As i heard Smiths tachometers have inductive RPM signal coupling.
There is a wire loop on the back side of the instrument.
This wire loop should be connected in series between the ignition coil
negative terminal and the distributor, so the current of the primary
winding of the ignition coil runs through the wire loop too.
This works only if you have points in the distributor and there's no
power module in the ignition system.
I think the instrument would work too if the wire loop is on it's back is connected
between the positive terminal of the ignition coil and the ignition activated supply.
In this case the ignition current will flow through the wire loop too making possible
the inductive coupling of the ignition pulses.
Of course the instrument needs positive supply and ground too in order to operate.
Last edited by LoneWolf33; 19-05-11 at 11:59.