May 18, 2021 | Articles

Dennis F. Andrews & Rudy Limpert


Motor vehicles are increasingly equipped with electronic controls to accomplish a large number of different safety, emissions and comfort functions. The first large truck electronic ABS controls were introduced in Mercedes-Benz trucks equipped with air brakes in 1976. Although FMVSS 121 required indirectly (only momentary wheel lock was allowed) ABS brake controls by 1975, wheel speed signal processing was done by relatively slow analogue circuitry. The ABS requirement was deleted until 1999. Currently, analogue signals from sensors are processed in the micro-processor. The micro-processor causes an analogue signal to be produced to actuate valves or other electric components.

With increasingly stringent emission controls placed on combustion engines, electronic micro-processors (ECM) were first used in the mid-80s and are standard equipment in motor vehicles today.

Today special expertise and equipment must be used to properly download stored data from micro-processors. Once downloaded, the data can yield valuable information about the operation of the vehicle at the time of a collision or emergency braking situation involving the vehicle.

Fundamental Considerations

Electronic Control Modules (ECM) often contains valuable data to assist in the analysis and reconstruction of large truck traffic accidents. ECM data are often compared to the data collection capabilities of Electronic Data Recorders (EDR) found in passenger vehicles. However, the two units are in fact different and designed with different algorithms and purposes of data collection.

The EDR is designed to monitor aspects of passenger vehicle operation and then capture and store a short period of vehicle operation data in the event of a collision requiring air bag deployment, usually about 5 seconds. Data captured usually consists of documenting vehicle speed, brake use, seat belt use and deceleration experienced by the vehicle as a function of time. For example, immediately before a collision, an EDR wakes from sleep mode to capture and store data when pre-determined pre-impact deceleration thresholds are met, or in the event of a collision, impact sensors are activated. When the program criteria for an airbag deployment are met, the airbags are deployed and the captured vehicle operation data is store in a locked file for download capabilities. This file is a permanent file and cannot be overwritten with new data.

The ECM in a large truck is designed to continually monitor vehicle operation and capture data when vehicle components fail to operate correctly. In addition to capturing data when a truck’s component fails to operate correctly, the ECM captures data when the large truck experiences hard braking, or hard deceleration. A hard brake or hard deceleration is recognized by the ECM when the drive wheels experience a change in wheel speed of about 7 mph/sec. On dry ground this change in wheel speed equates to about a 0.31 g deceleration of the vehicle. However, because a change in wheel speed is being measured, wheel speed changes on wet or icy roads may occur quicker when brakes are applied because of reduced friction between the wheel and the roadway, triggering a hard brake data capture by the ECM.

In the event of hard braking or deceleration, data pertaining to the operation of the large truck is captured and stored in a temporary file which can be overwritten. Types of data captured by the ECM include data pertaining to vehicle speed, brake use, engine load and rpm, throttle use, and clutch use. Data previously stored in the temporary data file is over written by newly captured data obtained by the most recent hard brake, or hard deceleration, experienced by the large truck.

The ECM is the main control unit of the large truck.

Everything about the large truck is programmed into the ECM. Remove the ECM from the side of the engine, the large truck becomes inoperable. In short, the ECM is the central nervous system of the large truck. Data collected by the ECM are collected to facilitate servicing of the large truck engine.

The ECM captures data under two conditions:

1. When a vehicle fault is detected and recorded as a fault diagnostic code

2. When the vehicle experiences a hard brake deceleration where the drive wheel speed decelerates at a pre-set change in speed, usually about 7 mph/sec

When a vehicle fault is recognized, a fault diagnostic code is set and the ECM takes a snap shot of the vehicle’s current parameters and performance. The snapshot records such parameters as wheel speed and engine RPM at the moment the fault was realized. Only a limited number of fault code snap shots are stored at any one time.

During hard brake applications, or hard deceleration, often referred to as critical braking events, the ECM takes a snap shot of the vehicles parameters and performance. The snapshot captures such parameters as vehicle speed, clutch use, engine RPM and throttle use. The data is stored in a temporary file and remains there until it is overwritten with new data captured and downloaded by the ECM during a new hard brake event experienced by the large truck.

The deceleration of a large truck’s drive wheels is measured by a Vehicle Speed Sensor (VSS) pulse generator and tone ring that is mounted on the tail-shaft of the transmission. The ECM measures the VSS pulse frequency, and then calculates vehicle speed.

Preset decelerations vary from manufacture to manufacture. For example, the deceleration threshold for a Detroit Diesel ECM is 7 mph/sec., about 0.31 g’s, while the deceleration threshold of a Mack ECM is set at 10 mph/sec., about 0.45 g’s.

Because the speed data collected are based on the semi-tractor’s driver axle wheel speed, proper investigation needs to include the size of the wheels on the semi-tractor at the time of the accident. The proper tire size for the semi-tractor is generally found inside the driver door on the door latch pillar or “B pillar”. After insuring the semi-tractor is equipped with the proper tire size, the investigator can take his investigation one step further by measuring the radius of the drive wheels. Doing so will allow for calculation of the wheel speed when coupled with the appropriate axle ratio, engine RPM and transmission gear position at the time of the hard brake event.

Time periods recorded during hard brake events generally range from 32 to 105 seconds before the hard brake event and 15 seconds after the hard brake event. Fault code snapshot times are typically 44 seconds before the fault code detection and 15 seconds after.

Snapshots taken during hard brake events and fault code detection are not taken for use in accident reconstruction. These snapshots are taken for vehicle servicing and fleet management. Accident reconstruction personnel have adapted the use of the collected data to accident reconstruction. Because the data are not collected as an accident reconstruction tool, care must be taken to associate the collected data with a complete reconstruction of the accident utilizing traditional reconstruction procedures. For example, speed obtained from an ECM data recorder MUST be consistent with the calculated speed based on crush damage, distance traveled, and associated drag factors.

Fault codes can occur at any time during the operation of the large truck. Fault codes of interest to the accident reconstructionist are codes set during a collision under investigation. If power is not lost during the collision, fault codes can be set during the collision. For example, if the collision results in the radiator being ruptured and radiator fluid is lost, a radiator fault code can be triggered that captures the speed of the large truck at that moment. Fault codes captured during a collision may provide wheel speed, engine RPM and other pertinent data associated with the collision, even if a hard brake event before the collision was not detected. For example, a collision that occurs without pre-impact braking may still yield valuable information from fault codes triggered by collision damage and detected during the collision by the ECM.

Care must be taken when attempting to download fault codes. Fault codes are very sensitive in nature. Many fault codes can be cleared by starting the semi-tractor’s engine. Others can be cleared by attempting to download the ECM data, which then clears all set codes to their default settings. Because fault codes are easily lost, care should be taken to insure the tech downloading the information has a good working knowledge of obtaining fault codes before attempting to download fault code snapshots.

Unlike EDRs, data obtained by the ECM is not stored in a locked file that cannot be over written. With each hard braking event, the most recent snapshot is recorded. ECMs will store a preset number of fault codes and then start overwriting stored codes with the most recent recognized fault code. Because fault codes can be captured before a collision, care must be taken to ensure the data being reviewed pertains to the current event under investigation.

Care must also be taken when evaluating hard brake data captures. If more than one hard brake file is stored, be sure the correct file is attributed to the accident under investigation. For example, Detroit Diesel ECMs stores the last two hard brake events and the last stop. The last stop recorded will be identical to the most recent hard brake stop recorded if the last stop was the hard brake event that triggered the snapshot. However, if the semi-tractor was driven and wheel speed reaches 1.5 miles per hour and is then braked to a stop, the last stop record will show this most recent braking event as the last stop instead of a stop record identical to the last hard brake event.

Before the ECM data were obtained, efforts were made to calculate speed using traditional energy calculations by estimating the deceleration experienced by the semi-tractor and trailer due to braking. Most calculations used 50 percent of a full road surface drag factor of 0.73 for a deceleration of 0.36 g.

Because ECM hard braking data is not capturing data specific to a collision occurring, the data analyzer needs to determine where the collision possibly occurred within the data.


As can be seen in the information above, large truck ECMs can contain valuable information to assist in the reconstruction and analysis of large truck collisions. But like any tool, the ECM information is not a tell-all about the collision. The information must be used in conjunction with a complete analysis of all the evidence and information available. The investigator should become familiar with the type of engine and ECM on the vehicle so proper precautions and procedures can be taken to insure the safe download and usability of any stored information on the ECM in question.

Testing to determine the reliability of ECM speed data recorded by the various ECMs shows a good correlation between the captured data and the actual measured data of the large truck. When proper analysis between the captured data and the physical evidence is performed, an accurate representation of the large trucks speed at the time of the accident can be given.

ECM data downloads are another tool in the arsenal of accident investigators resources to reconstruct an accident involving large trucks. When proper precautions are taken so data is not lost or destroyed, the reconstructionist can obtain valuable data to assist in the analysis and reconstruction of these complex traffic accidents.

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