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

What car?
What year?
What modifications?

Post a photo of back side of alternator and of the voltage regulator



Everything worked as expected till you changed the alternator and voltage regulator.

Now starting is acting like you do not have voltage from IGN 2 while cranking.

Give us some more info and we can move forward