Nice looking Tacho. First question that needs answering is, what did it come out of, a 4, 6 or 8 cylinder car. Tachometers in cars built on the production line are specific, not like aftermarket versions, which have a switch to select 4, 6 or 8 cylinders. If it's off a 4 cylinder you'll be ok, if off a 6 cylinder it will read 4/6th of the actual RPM and if 8, read only 4/8ths of the revs.
I once fitted, what I thought was a dedicated 4cyl tacho, but discovered later, was for a 6 to the 4 cylinder motor in my rally car. During the next rally I couldn't understand why the motor wouldn't rev much past 5000 rpm, turned out that was actually 7500 rpm. Thought at the time I was going pretty hard and motor was on song, just the wrong tacho for the motor.
I'll assume it is off a 4 cylinder. Here's a picture of my VDO tacho, similar with 3 electrical connections.
Your 3 wires present should be no different, being Power +, Earth - and finally the signal wire "G". The signal wire G goes straight from the tacho to the terminal on your ignition coil that connects to the points in the distributor. This senses the pulse each time the coil fires a spark plug and the gauge converts that into RPM
For the +, take power from something that is only live when the ignition is switched on, the back of the ignition switch would be the closest thing.
The earth can go to an existing earth wire under the dash, there are a few under there earthing the bodies of the speedo and temp gauge.
The instrument globe, not shown, will most likely have only one wire to power it as it will pick it's earth from the tacho body that is earthed elsewhere. Connect the power to the globe from one of the globe/s on the speedo or temp gauge so it turns on and off with your park/headlights.
Check the 3 terminals on the back of your tacho, I suspect 2 will be insulated where they pass through the gauge body. The 3rd hopefully just welded to the body will be your earth, hard to tell from the picture but think it might be the brown one. That leaves only 2 wires to figure out. My best guess is that the violet wire is the power and the green with red trace is the one to the coil.
The above does not apply to Smiths Tachos, that's another story
Cheers Stephen
