Quick answer: A commercial truck's "black box" is really two systems: the engine control module, which records speed, braking, and throttle in the moments around a crash, and the electronic logging device, which tracks the driver's hours behind the wheel. Federal rules require carriers to keep ELD driving records for six months, but engine data can be overwritten in weeks, so preserving it fast often decides the case.
What Is a Truck's Black Box?
People borrow the term from aviation, but a commercial truck does not carry one single black box. It carries several systems that record what the truck and its driver were doing, and together they often tell the story of a crash more honestly than anyone's memory.
The engine control module (ECM)
The ECM is the truck's onboard computer. Depending on the make, it records vehicle speed, engine RPM, throttle position, brake application, cruise control status, hard-braking events, and fault codes. After a crash, this data can show exactly how fast the truck was moving and when, or whether, the driver braked.
The electronic logging device (ELD)
Federal law requires most interstate commercial drivers to record their hours with an electronic logging device under 49 CFR Part 395. The ELD connects to the engine and automatically logs driving time, which makes it the primary record of whether a driver exceeded the federal hours-of-service limits that exist to prevent fatigued driving.
Cameras and telematics
Many fleets also run forward-facing or driver-facing dashcams, GPS tracking, and telematics platforms that score speeding and hard braking. Dispatch and messaging records show what the company was telling the driver before the crash.
What the Data Can Prove
- Speed. ECM data shows the truck's speed in the seconds before impact, which tests the driver's story against the machine's memory. Stopping distance arguments collapse when the data shows the truck was speeding.
- Braking. Late braking or no braking points to distraction, fatigue, or following too closely, the core of a rear-end truck case.
- Hours of service. ELD records reveal a driver who had been behind the wheel past the federal limits, and they expose log falsification when compared against GPS and fuel receipts.
- Company pressure. Dispatch messages and telematics alerts can show the carrier knew about unsafe driving and pushed schedules anyway, which supports claims against the company itself, as we explain in who is liable in a Texas truck accident.
How Long Does the Data Last?
Not long, and that is the trap. The FMCSA requires motor carriers to keep ELD records of duty status for six months, along with backup copies. ECM data has no such retention rule: depending on the system it can be overwritten in the normal course of driving within days or weeks, and it can be lost entirely when a wrecked truck is repaired, salvaged, or returned to service. Camera footage cycles even faster on many platforms.
Once a carrier knows a crash is likely to produce a claim, it has a duty to preserve relevant evidence, and a spoliation finding can punish a company that lets it vanish. The practical protection is a preservation demand, sent in writing within days of the crash, that puts the carrier on notice and freezes the truck, the ECM download, the ELD records, and the footage before any of it disappears.
The Company Already Has Your Crash Data. You Should Too.
Understand the asymmetry: the trucking company and its insurer can access this data the day of the wreck, and their rapid-response teams often reach the scene before the tow truck leaves. The injured person gets none of it without demanding it. That is why the first days after a truck crash matter more than any other stretch of the case, and why we treat evidence preservation as step one, not a follow-up.
Get the Data Before It Is Gone. Talk to Sgt. Pike
Sgt. Pike, a decorated Army Green Beret with 30 years in the courtroom, sends preservation demands the day he is hired and deploys his Truck Accident Response Team to secure the truck's data, the driver's logs, and the camera footage. The review is free and there is no fee unless he wins. Tell us what happened.
Frequently Asked Questions
Do semi trucks have black boxes?
Yes, in effect. Most commercial trucks carry an engine control module that records speed, braking, throttle, and hard-braking events, and federal law requires most interstate drivers to use an electronic logging device that automatically records driving hours. Many fleets add dashcams, GPS, and telematics on top.
What does a truck's black box record?
The engine control module can record vehicle speed, brake application, throttle position, RPM, cruise control status, and fault codes around a crash. The electronic logging device records driving time and duty status. Together they can show how fast the truck was going, when the driver braked, and whether the driver was over the federal hours-of-service limits.
How long is truck black box data kept?
The FMCSA requires carriers to retain electronic logging device records of duty status for six months. Engine control module data has no retention requirement and can be overwritten within days or weeks of normal driving, or lost when the truck is repaired or salvaged. A written preservation demand sent within days of the crash is the way to lock it down.
Can a trucking company delete black box data?
Once a carrier is on notice of a likely claim, it has a legal duty to preserve relevant evidence, and courts can sanction a company that destroys it. The danger is data lost before notice, through routine overwriting or vehicle repair. That is why an early preservation letter, sent before the truck goes back into service, matters so much.
No fee unless we win.