Tach issue when using trim

[WTodd]

OK been awhile since I was on this board so not sure what all I need to post BUT:

2018 Nitro Z-18 (HIN BUJ89099A818), Merc 150 4 stroke (S/N 2B476327)

Now the issue and yes I searched.

The tach and trim are on the same gauge and when I use the trim switch the tach will go to '0', then go to 7000, then back to '0', then to whatever RPM I'm traveling; this happens when I use the trim switch on the shifter or the one on the steering column and it happens every time. I replaced the gauge because the old one started sticking (both tach and trim) and have looked for bad wiring, bad connections, etc. but to no avail. I'm pretty sure I read a thread on this forum awhile ago that was similar to my problem but I can't find it. If someone has fixed this issue on your boat or knows how to fix it you're assistance would be greatly appreciated and please be specific on the fix; yes you can explain it to me like I'm 5 and I won't get offended

Thanks in advance

OK issue fixed; it was the alternator. Bass Pro couldn't or wouldn't diagnosis it correctly so I took it to another Merc dealer and voila fixed

[billnorman1]

Sounds like a weak ground for the gauge circuit or the entire system. Trim motors use a lot of juice and can drive the complete system inoperative. Watch your voltmeter when you use the trim circuit. Expect it to drop, but not below 10 volts. Check your battery cables.
The above is all a guess, I don't own a Mercury.