Force 90 HP 1991 Model Tachometer problem

When i got this boat, the tach worked. Now it does not ( several years later) needle jumps when I turn on the ignition, but the tach does not work.
Where do I start to test or check for the signal from the engine?

Thanks!

Back of the tach, check the connections.
There might be a 3 prong connector/plug on the tach? pull it and WD-40 it a couple times.
Pull it off and back on a few times.
If it has the plug? I believe the center wire is for the tach signal.
There's a Black, and Blue and Purple and maybe a Grey wire.
Check/clean these.
They used 2 systems, Prestolite and Mercury
Model# or a pic f the power packs.
outboardignition.com has test procedures for your motor.

Always include the model/serial # if possible, makes it easier to diagnose when you know
what your working on????

Oh yea a tiny hit of WD in the tach won't hurt.

Force 90 HP M# 908P91C, S# 026494, Prestolite,

Thanks for the suggestion link on testing