Tore apart a V8 tach to see why it wasn't working.
Turns out the transformer was shorted on the primary side.
Good news for the ignition system that it didn't fail by opening up.
The only reason I can see it's shorted was that it over heated and the insulation melted.
Maybe the PO left the ignition ON without the engine running.
Transformers are great with AC but sck with DC.
The only difference between a I6 tach and a V8 tach is a capacitator. one is 220nF and the other is 270nF.
Made a little schematic up for them