Tach not working

[ECU1984]

I have twin 1996 DT 140s that run awesome, but the port tach is not working.
I switched engines w/ it and the tach worked on the other engine. Three wires go to it (yellow is the signal, grey is the power and black is the ground).
The yellow tach signal wire comes from under the flywheel and appears to go into and out of the rectifier and then through the harness to the dash/gauge.
I switched the rectifiers and still no tach on the port engine. Harness plugs look great. I believe the signal is not coming from underneath the flywheel. How do i check the yellow lead with a meter? Also, everything is working so could I pull a tach signal from somewhere else to the gauge?
Thanks
Rick

[boats.net]

Tach signal comes from alternator(battery charge coil under flywheel).
Disconnect plug to cahrge coil and measure resistance between yellow wires, should be .02-.03 ohms between top wire on plug and each of bottom wires and .02-.03 ohms between 2 bottom wires. If out of specs battery charge coil may be bad.

[ECU1984]

Thanks alot. My schematic shows two "battery charge coils" under there.
I assume they look the same and i would need to ohm each out as you described? The charging system appears to be working because when i throttle it up a little, I see the volt gauge move slightly up. I quess it could be charging and not producing a tach signal?
Thanks for all your help!

Rick

[JUST-IN-TIME]

got a hand held voltmeter?

use it on the battery and see what it says in DC volts

also you need to rev engine up to around 1500 rpm