable LED.

Malfunction Indicator Lamp (ISO Engine Icon)
- Illuminates or blinks when cluster receives PCI Bus data request from the PCM.
- Non-replaceable LED.
- The PCM can command the cluster to display P-codes in the vacuum fluorescent (VF) display. The cluster will not display these codes if a RPM message greater than 0 (zero) is received. The displaying of P-codes will take priority over any Vehicle Status messages in the VF display.

Check Gauges
- Illuminates when cluster receives PCI Bus data from the PCM.
- Illuminates for engine temperature high and engine temperature critical.
- Illuminates for oil pressure low.
- Illuminates for charging system fault.
- Non-replaceable LED.

Cruise
- Illuminates when the cluster receives PCI Bus data request from the PCM.
- Represents the status of the Cruise Control being turned on (not the setting of the cruise control speed).
- Non-replaceable LED.

Low Fuel
- Illuminates internally from a calibration in the cluster EEPROM.
- The cluster illuminates the indicator when the remaining fuel is approximately between 1/8 and E as displayed by the Fuel gauge.
- If the cluster receives PCI Bus data indicating a fuel level sensor error, the cluster will position the gauge pointer needle to the E position and illuminate the Low Fuel indicator.
- Non-replaceable LED.

SKIM (If Equipped)
- Illuminates when the cluster receives PCI Bus data from the SKIM module.
- The SKIM module can command the indicator to illuminate or flash.
- Non-replaceable LED.

Upshift (If Equipped)
- Illuminates when the cluster receives PCI Bus data request from the PCM.
- Indicator operates in U.S. and Canadian clusters only.
- Non-replaceable LED.

INTERIOR/COURTESY LIGHTING
The Smart Cluster controls the interior/courtesy lighting. The lighting includes the dome lamp, 2 (two) courtesy lamps located on the lower instrument panel, and the hardtop rear lamp. The cluster controls the dome/courtesy lamp illuminated entry, theater dimming time-out, the door ajar time-out, and the ignition off illumination.

Illuminated Entry
- Operates when any door is opened (ajar switch closed to ground).
- The cluster will turn on all of the interior lamps.
- The interior lamps will remain on for a maximum of 20 (twenty) minutes or until the conditions change.
- After 20 (twenty) minutes, the cluster will turn off the interior lamps. The 20 minute time-out will be reset when any door or liftgate status changes.
- The interior lamps Will not time-out if the ignition is on and engine RPM is greater than 300.

Theater Dimming
- After the doors are closed, the cluster will turn off the interior lamps with theater dimming within approximately 5 (five) seconds.
- If the ignition is in the Run/Start position before the theater dimming time-out, the interior lamps will be turned off immediately.

Courtesy Lamps
- When the Panel Dimming Switch is adjusted to the interior lamp position, the cluster illuminates the interior lamps.
- The cluster will turn off the interior lamps after approximately 20 (twenty) minutes.
- If the vehicle conditions change, the cluster will reinitiate the 20 minute time-out feature.
- This feature will operate with ignition in the off or Run/Start position.
- This feature will not operate with the engine running.

Ignition-Off Courtesy Lamps (Illuminated Exit)
When the ignition switch transitions from the Run/Start to the Lock position, the cluster will illuminate the interior lamps for approximately 10 (ten) seconds.

ODOMETER/TRIP ODOMETER
The VF odometer illuminates after the ignition is turned on. The total and trip odometers use a common display. A trip reset button is used to toggle between the total and trip display modes. The trip odometer will roll over at 9999.9. The maximum value for the total odometer is 999999.9 miles/Km. The total odometer will not roll over but will remain latched at the maximum value.

Trip Select Button
- If the trip reset button is held for approximately 1 (one) second, the cluster will toggle between the total and trip odometers.
- Holding the trip reset button for approximately 2 (two) seconds or more, the cluster will reset the trip odometer.
- Holding the trip reset button while turning the ignition to the Run/Start position will cause the cluster to enter the self-test diagnostics. Refer to the Self Test Description.

Odometer On With Door Ajar
- The VF display will illuminate when the cluster receives a Driver or Passenger door ajar input with the key off, 0 (zero) MPH, and 0 (zero) RPM.
- The cluster will display the last function of the odometer, either total or trip.
- This feature will time-out after 20 (twenty) minutes.

BATTERY SAVER FEATURE
The Battery Saver feature is contained internally in the Instrument Cluster. This feature prevents battery run-down in the event of extended parasitic draw from the courtesy lamp circuit when the ignition is in the off position.

Power - Down
The cluster has a sleep mode to lower current draw during ignition off. When the cluster is in the sleep mode, portions of the cluster are powered down. Conditions that will prevent the cluster from entering the sleep mode:
- Ignition in the Run/Start position.
- Park/Headlamps on.
- Vehicle door(s) open (ajar switch closed to ground). Operates without the doors off vehicle jumper fuse removed.
- Cluster processing PCI Bus messages.
- Panel dimming switch set to illuminate interior lamps.

The cluster will enter the sleep mode approximately 5 (five) seconds after all of the above conditions are eliminated. If all of the conditions are met except the door ajar switches, the cluster will keep the courtesy lamps illuminated for approximately 20 (twenty) minutes. Then the cluster will turn off the courtesy lamps and then enter the sleep mode. While the cluster is in the sleep mode, an internal timer will wake the cluster microprocessor every 262 milliseconds to check input status. If conditions change the cluster will resume normal operation, otherwise it will stay in the sleep mode.

Power - Up
When the cluster is in the sleep mode, it will awaken to the operating mode with the following conditions:
- Any door opening (ajar switch closed to ground).
- Park/Headlamp switch input.
- Ignition switch transition to Run/Start.
- PCI Bus messages.
- Panel dimming switch transitions to interior lamp illumination.

GAUGE POINTER PICK - UP AND ZERO SET
During battery re-connect, the cluster Will return the Tachometer and Speedometer pointers to zero. When the ignition is turned off, the cluster will position all gauge pointers at zero or the minimum indication of the gauge.




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. The modules are wired in parallel. Connections are made in the harness using splices. The following modules are used on 2001 TJ.
- Airbag Control Module
- Mechanical Instrument Cluster
- Powertrain Control Module
- Sentry Key Immobilizer 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 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 zero and about seven and a half 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 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
- No response received from any module on the PCI bus (except 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 module 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 +A 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.

NOTE: COMMUNICATION OVER THE BUS IS ESSENTIAL TO THE PROPER OPERATION OF THE VEHICLES ON-BOARD DIAGNOSTIC SYSTEMS AND THE DRB III. PROBLEMS WITH THE OPERATION OF THE BUS OR DRB III MUST BE CORRECTED BEFORE PROCEEDING WITH DIAGNOSTIC TESTING. IF THERE IS A PROBLEM, REFER TO THE COMMUNICATIONS CATEGORY.
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