This emerging and growing technology isn’t really “new” any more, but some of you might not be very familiar with it. As more and more vehicles become “supported” by their manufacturers for this protocol, this will increasingly become an element of a thorough traffic crash investigation. The term “black box” is inaccurate and misleading, but seems to have stuck as the lay person’s reference term for this methodology.

The “black box”, in reality, is most often silver (metallic) in color, and is the vehicle’s Airbag Control Module. It is frequently located in the vehicle’s “center tunnel”, or center hump or console track, under one of the front seats, or occasionally in a front kick panel. It’s usually square or rectangular, and is bolted to the floor pan with 3 machine screws. It’s most often oriented with an embossed arrow pointed forward, and with the three screws and some detent pins on the bottom of the module to properly set the module’s orientation. Now, what is the module there for? Well, as you might expect, it’s primary function is to sense a crash and make an evaluation as to whether air bags are needed to protect the front seat occupants. This is the module’s most important task. Everything else, including recording vehicle dynamics data, is secondary.

As a function of the module’s evaluation as it senses the crash pulse, it may determine that air bags are not necessary, but that seat belt pretensioners need to be deployed. This counts as a “deployment” in terms of how the module responds to the crash pulse that it senses. It is becoming increasingly possible that some vehicles will evaluate and record Power Train Control module (PCM) data, or Roll Over Sensor (ROS) data. The PCM data recording and download availability is primarily a Ford application at present.

There are three different ways to access and attempt to download data that has been recorded by a vehicle’s ACM. The first, least destructive and most convenient and preferable, is through the vehicle’s DLC, or Data Link Converter. This is that same data port that a dealership usually uses for engine diagnostics, and is normally under the dash on the driver’s side of the vehicle.  This is usually possible when the damage to the front of the vehicle, and the power system, is minimal. The second method is to go to the ACM module itself (or the Powertrain Control Module if applicable), and connect directly to it with the CDR kit cables and computer software linking elements. The module is still affixed to the vehicle’s floor pan, and the only change to the vehicle is to remove obstructing fixtures and carpeting, and unplug the wiring harness. This is the second most preferred method, as it is unlikely that the module will be jostled when powered up and a false event introduced and over writing the data that may be contained in the module. The third method, and the least preferred, is to remove the module from the vehicle and perform a “desk top” or “bench top” download. Care must be taken that the vehicle is not powered up when the module is removed and handled, for the reasons just given.  Great care must be taken when downloading PCM data. This data is more perishable and vulnerable to overwrite than ACM data. PCM’s may be also downloaded through the DLC, or directly from the PCM itself using special available cables. As stated, PCM’s are particularly vulnerable to the “over-write” issue, if the power to a vehicle is not cut within a brief time after a collision event. It’s always worth checking, even if you doubt that usable data may be recovered. PCM data is particularly important when it can be obtained in police involved crashes, because of the frequent involvement of the Crown Victoria model.

Generally speaking, in terms of useful information, late model GM vehicles that are supported for CDR are the most desired for download data. This is because they frequently provide some pre-crash data, whereas Ford vehicles that are supported often provide only crash pulse, post impact data. All modules generally record Delta-V, or change of velocity, data. As stated above, many Ford models now provide PCM data. Some Chrysler vehicles are now supported for CDR as well. GM’s have been on line since the mid 90’s, and Ford has been supported since 2001. Chrysler is more recent.

As of this date, June 2011, Fiat has now come on line with some coverage, and coverage of some Toyota models is becoming available as I type this. The GM vehicles that provide pre-crash data normally provide from 2.5 to 5 seconds of data, in either one second or half second resolution. The data points, depending on the age of the vehicle and the module on board, may include speed in miles per hour, engine speed in RPM’s, accelerator pedal position in percent of application, and braking. Additionally, information is frequently provided about seat belt status, ignition cycles, change of velocity, crash pulse duration, and sometimes steering wheel position, yaw rate, and even ambient temperature. Possible event reports are deployments, in which seat belts are pretensioned and/or air bags deployed, and non-deployment or near-deployment event records. Normally, deployment records are locked and aren’t perishable. Non-deployment records are only locked under certain conditions, and the data is often vulnerable to over write.

If an officer, or a private practice technician, has to remove a module from a vehicle for download and preservation as evidence, several precautionary actions should be taken: Make sure that power is not applied to the vehicle ignition or the module.  If possible, unplug the wiring harness from the module rather than cutting it.  Cut the wiring harness only when necessary. Do not pry on the module to remove it from the floor pan. Remove the three machine screws with a socket set. After the module is removed, mark it with a permanent marker, preferably a fine point Sharpie, with identifying information. The markings should be a clearly legible VIN, the date, the case reference, and the technician’s initials or signature. If a module has been exposed to moisture, and especially if it has been submerged or frozen in a vehicle that has been in the open for some time, it should be thoroughly dried by time before a download is attempted.  DO NOT USE HIGH TEMPERATURE ARTIFICIAL HEAT TO DRY THE MODULE, AS THIS MAY DAMAGE OR RUIN THE MODULE AND DESTROY DATA. ALSO, THE MODULE SHOULD NOT BE DOWNLOADED WHILE WET OR EVEN DAMP.  DATA MAY BE LOST. It should not be assumed that if a vehicle is burned that the module is destroyed or useless. Unless the casing has literally been burned open and the contents destroyed, a download may be possible.

Written and submitted by  Cliff Prosser
Cliff is a police officer with the Birmingham, Alabama Police Department for 14 years (1972-1986). Previous to that, military police service in Georgia, Oklahoma and Vietnam. Academy instructor, trained in Traffic Accident Investigation and Reconstruction, SWAT, Dignitary Protection and Crash Data Retrieval. Published author, IPTM.