Air Bag Control Module: Description and Operation
AIRBAG CONTROL MODULEThe Airbag Control Module (ACM) is concealed underneath the plastic ACM trim cover (automatic transmission) or center console (manual transmission), directly below the instrument panel in the passenger compartment of the vehicle. The ACM is secured with screws to a stamped steel mounting bracket located under the instrument panel center support bracket on the floor panel transmission tunnel. 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 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. 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.
The ACM receives battery current through two circuits, on a fused ignition switch output (run) circuit through a fuse in the Junction Block (JB), and on a fused ignition switch output (start-run) circuit through a second fuse in the JB. 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 nut to a ground stud located on the forward extension of the left front fender wheel housing in the engine 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. The safing sensor is an electromechanical sensor within the ACM that is connected in series between the ACM microprocessor airbag deployment circuit and the airbags. 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 to deploy. The impact sensor and safing sensor are calibrated for the specific vehicle, and are only serviced as a unit with the ACM.