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You're wired correct. I'd go thru the basics again then see where you're at: batt in good shape (& all charged up/no bad cell), alt brushes etc tight (take em out & look at them), positive/neg circuit sides clean/tight/low resistance (you've checked the neg side). seperate/clean the bulkhead brass terminals (a small metal bristle brush on the male ones/NAPA 725147 brass male terminal to clean the female ones)



A bad battery can cause such issues? That's interesting. The battery in there now is not so great (it won't hold a charge for very long). I guess I'll need to try it with a known-good battery.

I bypassed the alternator gauge today by attaching the ring terminals of both wires that go to it to a single post on the back of the gauge, and that got rid of the rapid fluctuations in the voltage readings, but it is only showing a relatively steady 12.5 - 12.7 volts at the battery when the engine is idling. It is also only showing about 12.5 volts at the alternator.