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:

Notice: Do not open the sensing and diagnostic module (SDM) 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 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. Inflator Module Deployment: When a frontal crash requiring deployment is detected, the SDM provides enough current to deploy the inflator modules.
3. Energy Reserve: The SDM maintains approximately a 23 volt loop energy reserve (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 set DTCs when a malfunction is detected.
5. Driver Notification: The SDM warns the driver of the SIR system malfunctions by controlling the AIR BAG telltale 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.

The SDM is connected to the SIR wiring harness by a 18-way connector.

Ignition voltage is applied to the AIR BAG telltale lamp when the ignition switch is at the RUN or START position. The SDM controls the lamp by providing a signal to the lamp driver. The AIR BAG telltale lamp is used in the SIR system to notify the operator of the following:

1. Correct system operation by flashing seven times when the ignition switch is first turned "On" and then extinguishing.
2. Warn the vehicle driver of SIR electrical system malfunctions which could potentially affect the operation of the SIR system.

The AIR BAG telltale lamp is the key to driver notification of SIR system malfunctions.

Refer to SIR Component Views .

Refer to SIR Component Views .

Refer to SIR Component Views .

Refer to SIR Component Views .

The inflator modules consist of an inflatable bag and an inflator (a canister of gas generating material and an initiating device). When the vehicle is in a frontal crash requiring deployment, the SDM causes current to flow through the deployment loops. Current passing through the initiator ignites the material in the inflator. The gas produced from this reaction rapidly inflates the air bag.

There is a shorting bar on the driver inflator module. The shorting bar shorts across the diver inflator module circuits when the connector is disconnected. The circuit to the driver inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the driver inflator module, the steering column or other SIR system components.

There is a shorting bar on the passenger inflator module connector which connects the passenger inflator module to the I/P wiring harness. The shorting bar shorts across the passenger inflator module circuit when disconnected. The circuit to the passenger inflator module is shorted in this way to help prevent unwanted deployment of the air bag when servicing the passenger inflator module, the I/P or other SIR system components.

The steering column is energy absorbing and is designed to compress in a frontal crash to minimize injury to the driver.

The knee bolsters are used to absorb energy and control the forward movement of the driver and passenger during a frontal crash by limiting leg movement.