It varies depending on the year and make of the vehicle, and what country your in. I can only speak as to the US. I am involved in accident reconstruction, and work with the things from time to time so know a bit about them.
The ‘black box’, or airbag control module (ACM), is continually monitoring several data streams from the vehicle. After 2012, this is generally vehicle speed, % throttle, engine RPM, brake on/off, ABS on/off, driver/front passenger seatbelts on/off. Some makes may also report the amount of steering input, which is nice as it tells us if the driver tried to avoid the collision.
The ACM is located as close as possible to the center of mass of the vehicle. Usually it’s in the hump between the front seats around the cupholders. It looks like a silver box the size of two packs of cigarettes. They’re not indestructible – we’ve had them destroyed in collisions by impact and also fire. They may also not record data if power is cut during the collision sequence before the data can be recorded. In general, though, we can pull data from them.
In newer cars there are two accelerometers in the vehicle - one for lateral g’s, one for longitudinal g’s. If there’s a deceleration event sufficient to ‘wake up’ the ACM, it will lock in the last five seconds of data, usually in 0.5 second increments, along with the change in speed (delta V) for the lateral and longitudinal direction. A Ford ACM can get us data in 0.1 second increments, along with lat/long g’s on those same increments. Fords can also give us data sets up to 25 seconds prior to the collision through the power control module. We also get lat/long g data during the collision sequence (about 0.15 seconds) in a nice graph and table. If there’s more than one event (car hits car, then another car), we may also get both impacts recorded.
Interestingly, since Tesla uses a front-facing camera for auto-navigation, it records still images every second or so. That’s not part of its ACM data, but a separate telematics log that’s sent to Tesla. We can get that data with a search warrant and see the conditions leading to the collision.
The ACM won’t tell us when the collision happened, but it does record ignition cycles. So if we download the ACM and it reports a collision at cycle 2145, and the download at cycle 2147, we know the deployment is relevant to the crash under investigation. If the collision is at cycle 1932 and the download is at cycle 2147, then it’s likely from an earlier, separate collision event and the car was put back on the road.
IIRC, ACMs began as just the gizmo to deploy the airbag. Over time, in a quest to understand more about what happens in a collision, more data streams were recorded. However, what was recorded and for how long varied by manufacturer. In 2012 congress passed a law mandating certain items to be recorded and for how long, and so after that year what we get when we download an ACM is pretty consistent. Before that, there is a lot of variability in what is recorded, or if we can even download data from the unit. Europe has different laws, but European vehicles sold in the US must comply with the 2012 ACM requirements (may also be for all of North America – can’t recall).
I have no experience downloading from trucks (missed the class this month, dang it, and we’ve just not had those collisions in my office yet), but I’d expect much of the data to be the same.
Downloading the data requires the BOSCH CDR tool, a special gizmo you plug into the vehicle DLC port under the dash and a laptop which decodes the ACM data and prints it in a special report. If you can’t access through the DLC port due to crash damage, you need special cables to plug into the ACM directly. We have a rolling suitcase crammed with different cables because manufacturers can’t seem to decide on a standardized cable, even within their own damn company.