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Tachometer Signal Wire MYSTERY

Old 03-14-2021, 09:29 PM
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Question Tachometer Signal Wire MYSTERY

Man do I have a mystery on my hands, any insight would be appreciated.


Just swapped a carb'd 454 into my 1999 sea ray sundeck 240 - Oh yeah, she rips


I'm usually a hot rod/motorcycle guy but being stationed in Florida, I wanted to build a boat for the water and my youtube channel.


My ignition coil has 1 purple lead (+) and 2 gray leads (-)


The engine would not fire unless I disconnected one of the gray wires on the negative side of the coil (turns out, this was the tachometer signal wire)


The engine runs fantastic but as soon as I connect the signal wire - The engine will abruptly shut off.


Tach has 12v - new coil - continuity tests on tach and coil wires all good


Almost seems like the coil is grounding out through the tachometer.


Thanks!
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Old 03-15-2021, 05:32 AM
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My thought exactly with the tach having a short or grounding somehow. You said the tach checked out fine though. What happens if you disconnect at the back of tach while leaving both wires on the coil. I never had a tach go bad. Any other wires going to tach. If so possible something else further down the line. Just guessing.
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slickshifts (03-15-2021)
Old 03-15-2021, 05:38 AM
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It's for sure a 12 volt coil right! Everything all good at the distributor sounds like. Damm that seems strange. Try the above.
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Old 03-15-2021, 07:10 AM
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check the wiring and see if you pinched a wire while installing motor..
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slickshifts (03-15-2021)
Old 03-15-2021, 07:15 AM
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yes, that can happen. Bad tach’s csn cause misfiring and sometimes no start.

More common is someone installing a new tach but putting wires on in position of old tach, not realizing new tach has posts in different positions.

Also I have seen twice, people hooking grey tach wire to ‘ignition’ post of the tach. Ignition is ignition switched power, usually purple wire. Gray tach wire should go to ‘sender’ post.
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Old 03-15-2021, 08:34 AM
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Good points. I'm just guessing however I doubt tach was even touched or wired with repower.
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Old 03-15-2021, 08:37 AM
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Wow wasn’t expecting replies this quick thanks everyone.

regarding a pinched wire - eliminated the potential of this problem by running a wire from the tach all the way to the coil - same problem

regarding tach wiring - ordered all new gauges (mercruiser brand) and tach has the same 3 posts as the other labeled - power, ground, and signal

i will take a picture when I get home from work - wiring isnt the best I picked this boat up in Panama City for 2300 and did a full resto

The best comparison I can make concerning the condition of the boat when I picked it up would be a crack house that’s actually a boat
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Old 03-15-2021, 08:53 AM
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That's a new one. 😂 You've definately done your troubleshooting. Ground from tach goes a short distance to ground block under dash correct. Just making sure it's not touching 12 v anywhere.
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Old 03-15-2021, 08:58 AM
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I know this sounds highly unlikely but did you try another coil. This past summer I chased a small engine issue for what seemed like forever. Somehow the coil checked out good however after replacing problem solved. Mine was intermittent though which I didn't even realize a coil was capable of.

Last edited by getrdunn; 03-15-2021 at 09:01 AM. Reason: Sp
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Old 03-15-2021, 10:51 AM
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Replaced the coil 3 months ago before I dropped the motor in

could be bad batch though

I’ll take some pictures after work - hopefully that helps
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