Air Bag Control Module: Description and Operation

Airbag Control Module:

ACM Bracket:

The Airbag Control Module (ACM) is secured with three screws to the top mounting surface of a stamped steel bracket that is welded onto the floor panel within the seat riser 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 integral snap features to a flat metal base plate sandwiched between the bottom of the ACM and the mounting bracket on the floor panel. This cover is designed to shield the ACM and its wire harness connection from other electrical components and wiring located within the seat riser area beneath the driver seat, and must always be reinstalled following service removal.

Concealed within a hollow in the center of the die cast aluminum ACM housing is the electronic circuitry of the ACM which includes a microprocessor, an electronic impact sensor, an electronic safing sensor, and an energy storage capacitor. A stamped metal cover plate is permanently secured to the bottom of the ACM housing to enclose and protect the internal electronic circuitry and components.

An arrow printed on a label on the top of the ACM housing near the rear provides a visual verification of the proper orientation of the unit, and should always be pointed toward the front of the vehicle. The ACM housing has integral mounting flanges on three corners. A molded plastic electrical connector receptacle containing numerous terminal pins is integral to the left facing side of the ACM housing. These terminal pins connect the ACM to the vehicle electrical system through a dedicated take out and connector of the vehicle wire harness.

The impact sensor internal to the ACM are calibrated for the specific vehicle, and are only serviced as a unit with the ACM. In addition, the ACM must be electronically programmed for the correct standard and optional supplemental restraint system components installed in the vehicle. The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.

The microprocessor in the Airbag Control Module (ACM) contains the supplemental restraint system logic circuits and controls all of the supplemental restraint system components. The ACM uses On-Board Diagnostics (OBD) and can communicate with the DRBIII(R) scan tool using a diagnostic Serial Communication Interface (SCI) data bus line for supplemental restraint system programming or diagnosis and testing through the 16-way Data Link Connector (DLC) located on the dash panel below the driver side end of the instrument panel. A hard wired output from the ACM is used for control of the airbag indicator in the Electro Mechanical Instrument Cluster (EMIC).

The ACM microprocessor continuously monitors all of the supplemental restraint system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault, it sets an appropriate Diagnostic Trouble Code (DTC) and sends an output to the EMIC to turn on the airbag indicator. The ACM illuminates the indicator for about four seconds each time the ignition switch is turned to the On position as a bulb test. If the indicator remains illuminated for about ten seconds after the ignition switch is turned to the On position, the ACM has detected a non-critical fault that poses no danger to the vehicle occupants. If the airbag indicator illuminates solid (not flashing) while driving or stays on longer than ten seconds following the bulb test, the ACM has detected a critical fault that may cause the airbags not to deploy when required or to deploy when not required. An active fault only remains for the duration of the fault, or in some cases, for the duration of the current ignition switch cycle, while a stored fault causes a DTC to be stored in memory by the ACM.

The ACM receives battery current through a fused ignition switch output circuit. The ACM receives ground through a ground circuit and take out of the vehicle wire harness. This take out has an eyelet terminal connector secured by a nut to a ground stud on the floor panel directly below the ACM within the driver side seat riser. A case ground is also provided for the ACM through a ground circuit and eyelet terminal connector secured under the left rear ACM mounting screw. These connections allow the ACM to be operational whenever the ignition switch is in the On position.

The ACM also contains an energy-storage capacitor. When the ignition switch is in the On position, this capacitor is continually being charged with enough electrical energy to deploy the supplemental restraint components for up to one second following a battery disconnect or failure. The purpose of the capacitor is to provide backup supplemental restraint system protection in case there is a loss of battery current supply to the ACM during an impact.

Two sensors are contained within the ACM, an electronic impact sensor and a safing sensor. These electronic sensors are accelerometers that sense the rate of vehicle deceleration, which provide verification of the direction and severity of an impact. On models equipped with optional side curtain airbags, the ACM also monitors inputs from two remote side impact sensors located within the left and right front door step wells to control deployment of the side curtain airbag units.

The safing sensor is an electronic accelerometer sensor within the ACM that provides an additional logic input to the ACM microprocessor. The safing sensor is used to verify the need for a supplemental restraint deployment by detecting impact energy of a lesser magnitude than that of the primary electronic impact sensors, and must exceed a safing threshold in order for the airbags to deploy. Vehicles equipped with optional side curtain airbags feature a second safing sensor within the ACM to provide confirmation to the ACM microprocessor of side impact forces. This second safing sensor is a bi-directional unit that detects impact forces from either side of the vehicle.

Pre-programmed decision algorithms in the ACM microprocessor determine when the deceleration rate as signaled by the impact sensors and the safing sensors indicate an impact that is severe enough to require supplemental restraint system protection. When the programmed conditions are met, the ACM sends the proper electrical signals to deploy the front airbags and seat belt tensioners and, if the vehicle is so equipped, either side curtain airbag unit.

The ACM also provides a hard wired electrical crash signal output following a supplemental restraint deployment event. This output is used to signal other electronic modules in the vehicle to provide their enhanced accident response features, which include automatically disabling the engine from running and unlocking all of the doors. However, these responses are each dependent upon the circuits, components, and modules controlling these features remaining intact from collateral damage incurred during the vehicle impact.

A single ACM is used for all variations of the supplemental restraint system available in this vehicle. This ACM is programmable and in order to function properly it must be programmed for the correct vehicle supplemental restraint system equipment using an initialization procedure. The initialization procedure requires the use of a DRBIII(R) scan tool. Refer to the appropriate diagnostic information. The hard wired inputs and outputs for the ACM may be diagnosed and tested using conventional diagnostic tools and procedures. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the ACM or the supplemental restraint system. The most reliable, efficient, and accurate means to diagnose the ACM or the supplemental restraint system requires the use of a DRBIII(R) scan tool. Refer to the appropriate diagnostic information.