I recently bought a "new to me" boat with a 1997 Merc 200 EFI. The motor runs like a champ but the tach doesn't want to work right. Most of the time it indicates nothing. Every once in a while it will indicate what appears to be the correct RPM, but at other times the RPM indicated is wrong (i.e. indicating 3000 RPM at idle) I also had an issue that the original sonar unit at the console would give the message "over voltage condition" when running at fairly high rpms. This unit was wired to the cranking battery. I've replaced that unit with a Hbird 998 which I wired to the trolling batt. I'm wondering if this tach/over voltage issue might be related and if anyone has an idea of what it could be? The cranking batt stays fully charged so no issue there.

Any help is much appreciated.