If you have 12 volts at the negative side of the coil when the points are closed, there is no connection between the coil and ground. 3B is correct about a ground wire between the breaker plate and the distributor case, although originally the distributor relied on the vacuum advance linkage for the breaker plate ground. That was unreliable as the engine accumulated mileage. If the wire is missing or broken, the breaker plate probably isn't being grounded in your situation. Measure voltage between the points/breaker plate and the distributor case- key on, points closed. 12 volts indicates an open circuit between the breaker plate and the distributor case. I would suggest changing the ground wire even if it looks good, because they tend to loosen in the crimp at one of the ring lugs. You can also solder the lug and wire if you want to do so, but don't replace the wire with normal stranded wire- that is not flexible enough to withstand the breaker plate movement, and it will likely break before long.
You should be able to find the proper ground wire for your distributor, but if necessary, Ford distributors use the same type of ground wire, for the same reason of ensuring a good ground for the points.
Harvester
