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The patient: a JD 4010 w. Delco 24v starter, 2 x 12v batteries + Delco alternator, using a Delco-style series/parallel relay. Had been starting fine until I had the bright idea to replace the wiring harness. After replacement the starter barely turned- apparently receiving 12v only. Everything checked out, wired to spec, batt's good, etc. Start problem apparently resolved by running small (10ga) jumper from Bat B + to starter ground (-) terminal. No idea why this worked- but P.O. had a pencil sketch illustrating setup so I followed it. Starter spun just great. But then, after a week working fine, it stopped starting again. The series/parallel relay energizes fine, but fuse on + jumper fries instantly when batteries connected.

Suspicion went to starter solenoid. Sure enough, on disassembly solenoid contact plate had clearly welded to contacts at some point, so not releasing. Ordered new solenoid & replaced. But now the fuse still blows, starter doesn't spin.

Voltmeter shows:
12v at batteries 24v at series/parallel relay when starter key turned. The relay energizes as it should.
24v at starter solenoid (trigger posts) when key turned
24v at starter battery terminals when key turned. All wiring seems correct.
Still, while the series/parallel relay engages, the starter solenoid doesn't and the starter either doesn't spin, or barely spins. Was spinning fine last week.

Any ideas/recommendations that may not have already offered?? After several weeks of trying to diagnose the fault without success, I'm getting darn frustrated.

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