Air Bag Control Module: Description and Operation

Fig. 5 Airbag Control Module:

The Airbag Control Module (ACM) is secured with screws to a stamped steel mount that is welded onto the floor panel underneath the driver side front seat in the passenger compartment of the vehicle. A molded plastic protective cover is installed over the ACM and secured with double-faced acrylic foam adhesive tape to the floor panel. This cover is designed to shield the ACM and its wire harness connection from excessive moisture exposure, and must always be replaced with a new cover following service removal. The ACM contains an electronic microprocessor, an electronic impact sensor, an electromechanical safing sensor, and an energy storage capacitor. The ACM is connected to the vehicle electrical system through a dedicated take out and connector of the instrument panel wire harness.

The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.

The microprocessor in the ACM contains the airbag system logic circuits, and it monitors and controls all of the airbag system components. The ACM also uses On-Board Diagnostics (OBD) and can communicate with other electronic modules in the vehicle as well as with the DRB III scan tool using the Chrysler Collision Detection (CCD) data bus network. This method of communication is used for control of the airbag indicator in the ElectroMechanical Instrument Cluster (EMIC) and for airbag system diagnosis and testing through the 16-way data link connector located on the lower left edge of the instrument panel. (Refer to INSTRUMENT CLUSTER/AIRBAG INDICATOR - OPERATION). The ACM microprocessor continuously monitors all of the airbag system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault, it sets an active Diagnostic Trouble Code (DTC) and sends messages to the EMIC over the CCD data bus to turn ON the airbag indicator. If the airbag system fault is still present when the ignition switch is turned to the OFF position, the DTC is stored in memory by the ACM. However, if a fault does not recur for a number of ignition cycles, the ACM will automatically erase the stored DTC.

For models equipped with the passenger airbag ON/OFF switch (all without factory-installed or approved rear passenger seating), the switch is installed in series in the circuits between the ACM and the passenger airbag. If the passenger airbag ON/OFF switch is set to the OFF position, the switch opens the circuits to the passenger airbag to disable the passenger airbag from being deployed if an impact is detected that is sufficient for an airbag deployment, and closes the circuits from the ACM through an internal resistor to prevent the ACM from setting a DTC for the open passenger airbag circuits. The ACM will also set and/or store a DTC for faults it detects in the passenger airbag ON/OFF switch circuits just as though the switch were the passenger airbag, and will turn ON the airbag indicator in the EMIC if a fault has been detected.

The ACM receives battery current through two circuits, on a fused ignition switch output (run) circuit through a fuse in the fuse block, and on a fused ignition switch output (run-start) circuit through a second fuse in the fuse block. The ACM is grounded through a ground circuit and take out of the instrument panel wire harness. This take out has a single eyelet terminal connector secured by a ground screw to the back of the left end of the main instrument panel cross body support in the passenger compartment. Therefore, the ACM is operational whenever the ignition switch is in the Start or ON positions. The ACM also contains an energy-storage capacitor. When the ignition switch is in the Start or ON positions, this capacitor is continually being charged with enough electrical energy to deploy the airbags for up to one second following a battery disconnect or failure. The purpose of the capacitor is to provide backup airbag system protection in case there is a loss of battery current supply to the ACM during an impact. The capacitor is only serviced as a unit with the ACM.

Two sensors are contained within the ACM, an electronic impact sensor and a safing sensor. The electronic impact sensor is an accelerometer that senses the rate of vehicle deceleration, which provides verification of the direction and severity of an impact. A pre-programmed decision algorithm in the ACM microprocessor determines when the deceleration rate as signaled by the impact 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 airbags and the seat belt tensioners. The safing sensor is an electromechanical sensor within the ACM that is connected in series between the ACM microprocessor deployment circuit and the airbags/seat belt tensioners. The safing sensor is a normally open switch that is used to verify or confirm the need for an airbag deployment by detecting impact energy of a lesser magnitude than that of the electronic impact sensor, and must be closed in order for the airbags/seat belt tensioners to deploy. The impact sensor and safing sensor are calibrated for the specific vehicle, and are only serviced as a unit with the ACM.