Event Data Recorder (EDR) technology has been incorporated into the Electronic Control Modules (ECMs) of many on-highway heavy trucks. One benefit of this technology is its applicability to vehicle collision investigation and reconstruction ( Goebelbecker & Ferrone, 2000 ; van Nooten & Hrycay, 2005 ). However, collisions that cause extensive damage to the truck may cause a loss of electrical power to the ECM, which might interrupt the data storage process. This research is an attempt to determine the effects of power loss on heavy vehicle ECMs 1 , and the associated effects on data collected by the EDR function. Controlled testing was conducted with Detroit Diesel, Mercedes, Mack, Cummins, and Caterpillar engines, and power failures were created by artificially interrupting power between the vehicle's battery and ECM at predetermined intervals. EDR data from the test vehicles were extracted after each test, and the presence or absence of new data was examined. New data were compared to an independent measure of the vehicle's kinematics during the test run. The tests demonstrated that each ECM/EDR combination had different requirements for uninterrupted power supply and that several of the combinations required other inputs, such as cycling the key off or applying the park brake, to preserve event data. Finally, the testing showed that, if data were recorded, their accuracy was not affected by power loss. When data were collected and stored, they reflected the motion of the vehicle with the same accuracy as in cases where no power failure occurred.