Need Help Wiring Tach

I have an old Radson Tach with one each - red, yellow, and green wires. The wires are not marked and I need to know where to connect them.
Thanks

Not familiar with that brand but the common wiring for tachs was:

yellow or white is the back light
red is switched 12VDC
green is the trigger (coil -)
black is ground

Take your ohm meter and see if there is continuity from the yellow
wire to the case....it may be grounded thru the case....somewhere
the thing has to be grounded!!!

mike in tucson

Thanks for the quick responses guys.
Harry F