Hi, anybody know how I can check a revcounter/tacho to see if it's working. May sound stupid, but they, (two), are on my boat, and are for petrol engines, but are reading diesel engines through a signal transmitter that uses four black and white segmants on the crank pulley, that represent the signal from a coil. The signal transmitters are working as they should, but the revcounters don't even twitch. Any help much appreciated. Cheers Ray

Are they easily removable? If so, can you not test them on a petrol engine on your car?

Some tachometers (mostly older ones) rely on the high voltage pulse from the ignition coil primary to operate.