I'm assuming you have a diesel
, and being so you probably have the magnetic pick up.
There are 4 kinds of tach's . One is mechanical that runs off of a cable like a speedometer.
Two, is the electrical
which counts the time the coil sparks on gas engines (not likely yours).
Three is the impaluse type that runs off of the altenator and counts the cycles.
And four, is the mag. pick up type which counts the number of teeth on the flywheel. (most likely yours).
One is also made for outboards.
Look around the area where your flywheel starter gear
would be. Most likely oppisite side from the starter. Chances are it'll be a solid gray or gray striped wire.
If so, then there is an adjustment for the sending unit. On mine it is: With the motor
off, tighten it down until it touches then back off 1/2 turn and lock down.
Another thing, on the back of the tach there maybe a dial that adjust to the number of teeth of the flywheel. The tach has to match up to your motor
. If there's no adjustment then it maybe incompatable to yours.
New modern Teleflex tach is around US$120 if you shop around. Last May I just replaced my old S&W that had rusted thru.