I have a 2000 7.4LMPI (310HP). Anyone know what the gray wire is that comes out of the Mercathode wiring harness? It's right below the MEFI3 ECM. It is switched on (12V) with the ignition switch.

I am adding a Gaffrig fuel sending unit and need the third "Hot" wire at the sending unit location on top of the tank. I was planning on running this gray wire down to the tank. Any advice?

But from what I remember on the last one I added gages to, the grey wire on a Mercruiser is from the "Tach" side of the ignition circuit... however if you have access to the terminal strip under the dash that bridges the wiring from the motors, to the dash & gages, 12 V etc.... depending on who rigged the boat, most will have labeled the wiring at that point. You NEVER want to pull voltage from anything related to your ignition system, because if what you hooked up fails, your paddlin... Check the trim solenoind for a key on power source, or better yet, go from the terminal strip that supplies the key on accys or gages. Also that gives you a fuse protected circuit. BE careful, because it is possible to see 12 volts from the negative side of the ign coils... and you may as well plan on those little things we all ***** about happening... (like the wife whining when the boat becomes a small barge) Its a pain to run a wire clear down the side of the boat, but it's the right and safe way to do it. A inexpensive electricians snake will make your life easier doing that, plus theres all kinds of cool things you can do with one when your done...lol Good luck!

I was thinking about running a wire down from the gauges anyways, but when I found this gray wire just sitting there waiting for something to be plugged into it, I had to research it.

What I don't get is that my new Mercruiser manuals for this motor don't show this gray wire anywhere. What the hell?

I have a couple of unused wires on the engine harness on my '99 502 EFI. I have a service manual and traced one to an unused transmission temperature sensor.

Ron

Instead of looking at the engine wiring in the manual look at the fuel injection wiring. It is sometimes shown seperately. As FindMe posted earlier, the gray wire is normally used for the Tach. reference signal.

Good Luck,
Todd