I have Stewart Warner Tachometers PN 811531 which is 2 wire
and managed to get a mechanical sender stewart warner 82623B which is 3 wire
Instructions appear clear however those two wires on the backside are both black, neither + or - with ignition on or engine running. Nothing is marked for signal. The sender has +, -, and white signal. + & - I can do, it is the signal wire not clear on. using ohm meter and length of wire I identified one of the two as coming from sender location, but as yet not the other one.
Any thoughts which of two wires at sender to attach signal and then what to do with 2nd wire. Continue with trial and error?