The “Black Box” continue...

The “Black Box”

Beginning with the 1999 model year, General Motors added the capability to record vehicle systems status data for several seconds before impact to many of its vehicles. Vehicle speed, engine RPM, throttle position and brake switch (on/off) status are recorded for the five seconds preceding a deployment of non-deployment event. Essentially all General Motors vehicles now have that capability.

How the System Works

Automobiles that are equipped with airbags use an algorithm to determine when the airbags should be deployed. Generally, the air bag is designed to inflate in moderate to severe frontal or near-frontal crashes. The air bag will inflate only if the impact speed is above the system's designed "threshold level." If a vehicle goes into a wall that doesn't move or deform, the threshold level is about 9 to 15 mph (14 to 24 km/h). The threshold level can vary, however, with specific vehicle design, so that it can be somewhat above or below this range.

The SDM, which is controlled by a microprocessor, has multiple functions: (1) it determines if a severe enough impact has occurred to warrant deployment of the air bag; (2) it monitors the air bag's components; and (3) it permanently records information. The SDM contains software that analyzes the longitudinal deceleration of a vehicle to determine whether a deployment event has occurred, based on testing that has been done previously to determine what events would require protection by an air bag. When the SDM senses an event (either a deployment event or an event that is not severe enough to require an air bag--that is, a near-deployment event), that information is recorded to the microprocessor's electrically erasable programmable read-only memory (EEPROM). (When the air bag is deployed, the SDM records the event as a "Code 51.") Although certain diagnostic codes stored in the EEPROM can be erased, the SDM is specifically designed to prevent air-bag deployment data from being altered or erased.

Because SDM data is stored in hexadecimal format, it must ordinarily be converted to a decimal format before it can be analyzed. (A hexadecimal system is a numbering system that has 16 characters; that system employs the letters A through F, in addition to the numbers zero through nine, which are used in the decimal system.)

In mid-2000, a publicly available crash data retrieval tool (Vetronix equipment) was updated with new software to allow anyone with a Windows-based computer to download SDM data in an easy-to-read format.

As previously stated, there are two types of airbag module (SDM) recorded crash events. The first is the near deployment event. A near deployment event is an event severe enough to "wake up" the sensing algorithm but not severe enough to deploy the airbag(s). It contains pre-crash and crash data. The SDM can store up to one near-deployment event. This can be overwritten by an event that has a greater SDM recorded velocity change. This event will be cleared by the SDM after the ignition has been cycled 250 times. The second type of SDM recorded crash event is the deployment event. It also contains pre-crash and crash data. The SDM can store up to two different deployment events, if they occur within five seconds of one another. The first deployment event will be stored in the deployment file (this would have been the event that deployed the airbag) and the second deployment event will be stored in the near-deployment file. Deployment events cannot be overwritten or cleared from the SDM. Once the SDM has deployed the airbag, the SDM must be replaced.

Table 1 contains an abbreviated summary of the data recording capability of various General Motors airbag systems, based upon the year that they were introduced.

Table 1: Data Stored by Selected GM Airbag Systems

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