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!

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.

It's for sure a 12 volt coil right! Everything all good at the distributor sounds like. Damm that seems strange. Try the above.

check the wiring and see if you pinched a wire while installing motor..

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.

Good points. I'm just guessing however I doubt tach was even touched or wired with repower.

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

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.

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.

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