trigger for tachometer

HI:
Any idea how to connect an analog tachometer 12vdc from magnetic field of coil
sparker (BUT ONLY HAVING LITTLE VOLTAGE FROM CABLE WHO FEED THE COIL 2 or 3 amps).

THANKS

luisguadalajara@yahoo.com

In a car it is normally connected to the negative or interrupted side of the ignition coil which goes to the points or hall sensor.
The 3 rd lead goes to the tacho and will provide the impulses for the indication.

Some older Smiths tacho's have the actual coil negative wire going through a loop at the back of the tacho and then back to the interruptor (points) and sense the actual ac component of the current, from the ignition circuit.