Air Bag Control Module: Description and Operation

WARNING: During service procedures, be very careful when handling a sensing and diagnostic module (SDM). Never strike or jar the SDM. never power up the sir system when the SDM is not rigidly attached to the vehicle. All SDM and mounting bracket fasteners must be carefully torqued and the arrow must be pointing toward the front of the vehicle to ensure proper operation of the sir system. The SDM could deploy the AIR bags resulting in personal injury if powered and not rigidly attached to the vehicle. Be sure that the ignition is "off" and the SDM connector disconnected before mounting or dismounting the SDM.


The sensing and diagnostic module (SDM) is the control module for the Supplemental Restraint System (SIR) and contains a sensing device which converts the vehicles acceleration changes to an electrical signal. The electrical signal generated is processed by the SDM and then compared to values stored in electrically erasable programmable read only memory (EEPROM). When the proper thresholds are exceeded, the SDM will cause current to flow through the inflator modules deploying the air bags.


NOTE: Do not open the ADM case for any reason. Touching the connector pins or soldered components may cause electrostatic discharge damage. Repair of a faulty SDM is by replacement only.

The sensing and diagnostic module (SDM) is designed to perform the following functions in the SIR system:
1. Frontal Crash Detection: The SDM monitors vehicle acceleration changes to detect frontal crashes which are severe enough to warrant deployment.
2. Air Bag Deployment: When a frontal crash requiring deployment is detected, the ADM provides enough current to deploy the air bags.
3. Energy Reserve: The SDM maintains a 36 volt loop energy reserve (36 VLR) energy supply to provide deployment current if ignition voltage is lost in a frontal crash.
4. Malfunction Detection: The SDM performs diagnostic monitoring of SIR system electrical components and sets diagnostic fault codes when a malfunction is detected.
5. Driver Notification: The SDM warns the driver of the SIR system malfunctions by controlling the "Air Bag" warning lamp.
6. Malfunction Diagnosis: The SDM displays SIR diagnostic fault codes and system status information through a scan tool.
7. Crash Recording: The SDM records information regarding the SIR system status when deployment has been commanded.

NOTE: The SDM is connected to the SIR wiring harness by a 20-way connector. This connector has a shorting bar across two terminals which connects the AIR BAG warning lamp to ground when the SDM connector is disconnected. This will cause the AIR BAG telltale to come "On" steady whenever the ignition switch is at the RUN or START positions with the SDM disconnected.