Modernized Engine Wiring

KitCarlson said

Bill,
A second diode parallel in the reverse direction would also clamp voltage drop in reverse direction. However significant discharges without running alternator are often avoided to save the battery. It would be an extra expense without benefit.

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You need the 2nd parallel diode to protect the ammeter for current flows the other way, i.e. when current is drawn out of the battery. In the factory wiring, all battery current goes thru the ammeter, except for a few under-hood loads, like starter current and horn.

I gave a link to the diode I used. Probably not the optimal choice, but cheap on ebay and looked like the "nestle head to foot & solder" would work, which it did. I'll re-post later if it fails. I won't be driving the car until I paint and redo the interior (after home repairs, etc).
Disclaimer - copy me at your own risk. Man up and don't whine if you try and break something.