I have been looking to see if I can get my smiths current sensing tach to work with my ford engine (tfi ignition).
Any hints, the tach is a rvi 1439/00a
Thanks
Dale

If it is the tack I think it is, it runs off the coil. There is a company in New York called Nosinger or similar who can convert it to run on a computer controlled engine.
They also rebuild all Smiths and Jaeger instuments.

is it negative or positive earth?

negative earth: for current sensing tacho there will be a wire that that wraps anti-clockwise round a plastic former - this is the single turn of the primary transformer.

positive earth is other way round.

one end of the wire connects to the ignition switch, goes as described above, and to the ignition coil.

there will be two tabs - one insulated for the power and other is earth to the tacho - again you need to check if it is positive or negative earth; the former you would need to swap 2 wires inside.

02GF74 If this is correct what difference does it matter whether it's wrapped anti or clockwise? Also should it not be the wire from the trigger of the distributor (either elec or points) to the coil that is wrapped. It's this wire that pulses whereas the ignition live is constant?
I only ask because I'm using a Smiths tacho too......

The trigger wire will only be carrying a very small current, nowhere near enough to operate the Tacho. Either the feed wire to the ignition amplifier or the wire between the coil and the amplifier should be used, but there is no guarantee that these old current sensing tachos will work correctly with modern constant energy ignitions.

the direction of wrapping will determine the polarity of the pulse.

here, I am not making this up

AFAIK putting the tacho between igntion switch (12V) and coil or coil and points should not make a difference
since the tacho operates on change in current in the wire. MG may have wired it that way to use shorter wires perhaps?
Rescued attachment tacho.JPG