Air Bag Control Module: Description and Operation

CAUTION: Be careful when you handle a Sensing and Diagnostic Module (SDM). Do not strike or jolt the SDM. Before applying power to the SDM:
^ Remove any dirt, grease, etc. from the mounting surface.
^ Position the SDM horizontally on the mounting surface.
^ Point the arrow on the SDM toward the front of the vehicle.
^ Tighten all of the SDM fasteners and SDM bracket fasteners to the specified torque value.
Failure to follow the correct procedure could cause air bag deployment, personal injury, or unnecessary SIR system repairs.

The Sensing and Diagnostic Module (SDM) is designed to perform the following functions in the SIR system:
1. Energy Reserve - The SDM maintains a 36 Volt Loop Reserve (36 VLR) energy supply to provide deployment energy when ignition voltage is lost in a frontal crash.
2. Frontal Crash Detection - The SDM monitors vehicle velocity changes to detect frontal crashes which are severe enough to warrant deployment.
3. Air Bag Deployment When a frontal crash of sufficient force is detected, the SDM will cause enough current to flow through the inflator modules to deploy the air bags.
4. Frontal Crash Recording - The SDM records information regarding the SIR system status during a frontal crash.
5. Malfunction Detection - The SDM performs diagnostic monitoring of the SIR system electrical components and sets a diagnostic trouble code when a malfunction is detected.
6. Malfunction Diagnosis - The SDM displays SIR diagnostic trouble codes and system status information through the use of a scan tool.
7. Driver Notification - The SDM warns the vehicle driver of SIR system malfunctions by controlling the "AIR BAG" warning lamp.

The SDM is connected to the SIR wiring harness by a 20-way connector. This harness connector uses a shorting bar across certain terminals in the contact area. This shorting bar connects the "AIR BAG" warning lamp to ground when the SDM harness connector is disconnected.

This will cause the "AIR BAG" warning lamp to come "ON" steady whenever the ignition switch is at the RUN, BULB TEST or START positions with the SDM disconnected.