Cranks but won't start. Unplug ground at field spade on Alternator and it starts?

Dana67Dart said
This suggests that IGN 2 is not able to provide the bypass it is designed to do.

Can't say for sure but...

99% of grounded field (round back) alternators are coming from the rebuilders as a isolated field (square back) with one terminal internally grounded. They clip off the spade terminal but leave enough of a terminal there that many have put field terminal on it and fried their voltage regulator.

Arrow points to trimmed terminal on the GROUNDED field terminal.

View attachment 1716271029

I ran alternator with one spade grounded other hooked to field voltage regulator, reversed grounded other spade and hooked one that was grounded to field wire. This was done when everything was working two weeks back. So it looked to me that neither spade was grounded based on those tests.