Retrieving EDR Data
Retrieving EDR data for forensic purposes is more complicated than reading the ECM in a repair shop.
If accepted protocols are followed by trained forensic investigators / engineers, the data will not be altered when accessed through the in-cab (Deutsch) connectors and should remain the same for the life of the ECM battery.
In the experience of the authors of this site, there is a much higher likelihood of missing, erased, corrupted, or incomplete data when the download is performed by an “independent” party that is not trained in the forensic use of HV EDR technology.
Overwriting EDR Data
EDR data are overwritten by every subsequent event. For example, Detroit Diesel’s Last Stop Record can be overwritten by driving the tractor onto a flatbed wrecker to tow it from the scene. In general, every subsequent event overwrites the data from the oldest event saved. This means that two or three relatively hard stops can overwrite all of the relevant crash data.
Keep in mind that a skidding tractor with well adjusted brakes can generate a drag factor of -16.1 fps or higher. Most HV EDRs overwrite an old event and create a new one when they calculate a reduction in wheel speed of -9.7 fps.
Benchtop downloads will overwrite certain fault code data that are stored in the ECM. In many cases, pre-accident fault codes are not materially significant to the crash. In some cases, the damage to the tractor is so severe that removing the module is the only realistic option. Nonetheless, a benchtop download should only be conducted with a clear understanding that some fault code data will be altered.
Can a Mechanic Perform a Download?
Most diesel mechanics are well versed in ECM software for the purpose of maintaining an engine or diagnosing a problem. Performing a forensic download is different. Some ECMs have preset defaults that tell the computer to erase or reset all data after a download is performed. Although this improves a mechanic’s efficiency, it is undesirable for forensic purposes. An additional problem occurs because the ECM was not primarily designed with forensic use in mind. For this reason, a Detroit Diesel download requires the investigator to save 10-20 screen captures of data, which are not otherwise saved in the download. Likewise, Cummins engines require several pages to be printed as PDF files or saved as screen captures. In addition, forensic Cummins downloads require two separate software packages, only one of which a mechanic is likely to have. With Caterpillar engines, a large amount of potentially valuable data must be exported to an Excel or comma-delineated file.
Having a mechanic physically perform the download may have the benefit of bringing a third party to the event, but it should only be done with assistance and instruction from a qualified forensic investigator. Common pitfalls when relying on a mechanic without the assistance of a forensic expert include the possibility that the mechanic will not be available for a trial, may not be qualified by a judge as an expert in downloading and interpreting the data for forensic purposes, or that the mechanic may not be trained and/or comfortable with testifying in a trial or deposition.
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