i am not 100% certain on fourtraks, but normally on diesels the rev counter is powered by a single wire to the alternator, so your first check should be that there is no loose or coroded wires on the alternator. if the wires are ok then it will possibly be the alternator, but it could be the rev counter itself or any fuses that are protecting the circuit. from experience i normally find it to be the alternator, but make sure you rule out the other options before buying a new alternator ( even if it is the alternator, this doesnt mean that the alternator isnt charging the battery, or is a problem for normal driving, remember the rev counter is nothing more than a toy and not an essential for driving, although its nice to have it working) good luck and i hope you get it sorted