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In the early days of air bag installation and use, modules, which contained large capacitors, were included with the air bag installations. The purpose of those modules was to fire the air bags from the electrical energy stored in the capacitors in the event that impact damages took the vehicle's battery out of service early in the collision sequence. Those modules typically contained a small memory chip, which would record certain parameters at the time of the crash for the purpose of allowing post-crash investigators to evaluate the efficacy of the air bag system and other safety-related components. In General Motors vehicles, that initial module has evolved into the Sensing Diagnostic Module (SDM). In motor vehicles, which have such recording capability, the generic term is Event Data Recorder (EDR). These recorders are generally, but not universally, included as an element of the Airbag Control Module (ACM), whose primary function is to fire the airbags when needed; data collection/storage is a secondary function of the ACM.
Since air bag deployment needs to be anticipatory rather than reactive, the SDM uses an algorithm to decide if the air bags will be needed and deploy them when appropriate conditions have been met. When the vehicle's parameters are such that the algorithm "wakes up" and "starts looking" to evaluate the severity of the event, the SDM can also record data. The GM SDM can record data for a non-deployment event, in which the algorithm was enabled but the airbag's were not fired, and it will (in most cases) record a very useful collection of data when the algorithm has fired the bag's. The instant when the algorithm "wakes up" is referred to as "algorithm enable" (AE). Most data values are recorded in terms of seconds or milliseconds before and/or after AE.
What does that mean to accident reconstruction? In many cases, the data contained in the SDM can significantly augment or reinforce the observations, opinions, and conclusions derived from a thorough investigation and reconstruction. The data stored in the SDM should never be used as a substitute for an accident reconstruction, but it contains information that may not be verifiable by post-collision examination.
For instance, it typically records whether or not the driver's seat belt was buckled, whether or not there were diagnostic trouble codes present before the crash, and whether or not the brake pedal was applied during a time period preceding AE.
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