Air Bag Control Module: Description and Operation

The Sensing and Diagnostic Module (SDM) performs the following functions within the SIR system:

^ Energy Reserve - The SDM maintains a 23 Volt Loop Reserve (23 VLR) energy supplies in order to provide deployment energy. If ignition voltage is lost during a frontal crash, the 23 VLR supply will deploy the air bags.
^ Frontal Crash Detection - The SDM monitors the vehicle velocity changes in order to detect frontal crashes which are severe enough to warrant deployment.
^ Air Bag Deployment - When a frontal crash of sufficient force is detected, the SDM will cause enough current to flow through the inflator modules in order to deploy the air bags.
^ Frontal Crash Recording - The SDM records information regarding the SIR system status during a frontal crash.
^ Malfunction Detection - The SDM performs diagnostic monitoring of the SIR system electrical components. The SDM sets a diagnostic trouble code when the SDM detects a malfunction.
^ Malfunction Diagnosis - The SDM displays SIR diagnostic trouble codes and system status information through the use of a scan tool.
^ Driver Notification - The SDM warns the vehicle driver of SIR system malfunctions by controlling the AIR BAG warning lamp.

The SDM connects to the SIR wiring harness using a 12-way connector. The SDM harness connector uses a shorting bar across certain terminals in the contact area. If you remove of the SDM Connector Position Assurance (CPA) or the harness connector itself, the AIR BAG warning lamp will connect to ground through the shorting bar. The AIR BAG warning lamp will come ON steady with the power applied to the SDM when either one of the following two conditions exists:

^ You remove the SDM CPA
^ You disconnect the SDM harness connector

The SDM receives power whenever the ignition switch is at the ON or the START positions.