Tachometer help please

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Purchased this tach off ebay. ID as a 1970 MG Midget SN-5230/05S1376

On a previous thread it was explainned how to change from pos. ground to neg. ground. This tach seems different.

Is it already Neg. ground from factory?
If not how to I change to neg. ground

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Also since I am rewiring from scratch, how is this year tach wired


I do not have car wired like the factory all new wires, so I need to know what each wire does. as in power, coil side dist. etc...

Neg from factory. It would SAY Positive Earth on the face.

Thanks for clearing that up. Anyone know about the wires,
1. The black I have grounded to the Tach
2. I have a male spade
3. A male bullet (metal) connected to white wire in tach
4. A female bullet (metal) connected to white wire in tach

That is still an RVI tach so it must "pass" the current going to (or from) the coil.

The gauge case is the earth connection.
The male spade on the insulated plate is for switched 12V power in (typically a dark green wire).

The two bullets are for coil power. Since you are re-wiring from scratch, the easiest method is to take the wire from the ignition switch which NORMALLY goes to coil (+) and run it to one of the bullet connectors on the tach. You then run a second white wire from the remaining tach bullet terminal to coil (+). If the tach doesn't behave correctly, swap the two white wires on the back of the tach and try again.

Basically, every time the points open and close there is pulsed current flowing though that ball of white wire you see inside the tach. The remaining wires you see there are essentially a transformer which couples the pulse from the coil power to the circuitry inside the tach.

Thanks. When I say the white wires I figured it was for power/ignition but you made it all make sense. Hopefully I am ready for Monday my day off and to get this tach. installed and finalize my no charging issues.

My $500.00 Midget project is seeing the light at the end of the tunnel. I can realizetical think about driving it on a paved road soon. Not that dirt is bad