At 02:49 AM 8/2/2020, Lorenzo Pellini in Italy wrote:
The tachometer you see was probably modified in Italy after my MGA 1957 was converted with alternator, and maybe some engine parts where the mechanical drive wire was not present. I've attached the picture of the Tachometer which was working, but after I've change the old wires with a brand new cable set I'm not sure which are the correct connections. I open it and took some pictures. No Idea which tachometer they used inside.

- I feel the YELLOW (Giallo) cable could be the white from the Ignition Coil (CB for Negative Earth)
- RED (Rosso) should be the Positive. Any suggestion where to take the positive from? From the fuse box? From the ignition switch?
- Earth was attached to where I put Green "Massa" but probably should arrive to the Blue one.

Any tips and suggestion would be great! -- Cheers from Italy, -- Lorenzo




Cute. The case and face are original MGA Jaeger tachometer, but much of the mechanical driver has been replaced with electronics. So I don't have a clue without instructions from the source of the modification. Since it does not have an induction loop (like mid production MGB), I only assume that it is a voltage sensing tachometer. And there may be the rash assumption that it is negative earth system, but not even guaranteed that. It is possible to make custom built positive earth electronic circuits.

Wiring would be
1. battery power (white wire from ignition switch)
2. chassis ground (any black wire in the harness)
3, signal wire from the downstream side of the ignition coil (the wire running to the distributor), which will need a new wire not part of the original harness.

Beyond that, I don't know which terminal is which on your tachometer.
Ask the previous owner.
Ask the folks who modified the instrument.
Connect some wires and see if you can get it to work.
Without documentation, throw it out and start over.

If anyone has seen this electronic conversion before, and may have some clue how to connect the vehicle wiring, do tell. This is a good example of why you need to keep documentation of any non-standard modification to your car, and assure that the documentation can always follow the car, no matter where it goes.