I'm not positive, but I would guess that the diagnostic output from your motor is not analog, but digital, in some sort of proprietary format.
To get that to talk NMEA (either 0183 or 2000) you'd need to decode whatever is coming out, and convert it to the appropriate NMEA format. Not really all that small a task if starting from scratch.
The Command Link port on the newer engines is a NMEA 2000 compatible output, I believe.
jky
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