What is an Event Data Recorder and How Does it Operate?

A vehicle’s Event Data Recorder (EDR), often referred to as the “Black Box” is a device inside a vehicle that records data regarding vehicle dynamics, vehicle operation and safety system information in the moments surrounding a crash.  The technology to “download” the data from an EDR has been commercially available since 2000.  Since 2013, most vehicles sold in the United States record EDR data that can be retrieved following a crash.

The EDR is typically a part of a vehicle’s Airbag Control Module (ACM).  The ACM is an onboard computer which is constantly monitoring the output of safety sensors around the vehicle.  When the ACM detects a signal that is consistent with a crash, it “wakes up” and performs computations to determine if it needs to deploy supplemental restraints, such as airbags or seat belt pre-tensioners.

If the ACM determines that a deployment is necessary, the EDR records what is called a Deployment Event.  This information is permanently written to the EDR and cannot be overwritten.  Deployment Events are associated with moderate to severe crashes and, as a result, are often associated with severe vehicle damage.  In these circumstances power may be lost to the ACM and the EDR data must be imaged directly from the ACM, which must be extracted from the crashed vehicle.  In very serious crashes, the EDR may lose power before all the information can be recorded.

If the ACM “wakes up” but does not determine that a deployment is warranted, the EDR records what is called a Non-Deployment Event.  Non- Deployment Events result from rapid speed changes experienced by the vehicle.  These speed changes can be the result of minor to moderate impacts, and, depending on the sensitivity of the ACM, can be the result of more minor driving events, such as curb impacts.

Non-Deployment Events are recorded by the EDR.  Subsequent Non-Deployment Events will also be recorded by the EDR, but a subsequent Non-Deployment Event may overwrite a prior Non-Deployment Event. It is common for an EDR to have memory available to record three separate events, and, therefore, a Non-Deployment Event will be overwritten every third Non-Deployment Event.  Therefore, it is a good idea to download the data as close as possible to the crash date.  If an EDR has multiple events recorded, a skilled technician can use the Ignition Cycle (the number of times the ignition has been turned on) that was recorded at the time of the event to help determine which event pertains to the crash of interest.