In a passenger car or light truck / SUV, the EDR is contained within the Air Bag Control Module (ACM). Any data retrieved from this module is related to a sudden deceleration most likely caused by a crash. Therefore, the data usually relates to the crash event in question.
The EDR for a heavy vehicle such as a truck tractor is contained within the Engine Control Module (ECM) and is designed to take “snapshot” data under various scenarios relating to the truck’s operation. The ECM controls engine performance, emissions systems, fuel use, driver incentives, etc. and records data useful for safety management, etc. Its purpose is not designed to record “crash data”, but many times the “snapshot data” also relates to the crash event in question. The ECM “snapshot” is not triggered by a crash event like the Air Bag Control Module is.
As an example, “quick stop” events are used by the trucking company to judge a driver’s behavior behind the wheel. If a driver performs an excessive number of “quick stops”, the driver may not using good defensive driving techniques and may also be abusing the trucking company’s equipment. However, a “quick stop” event may also occur just prior to a collision and then this data can be valuable to a reconstructionist.
In summary, EDR data from an Air Bag Control Module usually relates to a crash event. The EDR data from an Engine Control Module may relate to a crash event.