Modern Tach won't work with points?

[markdtn]

The only 6V thing that goes through the ballast resistor is the coil. Everything else is 12V (lights, voltmeter, tach, electric fans). I didn't think the coil negative side/tach signal would care about voltage but perhaps it does.

Here is a wiring diagram in case anyone is interested. It does not show the tach but it gets 12V ignition power from the junction block as a yellow wire, ground from the dash next to it (with a star washer), and signal from the - side of the coil.