Aqua meter tachometer wiring?

[Dannav]

I recently purchased an older used aqua meter tachometer for my 1984 mercury 115 hp outboard. The Tach has only two wires on the back grey and black. Most Tachs I think have three ,12v ,GRD and send as I have seen. Does anyone know how this tach would be wired. I have a wiring diagram for the motor and have located the the send wire to the rectifier. Thanks, Dan

 

[Merc90HPnewbie]

Re: Aqua meter tachometer wiring?

I'm a boating newbie but I'm pretty good with electronics. I dont think a lighted tach can function with only two leads- if there is internal lighting in the meter then it at least needs a +12V to light the bulb, a sense lead for the ignition pulses, and a ground to return both signals. Look around on the meter, maybe the case or a non obvious screw is the ground and the other two are +12V and sense?<br /><br />If there's no bulb, I don't know, maybe there's a way to measure RPMs using just the spark pulses and a ground wire???