Air Bag Control Module: Description and Operation

AIRBAG SYSTEM / OCCUPANT RESTRAINT CONTROLLER SYSTEM
The Airbag system is equipped with a new type of Airbag Control Module (ACM) called an Occupant Restraint Controller (ORC). The ORC is secured to a bracket on the floor panel transmission tunnel below the instrument panel inside the vehicle. The ORC mounting bracket also serves as the instrument panel center support. The ORC contains a microprocessor, the impact sensor, and energy storage capacitors. The microprocessor contains the airbag system logic. The airbag system logic includes On-Board Diagnostics OBD capability, and communicates with the instrument cluster circuitry on the Programmable Communication Interface (PCI) data bus to control the airbag indicator lamp. The microprocessor in the ORC monitors the impact sensor signal and the airbag system electrical circuits to determine the system readiness. If the ORC detects a monitored system fault, it sends messages to the Mechanical Instrument Cluster (MIC) over the PCI data bus to turn ON the airbag warning lamp. A pre-programmed decision algorithm in the ORC microprocessor determines when the deceleration rate is severe enough to require airbag system protection. When the programmed conditions are met, the ORC sends an electrical signal to deploy the airbag system components. The impact sensor is an accelerometer that senses the rate of vehicle deceleration, which provides verification of the direction and severity of an impact. The impact sensor is calibrated for the specific vehicle, and is only serviced as a unit with the ORC. The ORC also contains an energy-storage capacitor. This capacitor stores enough electrical energy to deploy the airbags, for two minutes following a battery disconnect or failure during an impact. The purpose of the capacitor is to provide airbag system protection in severe secondary impact if the initial impact has damaged or disconnected the battery, but not severe enough to deploy the airbags. The ORC cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.

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 IN 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 OCCUPANT RESTRAINT CONTROLLER IS ACCIDENTALLY DROPPED DURING SERVICE, THE MODULE MUST BE SCRAPPED AND REPLACED WITH A NEW UNIT.

The airbag warning lamp 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 MIC performs a lamp check by turning the AIRBAG warning lamp ON for 6-8 seconds. If the lamp remains OFF, it means that the ORC 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 ORC system or the MIC lamp circuit may be internally shorted to ground.

SIDE IMPACT AIRBAG CONTROL MODULE (SIACM)
Vehicles equipped with side impact airbags use two side impact airbag control modules. One is located on each side of the vehicle within the body B-pillar.

Each Side Impact Airbag Control Module (SIACM) serves as the impact sensor for its seat mounted airbag. The right side SIACM controls the right seat airbag. The left side SIACM controls the left seat airbag. Each SIACM contains two accelerometers, both accelerometers must agree in order to deploy the side airbag. In the event of a side impact, the appropriate SIACM will send a electronic signal to its airbag, deploying the airbag. The SIACM communicates with the Occupant Restraint Controller (ORC) via the PCI bus circuit.

The SIACM perform self-diagnostics and circuit tests to determine if the system is functioning properly. If the test finds a problem, the SIACM will set both active and stored Diagnostic Trouble Codes (DTC's). If a DTC is active, the SIACM will request that the airbag warning lamp be turned on. The results of the system test are transmitted on the PCI bus circuit to the ORC once each second, or on a change in lamp state (ON/OFF). If the warning lamp status message from either SIACM contains a lamp ON request, the ORC will set an active DTC. At the same time as the DTC is set, the ORC sends a PCI bus message to the cluster requesting the airbag warning lamp to be turned ON. Observe all ORC WARNING and CAUTION statements when servicing or handling the SIACM. The SIACM's are not serviceable and must be replaced even if they are dropped.