Investigators of vehicular incidents often seek to recover data stored within on-board computer systems. For commercial vehicles, the primary source for this information is the engine control module (ECM). The data stored in these modules, not unlike passenger vehicles, varies widely among manufacturers, as do the hardware and software required to recover such data. Further, the options, and associated risks, involved with attempting to recover this data has a similarly wide variance relative to the engine manufacturer, incident related circumstances, and the tools currently available to perform such downloads. There are two primary paths available to obtain this data: (1) via the vehicle data bus (e.g. SAE
J1939
or
J1708
) or (2) direct-to-module (DTM) connection. When using the DTM method, power is applied to an ECM, and the module measures the various engine control and monitoring components for validity. If those systems are not connected or have malfunctioned, error codes, termed Diagnostic Trouble Codes (DTCs) are logged by the ECM and are time stamped. With many ECM systems, only the most recent time stamp is saved for each possible code, therefore it is at least desirable, if not imperative, to avoid logging new codes when performing a DTM download. If new codes are logged, this could result in less data available to be analyzed and potential evidentiary issues if there are any litigative matters stemming from the incident being investigated. In order to insure that new error codes are not introduced and/or existing data overwritten during an ECM download, the module must either be actually or virtually connected to the expected electrical/electronic environment. One option is to physically transfer the ECM to an exemplar, or sister, engine that has no error producing states with the monitored systems (i.e. an actual connection). Such downloads have been successfully performed by the authors of this study with a range of manufacturer's products; however, the availability of such exemplar vehicles is not always assured. If the ECM is instead virtually connected, the virtual electrical subsystems must replicate the actual electrical components. To accomplish this, each component must be analyzed and replicated either by a resistive load, voltage, pulse modulation, or another type of virtual system. The authors of this study hypothesized that a passive engine simulator module (PESM) could be constructed and connected to an ECM such that when a DTM download was performed, the ECM would sense the appropriate loads or signals, and no new error codes would be recorded or existing data overwritten. Utilizing manufacturer's published data, a number of PESMs have been constructed and DTM downloads performed successfully, without error code introduction. This study presents the methodology employed and demonstrates case examples contrasting the data extracted using such a PESM with that obtained during a bench download without the aid of a PESM. Due to the multiple engine manufacturers, engine models, and individual ranges of models and options, a specific PESM must be constructed and tested for each specific combination. The methodology presented in this study is applicable to other ECM-engine combinations and has been shown to produce valid download results without any compromise or alteration of the data stored at the time of the subject incident.