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Can anyone explain why the Gray wire off the rectifier runs up to the ignition switch? This is for a 1984 Mercury 50 outboard motor. 4 cylinder 2 stroke. No power trim.

I'm having to rewire the boat so I'm unsure of where this wire would even go or what it is accomplishing. It seems unnessecary and my thought is to leave it out. My only thought is it grounds out the stator and acts as a kill switch but from my reading if you allow continuity between the black yellow wire and the black wire, that will kill the engine. Those two wires run to the M terminals on the switch which are connected when the switch is in the off position. If that Gray wire is a backup kill switch then I wouldnt even know which terminal to attach it to on the ignition switch (assuming it terminated there).

2 Answers 2


The voltage is measured from that lead to battery ground and the tacho works out the engine speed based on the fixed drive ratio or trigger pulses and the number of poles in the alternator.

However, why the manfacturer did not provide a tacho feed directly from the switch box using the coil trigger info ...


So a boating forum informed me that the gray wire is for the tachometer. Apparently the tachometer is measuring AC waves and calculating RPM.

Onto the electrical stackexchange to ask how you can measure voltage from one lead...

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