Aftermarket tachometer & yamaha xj 750 1982

Hii,

I'm working on my xj 750 maxim and turning it into a caferacer.
Almost done so now i'm doing the smaller things like the tachometer.
I bought a cheap aftermarket one like this :

Now when connecting it doesn't give a reading.
I'm sure i connected everything properly ( tried multiple configurations )
I measured the voltage from my coils and they give a proper reading (4V and above when revving up)
So that shouldn't be the problem.
How can i be sure that the tachometer is faulty ? Can i test it with something ..?
Is there a way to connect it to give a reading for testing purpose ?

Wiring is like this :
Yellow = coil
green is power negative ( ground)
Black is power positive

And there is also a red & black wiring for the light ( that is working)



Thanks in advance

 

Can only guess since you didn't provide a link or supplied wiring diagram

There are two types of tach detection circuits:

One is a pulse detection with a preset bias, usually just a diode drop and any pulse above that drop is detected as a pulse - this works with the primary coil output from the TCI.

Two, a zero crossing detector where each time the signal crosses zero a pulse is detected - this works with the pickup coil output.

So, could be if you have it wired per the directions it is looking for a zero crossing pulse, so perhaps try connecting it to the pickup coil output

 

Okay after long searching and trying i found out that the manufacturers wiring information was incorrect.
For others interest in the future :
black = power
yellow = ground
green = coil signal
Works like a charm now