Hey Surf... There are various ways tachos get their signal.
Alternator driven, where an "AC" tap is available. Most alts have 12 pole but there are variations and pully ratios. Common for some perkins
, Kabota, Mitsubishi
Inductive, where a magnetic pickup typically counts flywheel or other gear
teeth. Common in Yanmar
, Cummins, Volvo
sender, where a small signal generator
is installed typically on a cam gear
. Older Cat, Detroit,
Most tachs have some way of adjusting for the various inputs. Some OEM types are fixed (Ususlly indictive)
What is you engine? This will give us a hint.