If the black jumper wire fixes the issue the ground is not a cause. The issue is a bad path from the battery to the reg on the black wire. wire. Nothing wrong with 14.6 volts at the battery.Well now, with the wire attached, the meter reads 14.6 volts, + or -. And you're right, it bypasses the ignition switch.
However, without the jumper, it's charging 16 volts, + or -. So I've got it back on the charger and I'll do the test again, after it goes into float mode. It took some cranking to get it to start so I figure the battery may be down a bit.
So, it seems there is a discrepancy, but maybe not enough. Where are the grounds you refer to, Mike?