Your Healey stops running, sudden ignition failure. If you trace the coil circuit to the battery switch in the trunk you will find it grounds at the switch. Seems like this feature causes more failures than preventing theft. Here is Doug Reids Tech Tip to prevent it. -jw-
Battery Cutoff Switch Tech Tip
By Doug Reid
I believe the position of the cutoff switch in 4 seater cars subjects it to the worst vibration, just like
if you ever sat in the very back of a school bus you got jolted the
most of all passengers due to the distance behind the rear wheels.
This can be pretty hard on the switch.
Most of the time when I connect that white wire with the black tracer to the
cutoff switch I don't bare any copper wire, so the holding screw bears on
insulation and makes it look correct without the wire actually being
connected.
Doug Reid, 18G Motorworks