I have 3 of the 4 gauges that were not working correctly, working (trim
sending units are a b*&^% to replace on an I/O). The last one is the
Tach. Here's what I've done and found thus far. I have good ground
at the gauge and a good 12v power supply. The sender wire is the
standard gray wire that goes to the electronic ignition module. With
the motor running, and using a shop tach, at 2k rpm my voltage on the
sender wire is 1.7 +/- and doesn't really change any from that no
matter what the rpm. With the key on the Tach needle goes right to 0
and stays there. According to my findings, I should see a minimum of
3.5v with the motor running and more as the rpm's go up.

I went ahead and replaced the tach gauge itself as I had replaced the
speedo for a more appropriate mph range and wanted the tach to match in
styling. Both tachs react the same way, so I'm convinced that it has
to do with the signal it's receiving.

I began tracing the wires and have good continuity all the way up to
the ignition module which is where I'm stuck. The tach receives
it's signal from the module via this single gray wire and that wire
does not get fed from nor does it feed anything else. It goes through
two connectors from the engine compartment to the gauge and all check
good.

Would I be correct to assume that since the other gauges all work fine,
the engine runs good (meaning the ignition system is working), that I
probably have a semi-faulty electronic ignition module since it will
run the boat, but is not giving out a tach signal?

I've noticed that a lot of tachs will get their signal from the neg
side of the coil. Could I just run a wire from there up to my gauge and
be done with it?

This is on a 93 OMC I/O GM 4.3.

Thanks in advance.