After a serious truck crash, people often ask whether the truck had a "black box" and what it recorded. The phrase is informal. A commercial truck can have more than one electronic system, and they do not all keep the same information or keep it for the same length of time.
This page is about the electronic records. For the broader case, see our page on commercial truck crashes in Oklahoma.
An electronic logging device, or ELD, is the hours-of-service tool. 49 CFR 395.2 defines an ELD as a device or technology that automatically records a driver's driving time, helps record hours of service accurately, and meets the requirements of subpart B of Part 395. The Federal Motor Carrier Safety Administration describes an ELD as synchronizing with a vehicle engine to automatically record driving time. An engine electronic control module, often shortened to ECM and sometimes also called a black box, is a different part of the truck. The ELD rules use "ECM" to mean the engine's electronic control module, and they require many later-model trucks' ELDs to link to that module and receive listed operating data from it. That is not the same thing as a passenger-car event data recorder.
What an ELD records
Under 49 CFR 395.8(a), except for a private motor carrier of passengers (nonbusiness), a motor carrier subject to Part 395 must generally require each of its drivers to use an ELD to record duty status, subject to the exceptions in 395.8(a)(1)(ii). Those exceptions include, among others, a driver who must complete a record of duty status on not more than 8 days in any 30-day period, certain driveaway-towaway operations, and a commercial motor vehicle manufactured before model year 2000. This page does not decide whether a particular trip was required to use an ELD.
When an ELD is in use, 49 CFR 395.26(b) requires it to automatically record: date; time; commercial-motor-vehicle geographic location; engine hours; vehicle miles; driver or authenticated-user identification data; vehicle identification data; and motor carrier identification data. The ELD records those elements when the driver changes duty status, at least once an hour while the vehicle is in motion if no other recording has been made, when the engine is powered up or shut down, and in the other situations listed in 395.26. Location recorded during authorized personal use of the vehicle is logged at a coarser resolution (approximately within a 10-mile radius), and engine hours and vehicle miles are left blank on that intermediate recording.
Those ELD fields can help show where the truck was, how long it had been running, and how the driver's duty status was recorded around the time of the crash. They should be compared with other evidence. A single data point does not, by itself, establish how a crash happened or whether someone was negligent.
What the engine module has to do with the ELD
Appendix A to Subpart B of 49 CFR Part 395 defines ECM as "Electronic Control Module." For a model year 2000 or later commercial motor vehicle that has an engine electronic control module, section 4.2 of that appendix requires the ELD, when the engine is powered, to establish a link to the engine ECM and automatically receive the engine's power status, the vehicle's motion status, miles driven, and engine hours, through the serial or Control Area Network protocols supported by the engine ECM or the vehicle's databus. If the ELD is required to have that link, section 4.3.1.2 of the appendix also requires vehicle speed information to be acquired from the engine ECM or the vehicle's databus.
That rule describes what the ELD must obtain from the engine module or databus in order to keep hours-of-service records. It does not say that every truck stores a complete crash-event file in the module, or that the same fields will still be there after ordinary use, a later trip, a repair, or a software process. We did not find a federal minimum retention period in 49 CFR Part 395 or Part 379 for a truck's raw engine-module files or a crash event-data file as such. If someone is using "black box" to mean a passenger-car event data recorder, that is a different set of rules and is not covered here. See a short explainer on what a truck's black box records.
How long a carrier must keep ELD records
49 CFR 395.8(k)(1) requires a motor carrier to retain records of duty status and supporting documents required under Part 395 for each of its drivers for a period of not less than 6 months from the date of receipt. The driver must keep a copy of each record of duty status for the previous 7 consecutive days, in the driver's possession and available for inspection while on duty (395.8(k)(2)).
Separately, 49 CFR 395.22(i)(1) requires a motor carrier to retain for 6 months a back-up copy of the ELD records on a device separate from the device that stores the original data. Section 395.22(i)(2) requires the carrier to retain a driver's ELD records so as to protect the driver's privacy in a manner consistent with sound business practices.
Those six-month periods are federal minimums for the records they name. They start from the date stated in the rule (for duty-status records and supporting documents, the date of receipt). They are not a promise that every other electronic file on the truck — including engine-module data, cameras, or telematics — will still exist at the end of six months. 49 CFR 379.1 applies Part 379 to motor carriers, brokers, and household goods freight forwarders. 49 CFR 379.3 states that after the required retention periods, the records listed in appendix A to Part 379 may be destroyed at the discretion of the company's management. That does not excuse compliance with the lawful requirements of any other governmental body prescribing a longer retention period.
49 CFR 395.8(e) also provides that no driver or motor carrier may make a false report in connection with a duty status, or disable, deactivate, disengage, jam, or otherwise block or degrade a signal transmission or reception, or reengineer, reprogram, or otherwise tamper with an ELD so that the device does not accurately record and retain required data. No driver or motor carrier may permit or require another person to do those things.
Why this matters after a truck crash, and how to ask that the data be preserved
ELD records and related engine-module data can be useful when the account of a crash is disputed, when hours of service are in question, or when it is unclear where the truck was in the hours leading up to the collision. They can also be gone, incomplete, or harder to obtain if the truck is put back in service, repaired, or salvaged, or if ordinary retention settings overwrite a file that is not covered by the six-month rules above.
A written preservation request can identify the truck, the date of the crash, and the records to keep. That list commonly includes the driver's records of duty status and supporting documents, ELD records and the required back-up copy, and a request that engine-module data, cameras, and other electronic files also be preserved and that the tractor and trailer not be repaired, salvaged, or sold before they can be inspected. Our page on spoliation letters after a truck crash explains what that letter is, why timing matters, and which records the letter typically asks the company to keep. This page does not reprint that letter, and it does not say that sending a letter, by itself, creates a legal duty to preserve.
If you are dealing with a shifting, overweight, or hazardous-materials load, see our page on cargo, overweight, and hazmat truck crashes. Hours-of-service questions are also discussed on our truck driver fatigue page.
FAQs
What is the difference between an ELD and a truck "black box"?
An ELD is the device that automatically records a driver's driving time and hours of service. People often use "black box" for the truck's engine electronic control module. They are not the same system. For many model-year 2000 and later trucks, the ELD must link to that engine module and receive listed operating data from it.
How long must a carrier keep ELD records?
A motor carrier must keep each driver's records of duty status and supporting documents for at least 6 months from the date of receipt. It must also keep a back-up copy of ELD records for 6 months on a separate device. Those are federal minimums, not a promise that every other electronic file on the truck will remain.
Does an ELD record speed and braking the way a car's event data recorder might?
An ELD automatically records date, time, location, engine hours, vehicle miles, and identification data. When the engine-module link is required, it must also obtain vehicle speed from the engine module or databus. That is not the same as a passenger-car event data recorder, and it does not mean every crash leaves a complete engine-module event file.
How do I ask that this data be preserved?
A written preservation request can identify the truck, the crash date, and the records to keep, including ELD records and back-up copies, records of duty status, supporting documents, and engine-module or other electronic data. Our spoliation-letter page explains what that letter typically covers. Sending a letter, by itself, is not described here as creating a legal duty.
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Sources
- 49 CFR 395.2 (definitions of "Electronic logging device (ELD)" and "ELD record"). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/section-395.2.
- 49 CFR 395.8, including (a) (ELD use; the (a)(1) lead-in exception for a private motor carrier of passengers (nonbusiness); and the listed (a)(1)(ii) exceptions), (e) (false reports and ELD tampering), and (k) (retention of records of duty status and supporting documents for not less than 6 months from the date of receipt; driver's 7-day copy). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/section-395.8.
- 49 CFR 395.22, including (i) (6-month back-up copy of ELD records on a separate device; privacy). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/section-395.22. [80 FR 78385, Dec. 16, 2015, as amended at 88 FR 80192, Nov. 17, 2023; 91 FR 37053, June 22, 2026].
- 49 CFR 395.26 (data an ELD automatically records, including the (b) element list and the events in (c)–(j)). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/section-395.26.
- Appendix A to Subpart B of 49 CFR Part 395, including the abbreviations list (ECM means Electronic Control Module), section 4.2 (required engine-ECM link on model year 2000 or later CMVs that have an engine ECM; power status, motion status, miles driven, and engine hours), and section 4.3.1.2 (vehicle speed from the engine ECM or databus when that link is required). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/part-395.
- 49 CFR Part 379, including 379.1 (applicability to motor carriers, brokers, and household goods freight forwarders, as amended at 91 FR 7859, Feb. 19, 2026) and 379.3 (companies subject to this part retain records for the minimum periods in appendix A; after those periods the records may be destroyed at management's discretion; this does not excuse compliance with the lawful requirements of any other governmental body prescribing longer retention periods). Federal Motor Carrier Safety Administration. https://www.ecfr.gov/current/title-49/part-379.
- FMCSA, "Electronic Logging Devices," https://www.fmcsa.dot.gov/hours-service/elds/electronic-logging-devices. Last updated Monday, October 2, 2023.