My starter motor has shorted. The resistance from the hot lead to ground is something like 0.5 ohms. If I press the contacts on the switch that the solenoid hits, the resistance goes to about 3.5 ohms.
I connected the starter (clamped in my vise) with jumper cables to a known good battery, and all I get are sparks. the solenoid will pull if I push it closer to the armature, but it will not fire on its own.
I took the starter apart, and there is a short from the power connection to any of the four brushes. Logic says the brushes on the ground side should be an open circuit, or at least high resistance.
I still have my starter out on the bench, and I need confirmation that I tested it well.