Grand Cherokee 2WD Limited V8-4.7L (2002)
Body Control Module: Description and Operation
Body Control System
System Description
SYSTEM DESCRIPTION
The body system on the 2002 WJ/WG consists of a combination of modules that communicate over the PCI bus (Programmable Communication
Interface multiplex system). Through the PCI bus, information about the operation of vehicle components and circuits is relayed quickly to the
appropriate module(s). All modules receive all the information transmitted on the bus even though a module may not require all information to
perform it's function. It will only respond to messages "addressed" to it through a binary coding process. This method of data transmission
significantly reduces the complexity of the wiring in the vehicle and the size of wiring harnesses. All of the information about the functioning of all
the systems is organized, controlled, and communicated by the PCI bus, which is described in the Communication Section of this general
information.
Part 1 of 3
AIRBAG SYSTEM / OCCUPANT RESTRAINT CONTROLLER SYSTEM
The 2002 WJ/WG Airbag System contain the following components: Occupant Restraint Controller (ORC), Airbag Warning Indicator,
Clockspring, Driver and Passenger Airbags, Driver and Passenger Hall-effect Seat Belt Buckle Switches (SBS), Driver and Passenger Front and
Side Impact Sensors, curtain Airbags, and front impact sensors.
The Occupant Restraint Controller (ORC) is a new type of Airbag Control Module (ACM). The new ACM supports staged airbag deployment and
remote impact sensing. Staged deployment is the ability to trigger airbag system squib inflators individually as needed to provide the appropriate
restraint for the severity of the impact. The ACM has four major functions: PCI Bus communications, onboard diagnostics, impact sensing, and
component deployment. The ACM also contains an energy-storage capacitor. This capacitor stores enough electrical energy to deploy the front
airbag components for two seconds following a battery disconnect or failure during an impact. The ACM is secured to the floor panel transmission
tunnel under the console. The ACM cannot be repaired or adjusted.
The ACM sends and/or receives PCI Bus Messages With The Instrument Cluster (MIC), Body Control Module (BCM), and Impact Sensors
Diagnostic trouble codes will be set if the communication with these modules is lost or contains invalid information.
The microprocessor in the ACM monitors the impact sensors signals and the airbag system electrical circuits to determine the system readiness. If
the ACM detects a monitored system fault it sends a message to the instrument cluster via PCI bus to turn ON the airbag warning indicator. The
ACM can set both active and stored diagnostic trouble codes to aid in the diagnosing system problems. See DIAGNOSTIC TROUBLE CODES.
The ACM uses two front impact sensors and ACM Accelerometer to sense the rate of vehicle deceleration, provide verification of the direction
and severity of an impact. A pre-programmed decision algorithm in the ACM microprocessor determines when the deceleration rate is severe
enough to require airbag system protection. The ACM also uses the driver and front passenger seat belt switch status (buckled or unbuckled) and
crash severity to determine the level of driver and front passenger airbag deployment, low medium or high. When the programmed conditions are
met, the ACM sends an electrical signal to deploy the appropriate airbag system components.
WARNING: THE AIRBAG SYSTEM IS A SENSITIVE, COMPLEX ELECTROMECHANICAL UNIT. BEFORE ATTEMPTING TO
DIAGNOSE OR SERVICE ANY AIRBAG SYSTEM OR RELATED STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT
PANEL COMPONENTS YOU MUST FIRST DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE.
WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE FURTHER SYSTEM SERVICE. THIS IS THE
ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO DO THIS COULD RESULT IS ACCIDENTAL AIRBAG
DEPLOYMENT AND POSSIBLE PERSONAL INJURY. NEVER STRIKE OR KICK THE AIRBAG CONTROL MODULE, AS IT
CAN DAMAGE THE IMPACT SENSOR OR AFFECT ITS CALIBRATION. IF AN AIRBAG CONTROL MODULE IS
ACCIDENTALLY DROPPED DURING SERVICE, THE MODULE MUST BE SCRAPPED AND REPLACED WITH A NEW UNIT.
The airbag warning indicator is the only point at which the customer can observe symptoms of a system malfunction. Whenever the ignition key is
turned to the run or start position, the ACM performs a lamp check by turning the airbag warning indicator ON for 6-8 seconds. After the lamp
check, if the indicator turns OFF; it means that the ACM has checked the system and found it to be free of discernible malfunctions. If the lamp
remains ON, there could be an active fault in the system or the MIC lamp circuit may be internally shorted. If the lamp comes ON and stays ON
for a period longer than 6-8 seconds then goes OFF, there is usually an intermittent problem in the system.
Driver Airbag
The airbag protective trim cover is the most visible part of the driver side airbag system. The protective trim cover is fitted to the front of the
airbag module and forms a decorative cover in the center of the steering wheel. The module is mounted directly to the steering wheel. Located
under the trim cover are the horn switch, the airbag cushion, and the airbag cushion supporting components. The airbag module includes a housing
to which the cushion and hybrid inflator are attached and sealed. The 2002 WJ/WG is equipped with driver airbag with dual stage inflators. When
supplied with the proper electrical signal, the inflator or inflators discharge the gas directly into the cushion. The airbag module cannot be
repaired, and must be replaced if deployed or in any way damaged.
WARNING: THE DRIVER AIRBAG MODULE CONTAINS ARGON GAS PRESSURIZED TO OVER 17236.89 Kpa (2500 PSI). DO
NOT ATTEMPT TO DISMANTLE AN AIRBAG MODULE OR TAMPER WITH ITS INFLATOR. DO NOT PUNCTURE,
