[Merc] Why does this wire connect the rectifier (AC side) to the ignition switch?

[imisswaves]

See image in the attached link.
Can anyone explain why the Gray wire off the rectifier runs up to the ignition switch? This is for a 1984 Mercury 50, 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 unnecessary and my thought is to leave it out. My only idea 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 wouldn't even know which terminal to attach it to on the ignition switch (assuming it terminated there).

Can someone shine a light on what I'm missing here?

 

[Scott Danforth]

grey wire is the tach wire.

 

[imisswaves]

Thanks Scott.
Not sure why that link got truncated, here is the actual diagram:
http://www.maxrules.com/oldmercs/Wiring/1966ona/60.jpg

Why is it coming off the rectifier? It looks like the AC side, and it's only one tap so it can't be voltage based, right? I'm confused by it.

 

[GA_Boater]

Welcome aboard, Missing.

The link was incomplete due using the Link icon beside the Camera icon. Linking to a sub-page on a website using the Advanced Editor link, as you did the second time, works much better. You'll get the hang of this forum in no time.

The tach does work off of AC cycles and counts the pulses per revolution. Also why tachs often have a setting for the number of poles a stator has to count properly. For example, your Merc has a 12 pole stator which gives 6 AC cycles per revolution. Either stator wire will work to supply the tach signal. Hope this helps.

The wire isn't connected to the ignition switch or shouldn't be. It passes through the remote control harness, to a tach wiring harness or directly to the tach.

 

[imisswaves]

Thanks GA_Boater!

I guess I'm showing my electrical ignorance in these questions. I didn't know you could measure voltage from only one wire. That's kinda twisting my mind.. It goes against all my learned knowledge. If I'm using a multimeter, there is no way to measure voltage of a circuit by just using one lead. By definition, voltage is the potential between _two_ points in a circuit. Do you happen to know how this wizardry works?

 

[GA_Boater]

Don't forget the tach also has a ground and voltage as well as the signal from the stator. And the key to it is the ground because the stator uses the same ground reference through the rectifier.

 

[imisswaves]

Ah. After reading back up on wave rectification I see where my misunderstanding was coming from. The stator and the battery must share a ground and therefore is the second point, as you said.

Thanks for clearing all this up guys. Very much appreciated, hope I see you on the water