Medium and Heavy Trucks
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There should be a BAT power pin to the ECM as I know Cummins, Cat, and Mack have ECM power from BAT and an inline fuse between the battery box and ECM. Unless of course there are changes to the MP8 which is possible but I doubt it. I would verify this when you have your diagram but I am pretty sure you should have a constant power unless this ECM has an internal battery. Since your check engine light is cycling with the key on it seems your ECM is communicating as it should. I would see if you can locate a test ECM before investing the money for a new one though. Do you have access to another truck with the same engine?