Heavy duty truck and engine manufacturers have been installing various forms of electronic Event Data Recorders (EDRs) on their products over the past decade, the most common being the Electronic Control Module (ECM). The primary purpose of the ECM is to control electrical and mechanical systems on the engine. As well, it monitors other vehicle systems. The ECM stores data in a manner that allows reports to be generated that aid fleet managers to monitor the performance of their equipment and that assist mechanics to diagnose problems. Some of the data in these reports can be of value in accident investigation and analysis even though these reports may not have been originally intended for this purpose. This paper discusses the specifics of data collection from the various ECMs and the reliability of such data when used for the purposes of accident analysis based on testing.