Hier nen Ausschnitt dazu aus dem englischen Mini Forum:
Warning: Technobabble to follow.
RVI tachs are often called impulse tachs. They are wired in series with current flowing through the ignition coil. They do not actually "touch" the ignition system. A loop of wire (either inside the case or literally looped on the back) inductively couples current pulses flowing through the ignition system wiring to the "counting circuit" inside the tach. (Failure of an RVI tach means the gauge may not work but your engine will still run).
RVC tachs are a later, more modern design. They use a single sense wire connection to the "low side" of the ignition coil. They count the voltage swings between 0V and 12V each time points open and close. (Failure of an RVC tach MAY result in an engine that won't run if the gauge experiences an internal short).
RVI tachs expect the ignition coil current pulse to be on and off for prescribed percentages of time. Because electronic ignitions don't turn on and off the same way points do, the RVI tachs are typically unhappy and refuse to work when wired with electronic ignitions. RVC types (because they effectively count voltage pulses) work happily with points and most electronic ignitions.
