Obviously don't do anything I said about touching live cables while starting a vehicle, *yawn* live wire.....gloves....elf n safety......leave to professionals....dont try at home....blah blah blah
But seriously, common sense is needed.
Not sure about V8 but Speedo on tdi gets its reading from the alternator, not sure if its mechanical or electrical tacho though but I have known alternators with failing bearings that give crazy readings to the Speedo still give good enough power for years. I have a feeling that the dead beemers...