Tach sometimes reads double actual rpm

[Bazooka Joe]

On a 2004 90 Yamaha, using an older Teleflex tach, I sometimes see an indicated tach readings which appears to be double the actual rpm.

Examples- it shows idle engine speeds at 1600 rpm, planes out 6600, and pegs the tach at full throttle. About everytime this has happened has been during warm up, although this situation does not happen every time (maybe 1/2 the time). After warm up, the tach indicates actual rpms and will usually work correctly for the rest of the outing. I noticed that sometimes it will be reading high until maybe 3000-4000, and then return to normal as if something kicked into proper function. Charging appears to be working, judging by the volt gauge readings.

What is the likely culprit? Rectifier, voltage regulator, wiring, tach, or?

I have not yet taken it to the dealer. Beings it is an intermittent problem, I thought I would try to check a few things and have some more information before taking it in. Nothing like hearing, "Hmmmm,,,, well, geez, it's working just fine NOW..."

Thanks in advance for suggestions.

[boats.net]

That usually is a bad tach we see that a lot.

[Bazooka Joe]

Thanks for the reply.

I'll probably just live with it for now, but it's good to know that it is most likely not associated with the motor's charging system.

Thanks again,

Bz Joe