Yamaha, F70LA, 2015

by Jerry
(Millfield, Ohio, USA)

When adjusting trim while under power my tachometer needle will drop to close to zero, the same for the voltage meter. On a trip to Canada August 13 - 27, 2021 the tach needle started sticking at around 1000 rpm ( was running at approx 5500 rpm based on my speed). At the same time the voltage meter dropped to around 10 volts and stayed there. Again, the cause/trigger seemed to be adjusting the trim. I was dealing with a low charge on the crank battery after fishing much of the day using a fish finder and the timed aerator in the boat without running the motor. The crank battery was low enough that I had to jump it from the trolling battery (first time this has happened). Replaced my crank battery with a fully charged one. Two day later, same thing happened. On the last day I started with a fully charged crank battery and the tach and voltage meter (charged at 13 volts) worked just fine. The only difference is I set trim before I got under speed.

Why does using the trim button cause the tachometer and voltage meter to drop out? Is this normal?

I assumed my alternator was dying. But why would it start working again? I'm enough of a mechanic to know these things don't fix themselves

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