I just bought this old RAC tachometer and want to install it on my 68 std with pertronix ignitor and flamethrower. My question is that im not sure how to wire it. It has 4 wires (green, black, yellow, gray). The yellow and gray wires go directly to a bulb assembly, the green and black go into the tach electronics. Im assuming green is negative terminal, but im not sure about the black.
Most aftermarket tachs require 3 wires for the tach: signal, power, and ground. I wonder if this is a current-sensing tach, which uses only two wires. One would go to the + side of the coil; the other to the ignition (run-only power). I can’t tell you which wire is which; usually that is determined by the connector gender: female would be output and male would be input.
Remember: female connectors are always hot when energized!
Here’s something I found on the Jalopy journal:http://www.jalopyjournal.com/forum/threads/tach-wiring-help.164893/
Like me in case you came across this post with no actual answer, the Black wire goes to ground, the green wire goes to the battery/dist side of the coil and the yellow/grey wires are for the light as the OP states. Sorry for bringing this up from the dead but i didn’t find the answer anywhere else and thought others might appreciate a definitive anwer. Works this way on a '66 200 6cyl mustang anyways.