I would look up digitalwrite in the arduino.cc playground, there is probably an example in
your arduino menu (that is how blink works), but you'll need to use a simple transistor circit to make a 12v signal out of it.
fyi, there may already be a factory installed "hillbilly magnet" if your ecu has rpm data. that could also make a 12v rpm signal with the addition of a transistor (no arduino or $30 elm required) and that wouldn't have any delays associated with layers of computer processing.
It is always nice to know what is trying to be accomplished too, especially if it relates to efficiency.
Welcome to ecomodder BTW
