G'day Old Fart
Mate, I have been having exactly the same issues with my damn Tacho..
No one seems to be able to tell me what to do with the loop behind the tacho.
Its ok to say " its just a loop" but some sort of voltage must be applied to it so that the tacho can read the "pulse".
That said, OK so I attach it to the Neg side of the coil... cool...
Where does the other end go ???
I tried the black wire with white trace... did nothing....
I tried earth... did nothing....
I tried taking it back to the Pos side of coil... did nothing....
I can't put it to the Dist ( as has been suggested ) as I have electronic ignition...

Now my head lights don't work ( must admit, they were dicky to start with )
And it won't start..... I didn't smell anything out of the ordinary tho
Mine is in an 81 ute and I don't believe the wiring harness supports it as standard.
As it is unlicensed, it will be a few weeks before I can get it to an auto elec to find out whats going on. If you are still having probs then, I will give you my solution.
In the meantime, if you find out with yours, can you let me know...
Very frustrating huh...

Cheers

Adam