Re: 86 140hp Evinrude Gauges
In my experience, what you're describing is almost always related to a loose or corroded ground connection. This can be at the gauge itself, where the ground wire from the gauge goes/connects to, at the battery, etc.
I will describe my setup, but yours may very well be different.
I have a "main" ground/negative terminal wire running from the battery to the front dashboard area. This is a black wire.
That black wire is connected to a terminal strip (a strip of wire connections) that is located under the dashboard. [I also have a power terminal strip under there- distinguished by the fact that each connection on that strip has a fuse and the main wire powering it is red as are all the wires terminating there]
Each of my gauges has a black wire running from the gauge to that grounding terminal strip. My other dashboard accessories are grounded there too, and are also powered through the power terminal strip, so there's actually a lot of wires running to them. This is pretty common way to do it so that if any gauge malfunctions, it is isolated from the others, and would be pretty straight forward to figure out.
Sometimes the ground wires from the gauges are wired in series. That means that one of the gauges is grounded somewhere (to the strip or some other nearby grounding source or maybe with a wire all the way back to the battery) and the other gauges have a wire running from their ground terminal to that gauges ground terminal, "daisy chained" so to speak.
A good guess for you, since there are two gauges that seem to be both malfunctioning intermittently, is that there is a ground that is common to both of them, and that wire has a bad connection.
That is what I'd look at first for yours.
If that doesn't do it, the culprit is most likely the source of the signal that the gauge gets, and that depends on the gauge. I'm not sure where your tach signal comes from, maybe the rectifier? Check that out for your particular engine. The trim gauge gets a signal from a "sender" (little movable feeler at the trim motor). Again, these have wires going to the gauge, so while you're at it, check the connection for all wires at the gauge .... if still not unsolved, then you'd have to trace the wires back to make sure they're intact, that the senders are working, good connections, etc.
After that, it would be the gauge itself, but that is not common at all.
But yours sounds like ground to me.
