This "black box" is just the engine control module that stores data for the BMW technicians. It stores data such as running time, maximum RPM, and other engine data. As I understand it, this data can be extracted in the event of a crash and vehicle speed can be determined from it. Vehicle speed can then be used in relation to the event or accident in question.

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG