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.

AIR BAG Warning Lamp
The ignition switch applies ignition voltage to the AIR BAG warning lamp. The AIR BAG warning lamp receives power whenever the ignition switch is at the ON or the START positions. The inflatable restraint SDM controls the lamp by providing ground with a lamp driver. The SIR system uses the AIR BAG warning lamp to do the following:
^ Verify lamp and SDM operation by flashing the lamp seven times when the ignition switch is first turned to the ON position
^ Warn the vehicle driver of any SIR electrical system malfunctions which could potentially affect the operation of the SIR system. These malfunctions could result in non-deployment in case of a frontal crash or deployment for conditions less severe than intended.

The AIR BAG warning lamp notifies the driver of SIR system malfunctions. For proper lamp operation, refer to SIR Diagnostic System Check.