What a Truck "Black Box" Actually Records — and How Quickly It Disappears

When people say "black box" on a truck, they usually mean one of two things: the Engine Control Module (ECM) or the Electronic Logging Device (ELD). They record different data, are controlled by different parties, and both can disappear fast.

The ECM (Engine Control Module)

Built into the truck's engine. Records a rolling window of operational data — speed, RPM, throttle position, brake application, hard-braking events, fault codes. In a hard-impact event, an ECM event-data recorder snapshot is often preserved for 30 seconds before and after.

The ELD (Electronic Logging Device)

Mandated by FMCSA since 2017. Records the driver's duty status — driving, on-duty not driving, off-duty, sleeper berth. Required to be retained for 6 months under 49 CFR §395.22(i), but in practice ELD records are frequently lost during carrier audits, equipment swaps, or device replacements.

How fast it disappears

ECM data can be overwritten on the next start cycle if the truck returns to service. ELD logs are technically retained for 6 months, but transferred or terminated drivers, fleet changes, and routine system upgrades regularly cause permanent loss. Dashcam footage often cycles within 7–30 days unless an event is "flagged" by the system.

The spoliation letter

Within days of a serious crash, our office sends a written demand to the carrier and its insurer to preserve every category of electronic and physical evidence. A defendant who destroys evidence after receiving spoliation notice faces adverse-inference instructions at trial — one of the most powerful tools in the plaintiff's arsenal.