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  1. #1
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    2019 Nitro Tachometer Jumping and Resetting When I Trim Motor

    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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  2. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #2
    I see this is your first post, so welcome to BBC!

    The issue is probably with your outboard electrical system, the battery or connections between the outboard and battery.

    A good first step would be to verify all battery cable connections are clean and snug, both at the battery and under the cowl.

    Next, voltage readings at the battery would be useful. Meter leads on the battery with alligator clips, and watch what it does when the tach jumps.

    If you’ll post your engine serial number, I’ll ping one the more knowledgeable Mercury guys to give their opinion. S/N is often important as year doesn’t define the engine well enough.
    If you have integrity, nothing else matters. If you don't have integrity,
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  3. Member
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    #3
    Appreciate the insight! I did just replace the cranking battery with a Tracker (Relion) LiFePO4 Starting Battery over the winter, and made sure terminal rings were clean, but I’ll confirm tightness and everything tomorrow when I’m back near the boat. I didn’t have the issue the first two times running with the new battery, but did on the third time.

    I can bring alligator clips and a volt meter with me next time out to check it out like you mentioned. I do have a battery voltage gauge on the cluster as well, but not quite as clear as a multimeter. I hadn’t thought to glance at that while the issue was occurring.

    Found a brand new replacement gauge for <READ ANNOUNCEMNETS> with the same part number, so I grabbed that last night and should have it by week’s end. However, it sounds like you’re thinking it’s from the engine. I was leaning toward the gauge since it only happened when trimming, but I’ll be able to narrow down a few things this weekend hopefully.

    Mercury Model: 1152F131D
    Mercury Serial: 2B616358
    Last edited by EuropeanAM; 03-22-2023 at 05:49 PM. Reason: See Forum Announcements- No 'bay-no.

  4. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #4
    Mercury experts, this was originally posted in the Electrical/Wiring/Trolling Motors forum. Figure it’s better answered here.
    If you have integrity, nothing else matters. If you don't have integrity,
    nothing else matters.​

  5. Member
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    #5
    check clean power harness and make sure it is clean and tight. does the lithium starting battery you install meet or exceed mercury recommendations for lithium batteries? if not that is more than likely your issue or your battery bms system is faulty. it sounds to me like the battery is shutting down for a extremely short time and causing the tach to act as if it was just keyed up

  6. Member
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    #6
    Sorry for the delayed reply. The tracker battery is the same as the Relion battery that Mercury now posts as compatible. I don't have the model numbers on hand at the moment, but wanted to get a reply on here so it didn't look like I disappeared.

    I did replace the gauge since I found a new one of the same part number for $70, and it ran this weekend perfectly. I'll keep an eye on it in the coming outings to make sure it doesn't become an issue again.

    Appreciate the insight from everyone.

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    Suspect the old tach has some corrosion on the pot switch. Often can be moved back and forth a couple positions, return to original setting and check to make sure it works properly.

    So perhaps don't throw the old one away.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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