2019 Nitro Z-18 w/ Mercury 150 Pro XS 4-stroke.
This started happening a couple weekends ago when I was out on the water. I noticed that when I would get up on plane and start trimming my motor, the tachometer would decrease by 500-1000 rpm, then shoot all the way to full, then reset back to zero and then return to an appropriate reading. All the while the engine is running just fine, no spiking or sputtering, so it seems like an electrical issue that's causing the tach to reset or something.
So to summarize how it goes:
1. Turn on boat and idle (tach reads normal idle)
2. Put boat into forward (tach reads normal idle in forward)
3. With motor trimmed all the way down, punch the throttle on the hot foot to get up on pad (tach reads normal and increases as the motor revs up)
4. Once I'm getting up on plane, start pressing the trim up a bit (within one second of pressing the trim up, the tachometer will jump down 500-1000 rpm, then shoot all the way to full, then drop to zero, and then return to 4000/4500 or whatever the motor is running at at that point.)
That entire sequence from when the trim button is pressed and when the tachometer starts acting goofy only takes about 2-3 seconds before it's back up and accurate.
I can make it happen on demand by just pressing the trim up button while running at higher RPMs. It doesn't seem to happen when I am in neutral/idle and press the trim up/down buttons. I can get the boat on plane without it happening if I leave the motor trimmed all the way down and don't touch the trim button. The only combination that I have been able to replicate the issue is when I am running the motor at higher RPMs in forward AND press the trim up button (I can do it repeatedly if I am running and trim up at any time).
The tachometer and trim gauge share a gauge (photo attached). My best guess is a short inside of the gauge causing some crossover/interference between the two inputs, so I was going to start looking for a replacement for that gauge. Figured I would post on here first in case anyone has seen something like this and knew the solution in case that doesn't solve it.
Anyone ever seen this before? I tried some googling, but difficult to figure out what terms to use so I didn't up come up anyone with the same situation. There is a single plug on the back of the tachometer which is secure and free of corrosion. The boat is a 2019 with 166 hours (I'm the original owner) and garage kept, so everything is pretty clean on it. Any help is appreciated.
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