What about that Black Box?
Since the 1990’s vehicles have had a black box or event data recorder (EDRs), the original purpose of which was to help manufacturers obtain information about their vehicle’s performance. The information stored on the EDR includes vehicle speed, brake application, seat belt use, and throttle position. When the airbag deploys, certain information is recorded electronically in a hexadecimal code which can be interpreted by a device. When downloading the data, the technician will first attempt to use the direct link to vehicles diagnostic link connector, which may be a problem if the vehicle has too much damage. A second option is to power the module through the fuse box, and the third option is to remove the module to obtain the information. There is a Standard Practice for such investigations.
The data is stored in a computer file with the extension CDRx which can be retained and is small enough to email. To properly interpret the data takes some training and experience since several vehicle operations could affect the EDR reported speed that must be ruled out along with equipment alternations before relying on the data. These facts include the vehicle being airborne, wheels scrubbing sideways, going in reverse or the tires not b
eing the proper size. The module can record up to three events, sometimes more, so it is necessary to determine if the data is related to the crash in question. If the airbag deploys the data is locked but if it is recording without airbag deployment, then the data can be overwritten. You will not get information about who was driving, the time of the crash or the location of the crash. You can get more data if the vehicle has OnStar.
In the investigation, you need to consider the application of the Driver’s Privacy Protection Act which is a U.S. federal statute and obtain an authorization from the owner or obtain a court order for the download.
EDR’s are all different, with different manufacturers using different EDR’s and even with the same manufacturer the same model of car, and over the years the EDR will change. With one model of vehicle, the type of EDR may change by the end of the year. So, the reports are all different, and they all record different information. You can get a sample report that will help you determine what you get before the downloading the information. There are also different modules including the air bag control module, roll over sensation, and power train module. Some reports like with Chrysler will be a large 40 to 50-page report, and others will only be a 3- or 4-page reports. Some EDRs will record a single event, some multiple events, some will record besides the deployment of an airbag the non-deployment.
Downloading the information from an EDR is also called imaging the data. After you download the information the first thing you must do is verify that the data is good or valid, does it match the crash you are investigating. If there are various event records, you must make sure the data you are looking for is the crash that is the focus of your inquiry. The technician will check the ignition cycles, how many cycles since the time of the event at the time of the download. You would expect maybe one or two. Did the system crash or is it reporting that the system operated properly? You much check to see if there were there multi-events. You check the odometer and the event km to make sure this is the same event.
The data retrieved is different from EDR to EDR but may include information about the steering wheel angle, delta v forces, how hard the car was hit, where, did the passengers have their seat belts on, how fast was the car going, what was it doing right before the crash, brake pressure, seat pressure (important if there are issues of who was driving), evasive actions are taken if any, brake touched or mashed, was the airbag warning on, was the
seat forward or back, and occupant size class.
The Delta V is measured both longitudinal and lateral in the instantaneous microseconds following impact. The velocity change measures how hard the vehicle was hit, what direct and how it speeds up or slowed down in milliseconds. It will measure the right to left forces. You can’t just add the data, but you combine the information with a formula. For example you don’t add the lateral and longitudinal forces but you square both and then add then use the square root of the sum to find out the actual impart, there are other math analysis that can be done in the typical crash to determine the immediate maximum engagement between the vehicles, change of direction and common speed between impact and the initial separation. Anything outside of 100 to 150 milliseconds is probably caused by ground forces or something other than the impact. It takes experience in interpreting the raw data and comparing it to the graph produced to determine when the change impact is over, and anything after that point will be discounted as caused by other events.
The report might have deployment command data should show when the airbag first popped, then if there was a second charge or a third. The sensors recognize the need for addition inflates to cushion the impact. The report will show if there was a trouble code for the car condition. A rollover graph measures in a ratio of degrees per second showing how far the car swung at impact and is used to show if there was a complete rollover. The EDR may record the brake speed, anti-lock brake system engagement and accelerator information. You can tell if the accelerator pedal is being pushed slowly or hard and the same information is available about brake pressure. It samples the information every .2 sec. This information can verify driver reaction to the hazard if any. It general it takes 1.5 seconds for a driver to react to a hazard and the EDR can verify what took place when the driver reacted. The steering information recorded is in degrees.
The EDR may record the wheel speed of each wheel, which is never the same even when the speed of the car is constant. It is measure often in kilometers per hour, and if the speed is within one km/hour of each other it’s all moving at the same speed, and this is a good check for the speeds in the report assuming the tires match the manufacturer specifications. The EDR will also show the tire pressure, and if the cruise control is on or off.
If you know the weight of the vehicles, the delta v, and charted the directions, you can use calculations to create the speed of the vehicle to confirm the data in the EDR and can use a computer stimulation to recreate the crash.
Finally, the report prints the hexadecimal raw data that the computer interprets and presents as a report and is a good check to make sure the data is legit. Use the latest version of the computer software, at this writing the latest version of Crash Data Retrieval Tool is 17.9.
If the module is taken out of the car, there are special protocols that must be followed because due to the sensors you can change the data by even bumping the module and creating a new event.
If you get a scale diagram and the data file off both vehicles with speed work up and the exact angle of approach and departure you can simulate the crash.