A heavy truck manufactured in the late 1990's is likely to be equipped with an electronic control module (ECM) which has the capability of being the truck's “flight recorder” in a serious accident. Extracting data from the ECM often answers critical questions regarding vehicle speed and the driver's actions leading up to, during and after a vehicle accident. This paper will briefly discuss the development of the diesel engine ECM from the late 1980's to the present with emphasis on the data recording capabilities related to vehicle accident reconstruction. In particular, vehicle diagnostic sensors which continuously monitor engine speed, vehicle speed, brake switch condition (on/off), clutch position (on/off), cruise switch condition (on/off), etc. will be discussed, as well as software capabilities which track rapid deceleration events (“quick stop occurrences”) and provide a “snapshot” of the vehicle's properties during the moments just prior to and after a collision. Recommendations on how one might interrogate an ECM and preserve post-crash data will be presented. Several case studies highlighting the use of data obtained from a truck's ECM after a crash will be discussed.