Body Control Module: Description and Operation

AIRBAG SYSTEM
The airbag system is designed to provide increased driver and passenger protection if the vehicle is involved in a front-end collision. The system is designed to be used in conjunction with the seat belt system. Currently, Chrysler Corporation uses two types of Airbag Control Modules (ACM). The ASDM system is a mechanically-triggered system utilizing two front impact sensors and a sensor inside the module. The AECM is an electronically-triggered system that uses only its own internal electronic sensor to trigger the airbags. The TJ Body uses only the AECM system.

Whenever the ignition key is turned to the RUN or START position, the airbag control module (ACM) performs a lamp check by turning the AIRBAG warning lamp on. The lamp stays lit for 6 - 8 seconds, then goes off. If the lamp remains off, it means the ACM has checked the system and found it to be free of discernable malfunctions. The airbag system is monitored by the ACM.

The ACM monitors critical input and output circuits within the airbag system, making sure they are operating correctly. Some circuits are tested continuously; others are checked only under certain circumstances. The ACM provides diagnostic information about the airbag system to the technician through the DRB via the CCD bus.

The AIRBAG warning lamp is the only point at which "symptoms" of a system malfunction can be observed by the customer.

The deceleration or g-force resulting from the impact of a front-end collision causes the electronic sensor inside of the ACM to be triggered. This causes the inflators to be actuated, thus deploying the airbags. The total time between determining to deploy and deflation of the airbag is 1/10th of a second.

2000 Wranglers Will be equipped with a passenger airbag ON/OFF switch. This switch has an LED that will light when the switch is in the "OFF" position. While in the "OFF" position a circuit board inside the switch will simulate a passenger airbag to prevent the ACM from logging a DTC. However it is important that the ignition be in the "OFF" before moving the ON/OFF switch to the "OFF" or "ON" position, otherwise the ACM may log a DTC and turn on the airbag warning lamp due to a momentary open of the switch contacts.

Use the test procedures in this book to find the cause of any customer complaint regarding the AIRBAG warning lamp such as:
- warning lamp does not come on at all
- warning lamp stays ON

INSTRUMENT CLUSTER
The instrument cluster used in the Jeep Wrangler contains six gauges and eleven warning lamps. The gauges are positioned with information received by the cluster over the CCD bus from the PCM. Some of the lamps are hardwired and some are controlled by the cluster using CCD bus information. The vehicle chime function is contained on the cluster circuit board as well as the timer for the rear window defogger. There is a self test available for the cluster that will actuate all the gauges and place them at their calibration points. The test will not run if the cluster sees engine RPM or vehicle speed. This test will light all CCD controlled lamps for verification of lamp operation. It also forces the odometer Vacuum Fluorescent display (VF) to count up from zero through nine. The self test routine can be entered by pushing and holding the trip reset button while rotating the ignition key from the "OFF" to the "ON" position.

The 2000 TJ cluster provides bus bias and the engine controller provides termination. If either of these components are disconnected, the bus will not be operational. Starting in 1998, the cluster also provides termination as a backup to the engine controller.

The cluster will be serviced as an assembly. There will be limited parts available, bulbs and sockets, the front lens, the trip odometer stem cover and the rear cover assembly. No internal parts of the cluster will be serviced.

Engine Temp Critical - when gauge reaches the high end of the red zone .
Oil Pressure Low - below 6 psi and above 300 rpm.
Volt Gauge Low - when engine controller sets the "charge fail" DTC.
Volt Gauge High - when engine controller sets the "voltage high" DTC.

Normal Operation: At key ON, the cluster will light the low fuel, seat belt, check gauges, airbag and/or smart key immobilizer and upshift lamps (if equipped). As the key is turned farther towards the start position, the ABS (if equipped) and brake warning lamp will illuminate. The vacuum fluorescent (VF) display will indicate the last selected mode, odometer or trip. If there is a bus failure and the cluster cannot receive distance pulses from the PCM, the cluster will display the last mileage stored until the ignition is turned off If the cluster is receiving bus messages but is unable to display odometer information because of an internal failure, the odometer will display dashes. VF dimming is handled by the potentiometer that controls instrumentation illumination. The odometer will remain in the last mode selected, trip or vehicle mileage after each key cycle.

Heater Rear Window Defogger
The timing circuit for the rear window defogger is contained in the cluster. The operation is as follows: With ignition on, the first push of the defogger button will start a 10 minute timer. After 10 minutes have elapsed, the defogger will be turned off. If the button is pushed a second time during the same ignition cycle, the timer will turn the defogger on for 5 minutes. Every consecutive push will operate the defogger for 5 minutes. When the defogger is ON, it can be turned off with a second push of the defogger button.

Chime
The chime function is a 1000 Hz tone contained with in the cluster. The cluster will not generate more than one chime per condition in the same ignition cycle. If a problem occurs that requires a second chime while the first one is currently being generated, the second chime request will be ignored.

The chime will sound for the following conditions:
- Seat Belt Warning - 6 seconds after key ON
- Key in Ignition Warning - door open with key in ignition
- Engine Temp High - when gauge enters red zone

Lamps (Hardwired)
The following lamps are hardwired in the cluster and are not part of the self-test.
- Full Time 4WD (export only)
- Part Time 4WD Full Time 4WD (export only)
- Part Time 4WD
- Brake WARNING (red)
- ABS (amber)
- High Beam
- Left Turn Signal
- Right Turn Signal
- Master Illumination (export only)

Lamps (CCD)
The following lamps are controlled by the cluster via CCD messages. They are actuated during the self-test procedure.
- Low Fuel Warning Airbag
- Seat Belt
- Upshift (if equipped) Check Engine
- Check Gauges
- Smart Key Immobilizer (if equipped)

Diagnostics
Gauges: The instrument cluster will not store any DTCs. Because the engine controller gathers all the gauge information and sends it to the cluster via the bus, it is the PCM's responsibility to monitor these inputs and store DTCs for them. For this reason the first item to be checked regarding a gauge problem (except oil pressure) should be the PCM for any DTCs relating to the gauge in question.

There are DTCs associated with all the gauges except for the oil pressure gauge. When diagnosing gauges, the input that is used for the gauge message (fuel level sender for example) must be valid. Because of OBD II requirements, most of the information used for monitoring the OBD II system happens to be the same information used by the PCM to formulate the gauge bus messages. OBD II requirements state that the fuel level sender must be checked for rationality. This includes determining if the sending unit voltage is too high or too low or if the voltage hasn't changed over time. If the PCM has determined that the sender information isn't valid, it will set a diagnostic trouble code. The presence or absence of the fuel level sending code in the PCM is an important factor to determine whether a fuel gauge problem is in the sending unit or the cluster.

This same situation is used in all cluster diagnostics related to gauge operation with the exception of the oil pressure gauge. OBD II requirements do not include oil pressure monitoring. Therefore, oil pressure diagnostics would dictate that a mechanical oil pressure gauge be attached to the engine for absolute verification of oil pressure readings.

Hardwired Lamps: The cluster contains several hardwired lamps described earlier. These lamps are set up in two configurations. Either the lamp is supplied power by the cluster and the ground is supplied by the circuit being monitored or the lamp is supplied by the ground in the cluster and power comes from a module or other device, Refer to the schematic for the specific details on the lamp in question. Verification of the bulb filament may be required in all cases of an inoperative lamp. This will require removal of the cluster and the bulb in question.

CCD Controlled Lamps: The cluster contains several CCD controlled lamps as described earlier. These lamps are part of the diagnostic self-test. When entering the self-test mode, operation of these lamps can be observed. If the bulb fails to operate during the self-test and the bulb filament is good, the cluster assembly will need to be replaced.

SENTRY KEY IMMOBILIZER SYSTEM (SKIM)
The SKIS is an immobilizer system designed to prevent unauthorized vehicle operation. The system consists of a Sentry Key Immobilizer Module (SKIM) and ignition key(s) called Sentry Key(s) which are equipped with a transponder chip. The SKIM communicates over the Chrysler Collision Detection (CCD) data bus network to the Powertrain Control Module (PCM), instrument cluster and/or the DRB scan tool.

When the ignition is turned to the RUN/START position, the SKIM produces a Radio Frequency (RF) via an antenna which is located around the ignition lock cylinder. The RF powers up the transponder chip inside the Smart Key. The transponder chip then emits a coded signal that is received by the SKIM antenna. If this coded signal is valid, the SKIM sends a "VALID KEY" message to the PCM. If the first transponder signal is not valid, the SKIM will attempt four more times to receive a correct transponder signal. If a valid signal is not received, the SKIM stores a diagnostic trouble code and sends an "INVALID KEY" message to the PCM. Then the PCM will allow the engine to start for approximately two seconds and stall.

The SKIS includes two valid Smart Keys from the factory. If the customer wishes, additional uncoded Smart Key blanks are available. These blank keys must be cut to match a valid key, but the engine will not start unless the transponder chip inside the blank key is programmed to that particular vehicle.

The Sentry Key Immobilizer System (SKIS) can be programmed to recognize a total of eight keys. For programming the Smart Key Transponder, refer to System Component Locations Maintenance and Service Information.

The SKIM also sends messages to the instrument cluster over the CCD bus to control the indicator lamp. The SKIM sends a message to the instrument cluster to turn the lamp on for about three seconds when the ignition is turned to the Run/Start position as a bulb test. If the indicator lamp comes on and stays on after the bulb test, it indicates that the SKIM has detected a system malfunction and/or that the SKIS has become inoperative. If an invalid key is detected in the ignition key cylinder with the ignition switch in the Run/Start position, or when the SKIM has entered its customer key programming mode, the SKIM will flash the SKIS indicator lamp ON and OFF.

VEHICLE COMMUNICATION
The Chrysler Collision Detection multiplex system (CCD bus) consists of a twisted pair of wires. These wires run from one module to another. They receive and deliver coded information between the modules. The information is coded to identify the message as well as the importance of the message. When there are multiple messages trying to access the CCD bus at one time, the code determines the message that has higher priority, which is then allowed to access the bus first.

The two wires of the twisted pair that make up the CCD multiplex system are called "bus(+)" (bus plus) and "bus(-) (bus minus) respectively. Each wire has a measurable voltage level of approximately 2.5 volts. In order to maintain the 2.5 volts on each line and provide a means of transportation for the coded messages, there is a "voltage divider network". This network consists of the Instrument Cluster module which supplies bias and the PCM which supplies termination. Bias is the part of the voltage divider network that places both bus(+) and bus(-) at 2.5 volts. Termination in the circuitry is required to complete the voltage divider network and also provides some electromagnetic protection for the bus.

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

Bus Failure Messages

Short to Battery - Either or both of the bus wires are shorted to the battery potential.

Short to 5 Volts - Either or both of the bus wires are shorted to a 5-volt potential.

Short to Ground - Either or both of the bus wires are shorted to ground.

Bus (+) & Bus (-) Shorted Together- The two bus wires are shorted together.

No Termination - The bus system has not lost connection with its terminator.

Bus Bias Level Too Low - Either or both of the bus wire potentials are significantly below their normal 2.5 volts.

Bus Bias Level Too High - Either or both of the bus wire potentials are significantly above their normal 2.5 volts.

No Bus Bias - The bus system has lost connection with the VTSS module which provides bias.

Bus (+) Open - The bus (+) wire has lost connection with termination and/or bias.

Bus (-) Open - The bus (-) wire has lost connection with termination and/or bias.

Not Receiving Bus Messages Correctly - The DRB cannot communicate over the bus and does not know why.

AIRBAG DIAGNOSTIC TROUBLE CODES
Airbag diagnostic trouble codes consist of active and stored codes. If more than one exists, diagnostic priority should be given to the active code(s).

Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of the trouble codes. It is necessary to perform all of the tests in this book to diagnose an individual code.

Always begin by reading the diagnostic trouble codes using the DRB. If more than one code exists diagnostic priority should be given to the active code(s).

Active diagnostic trouble codes for the airbag system are not permanent and will change the moment the reason for the code is corrected. In certain test procedures, diagnostic trouble codes are used as a diagnostic tool.

Active Codes
An active trouble code indicates an on-going malfunction. This means that the defect is currently there every time the airbag control module checks that circuit/function. It is impossible to erase an active code; active codes automatically erase themselves when the reason for the code has been corrected.

With the exception of the warning lamp trouble codes or malfunctions, when a malfunction is detected, the AIRBAG lamp remains lit for a minimum of 12 seconds or as long as the malfunction is present.

Stored Codes
Airbag codes are automatically stored in the ACM's memory when the ignition is turned OFF with the exception of the Loss of Ignition Run Only code, which is an active code only.

A "stored" code indicates there was an active code present at some time. However, the code currently may not be present as an active code, although another active code could be.

When a trouble code occurs, the AIRBAG warning lamp illuminates for 12 seconds minimum (even if the problem existed for less than 12 seconds). The code is stored, along with the time in minutes it was active, and the number of times the ignition has been cycled since the problem was last detected.

The minimum time shown for any code will be one minute, even if the code was actually present for less than one minute. Thus, the time shown for a code that was present for two minutes 13 seconds, for example, would be three minutes.

If a malfunction is not active while performing a diagnostic test procedure, the active code diagnostic test will not locate the source of the problem. In this case, the stored code can indicate an area to inspect.

If no obvious problems are found, erase stored codes, and with the ignition "ON", wiggle the wire harness and connectors, rotate the steering wheel from stop to stop. Recheck for codes periodically as you work through the system.This procedure may uncover a malfunction that is difficult to locate.

Airbag Trouble Codes
The Airbag Control Module may report any of the following diagnostic trouble codes.

For the following ACTIVE OR STORED codes, replace the Airbag Control Module (ACM) even if set intermittently:
- AECM Accelerometer
- Internal Diagnostic 1
- AECM Output Driver
- AECM Stored Energy Logic
- AECM Stored Energy Driver
- AECM Stored Energy Passenger
- Internal Diagnostic 2
- Internal Diagnostic 3
- Internal Diagnostic 4
- Safing Sensor Shorted

For these codes, refer to the appropriate diagnostic procedure:
- Driver Squib Circuit Open
- Driver Squib Circuit Shorted
- Loss of Ignition Run Only
- Loss of Ignition to Run/Start
- No Cluster CCD Bus Message
- No CCD Communication
- Passenger Squib Circuit Shorted
- Passenger Squib Circuit Open
- Either Squib Term Shorted to Battery
- Either Squib Term Short to Ground
- Warning Lamp Circuit Open/Shorted

SENTRY KEY IMMOBILIZER CODES
Typically, modules have Diagnostic Trouble Codes that are referred to as either active or stored. In the case of the Sentry Key Immobilizer Module, however, all codes are stored codes. The two various types of stored codes in the SKIM can be considered to be either current or pre-existing codes. If more than one code exists, diagnostic priority should be given to the current code(s). A current code indicates a malfunction that is CURRENT at the time of vehicle service. A pre-existing code indicates the malfunction is not currently present. If a malfunction is not detected after 80 ignition cycles, the code will automatically be erased.