Trouble Installing Tachometer

[Black70c10]

I can't believe I'm creating a thread for something so simple but I'm getting frustrated and figured I'd see what you all thought. Appreciate the input

Bought a sunpro tach and I've got the power running to an accessory fuse, the black on a ground under the dash (also tried directly to neg terminal), the white isn't hooked up yet, and I have the green hooked to the top of the negative coil. It's the most bizarre thing, when I start it up the dial definitely moves, but it moves right to zero and does not fluctuate at all. I thought maybe it wa ts the ground so I tried about 15 spots with the same result.
I even tried hooking the green up to the positive side of the coil and it shoots up to 3,000 RPM but does not fluctuate with throttle..
Seems like power is not an issue as its a 12V source, I can't imagine the ground is an issue..has something to do with the green wire but I can't figure out what in the world I'm doing wrong..

Any advice?