Help installing after market tacho

I need some help because I cant get the damm thing to work.

Bought myself one of these chines tachometer since i want to give my old KZ750-L4 from 1984 a face lift so to speak.

here is a wire schematic for my bike

As you can see there are five (5) wires for the gauge, two which are for the led backlight, which works so lets ignore them.
The other three is black(positive), green(negative) and black/yellow(signal) and I've connected to to the bikes brown(positive), black/yellow(ground) and black(negative from ignition coil). Identical as the stock using the same connector in the wire harness i might add.

It should work according to me but it doesn't, what is wrong?

do you have the manual for the tach?

It didnt come with one.
All the seller say is:
(Backlight)
long Red light +
long Black light -
(Tacho)
Short Black power/ignition lock +
Short Green Ground
Short Yellow engine signal line

Hook up the 12+ and the ground.
Then take the coil wire from the tach and touch the engine case rapidly.
Each time you touch and release the wire from the case the tach will think it's a plug firing.
The faster you can do it the higher the rpm.
This will give you some idea if the gauge works.
This is just a hunch though.
I'm not sure if the tach is looking for some sort of voltage spike on that coil wire.