Tachometer installation

I bought a tachometer the other day and I am having one little problem. I connected 3 of the 4 wires as illustrated in the directions. The last wire is a green wire that is supposed to connect to the negative treminal of the coil, the coil dosen't have anything Identifying what is what and I don't want to go cutting wires until I know what they are.

Thank you

What year and motor?

Sorry I forgot about that it's a 92' 4 litre HO.

Thanks