Inflatable Restraint Sensing and Diagnostic Module


The inflatable restraint sensing and diagnostic module (SDM) performs the following functions in the SIR system:

Energy Reserve
The SDM maintains 23 Volt Loop Reserve (23 VLR) energy supplies to provide deployment energy for the air bags. Ignition voltage can provide deployment energy if the 23 Volt Loop Reserves malfunction.

Crash Detection
The SDM monitors vehicle velocity changes in order to detect frontal crashes that are severe enough to warrant deployment.

Air Bag Deployment
During a frontal crash of sufficient force, the SDM will cause enough current to flow through the frontal inflator modules to deploy the frontal 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. Upon detection of a circuit malfunction, the SDM will set a Diagnostic Trouble Code (DTC).

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 in the instrument cluster via Class 2 serial data.

The SDM connects to the SIR wiring harness using the following connector(s):

The 18-way connector provides power, ground, and all the required interfaces for the frontal air bag deployment loops.

The SDM receives power when the ignition switch is in the RUN position.

Q: What data can be downloaded from the vehicle's air bag module?

A*: Recorded data depends on vehicle make, model and year. The following data is typical of what is found on a newer GM vehicle.

Vehicle speed (5 seconds before impact)
Engine speed (5 seconds before impact)
Brake status (5 seconds before impact)
Throttle position (5 seconds before impact)
State of driver's seat belt switch (On/Off)
Passenger's air bag enabled or disabled state (On/Off)
SIR Warning Lamp status (On/Off)
Time from vehicle impact to air bag deployment
Ignition cycle count at event time
Ignition cycle count at investigation
Maximum ΔV for non-deployment event
ΔV vs. time for frontal air bag deployment event
Time from vehicle impact to time of maximum ΔV
Time between non-deploy and deploy event (if within 5 seconds)

*Depending on the particular vehicle, all or a subset of this data may be available.

The recorders will trigger when preset physical conditions nearing those sufficient for the airbag to deploy are met—“waking-up” and recording data. Extraction of the data after the event is time sensitive. In general, if an airbag deploys, the unit will permanently write and capture data. Therefore, if that unit is to be replaced when the vehicle is repaired, measures must be taken to collect and preserve the unit and its data. Losing the unit loses the data. Likewise, if the vehicle is involved in an accident in which the airbag is not deployed, the recorder may still trigger during this “non-deployment event.” It will write data from a non-deployment event and retain that data for a limited time period. In some units, that period consists of 250 engine-ignition cycles. Low-impact collisions, therefore, will require early retrieval of the recorder to prohibit loss of the data upon ignition at the 251st start after the collision.

Air bags are typically designed to deploy in frontal and near-frontal collisions, which are comparable to hitting a solid barrier at approximately 8 to 14 miles per hour (mph). Roughly speaking, a 14 mph barrier collision is equivalent to striking a parked car of similar size across the full front of each vehicle at about 28 mph. This is because the parked car absorbs some of the energy of the crash, and is pushed by the striking vehicle. Unlike crash tests into barriers, real-world crashes typically occur at angles, and the crash forces usually are not evenly distributed across the front of the vehicle. Consequently, the relative speed between a striking and struck vehicle required to deploy the air bag in a real-world crash can be much higher than an equivalent barrier crash.
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Front air bags are not designed to deploy in side impact, rear impact or rollover crashes.
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The Crash Sensors are located either in the front of the vehicle and/or in the passenger compartment. Vehicles can have one or more crash sensors. The sensors are typically activated by forces generated in significant frontal or near-frontal crashes. Sensors measure deceleration, which is the rate at which the vehicle slows down. Because of this, the vehicle speed at which the sensors activate the air bag varies with the nature of the crash. Air bags are not designed to activate during sudden braking or while driving on rough or uneven pavement. In fact, the maximum deceleration generated in the severest braking is only a small fraction of that necessary to activate the air bag system.