On the port engine, the voltage seems to dance around at 1200 rpms. The faster the RPM's, the lower the output. But, at 800 RPMS is steady and higher.
Could it be the 2 smaller wires being reversed on the alternator?
Jim when I first got my current boat it did this but not as much bounce. The fault was corrioded battery cables, check them and the alternator connections.
Another possible fault may be the voltage regulator.
Verify all connections (including grounds) are clean & tight.
I'd suggest using a true voltmeter to check voltages at a few spots to isolate a true issue from a gauge issue.
If their isn't any bounce at the battery or alternator output, then you can; a) rest assured it's not a "true" problem with the charging circuit; and b) start tracing the issue as a faulty gauge/wiring issue.
Weird port engine Alternator reading
Linear Mode
