Replacement Ignition? (1979 A1)

SteveSmith wrote:

Excellent thanks.
I got to a similar place when I realised that
one terminal goes to earth via the resistor pack.
one terminal has voltage applied from the igniter piece.
when the voltage gets cut, the primary induces into the secondary which steps up the volts.

Not quite. The yellow wire with red stripe feeds 12 volts to the resistor pack. The resistor pack drops that voltage down to somewhere around 8 volts to supply the coils. The igniter opens and closes the ground from the primary coil circuit to create the spark.
The same wire that feeds power to the coils, also feeds power to the igniter. That power is supplied from the engine stop switch.