Where is the event data recorder?

Feb 15

We usually talk about Event Data Recorders (EDRs) as a physical device installed somewhere in the car - like the radio or the brake pedal. Event Data Recorders are better thought of as a memory chip that collects and saves crash data when it’s asked to by the car’s computers. This memory chip is almost always stored within the car’s Airbag Control Module (ACM) - sometimes called the Restraint Control Module, Airbag Control Unit, or similar. The Airbag Control Module does a good job of describing its function - it’s the brains behind the safety restraint system. The ACM takes in data from various sensors around the vehicle (crash sensors, occupant weight sensors, seat belt use sensors, vehicle speed, etc.) and makes decisions about if and when to deploy an airbag, seat belt pretensioner, or similair device. The ACM and EDR both use the same data sources (all those sensors) so it’s natural for the EDR to be stored within the ACM, which is already wired to receive all the data.

A Typical Airbag Control Module (or “Unit”)

Today’s post highlights two locations in a vehicle that collision investigators look for when recovering crash data:

The Data Link Connector (DLC) and;
The Airbag Control Module.

DLC - The Easy Way

The DLC, or Data Link Connector, is frequently referred to as the OBD II port (On Board Diagnostics). If you’ve ever had a Check Engine Light come on, you may have brought your car to a mechanic to have the Diagnostic Trouble Codes read and interpreted to figure out the problem. The mechanic uses a special tool that plugs into your car’s DLC to recover trouble codes in the same way that the collision investigator uses a special tool that plugs into the very same DLC to recover crash data. The DLC is the preferred way to recover data from the EDR because it is the simplest and fastest method. To recover EDR data using the DLC, the vehicle typically just needs to be powered on by pressing the ignition twice (without starting the engine) before a crash data retrieval tool can be plugged into the vehicle’s DLC to download the crash data from the EDR.

Direct to module

While the DLC is the preferred method of downloading crash data, sometimes it’s not an option and you have to go right to the source - the Airbag Control Module. Common reasons for needing to go direct to the ACM include:

A missing key - preventing the car from being powered up for the download.
Damage to the electrical system or harness - also preventing the car from being powered up for the download.
Damage to the vehicle’s communications network - preventing communication between the crash data retrieval tool and the ACM.

Regardless of the method used (DLC vs direct to module), the data recovered by the EDR will be the same. However, going directly to the module means that the investigator needs to find the ACM and plug into it - requiring more time and effort to recover the data. Fortunately, the ACM is almost always located under the center console regardless of the vehicle. Unfortunately, lots of other things are also located under the center console (e.g. vent ducts, shifter components, and lots of wiring and plugs). Sometimes, you get lucky and popping off a side panel from the center console reveals all the access you need to plug into the ACM. Other times, multiple panels will need to be removed to get access

Note: Center consoles, and other interior trim pieces, are designed to be removed and reinstalled in a non-destructive way to give access for the repair or replacement of parts over the normal course of the vehicle’s lifespan. At Evident, we take great care to skillfully disassemble only the components necessary to access the ACM before downloading the data and returning the console to its original, undamaged, condition.