Airbag System Description
AIRBAG SYSTEMThe Airbag Control Module (ACM) is bolted to a bracket on the floor panel transmission tunnel rearward from the gear shift selector inside the vehicle. The ACM mounting bracket is welded to the tunnel and is not serviced with the ACM. The ACM contains a microprocessor the impact sensor and energy storage capacitor. The microprocessor contains the airbag system logic. The ACM system logic includes On-Board Diagnostics (OBD) capability and communicates with the instrument cluster circuitry via the PCI data bus to control the airbag indicator lamp.
The microprocessor in the ACM monitors the impact sensor signal and the airbag system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault it sends messages to the instrument cluster on the PCI data bus to turn ON the airbag indicator lamp. A preprogrammed decision algorithm in the ACM microprocessor determines when the deceleration rate as signaled by the sensor indicates an impact that is severe enough to require airbag system protection. When the programmed conditions are met the ACM 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 ACM. A mechanical sensor called the safing sensor is also integral to the ACM.
The ACM also contains an energy-storage capacitor. This capacitor stores enough electrical energy to deploy the airbags for one second following a battery disconnect or failure during an impact. The purpose of the capacitor is to provide airbag system protection in a severe secondary impact if the initial impact has damaged or disconnected the battery but was not severe enough to deploy the airbags.
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 AIRBAG CONTROL MODULE 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 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 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 to ground. 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.
Perform the WARNING LAMP OPEN test to find the cause of any customer complaint regarding the AIRBAG warning lamp such as
- warning lamp does not illuminate
- warning lamp stays illuminated with no active DTC's
The ACM cannot be repaired or adjusted and if damaged or faulty it must be replaced