No spark

OK think of it as connecting two batteries together. Like a jumper battery. The alternator is one battery, and the 'battery battery' is the other "battery." The reason they melt under duress is that "originally" the battery source was on one side of the ammeter, and the alternator was on the other.

What causes flow through the ammeter is that one or the other (battery or alternator) becomes LESS positive than the other.

For example, you are at a light. The engine is idling slow, in drive. It's a rainy dark cold night, so you have "everything" on. Lights, heater, stereo, wipers, what all accessories you have added

The alternator output is sagging, lower and lower, at the low RPM, so the battery is not "more positive" and it becomes the primary power source. Power flows from ground (neg) through each and all accessories, through the fuse panel, through the fuse panel "hot" bus or through the key "accessory" bus and ends up at the BLACK "welded splice" where power continues through the BLACK ammeter wire, through the ammeter, out the RED, through the bulkhead, and to the battery.

Now the light turns, you give it some gas, the alternator speeds up and becomes higher voltage output. NOW the alternator is "more positive" and it becomes the source. Instead of the accessories now going clear back to the battery............they now go from the welded splice (BLACK) and Y off the splice to the BLACK alternator feed out through the bulkhead and to the alternator.

The battery is now acting as a LOAD, because it became low during the idle. Now the battery is being charged, and so flows current through the RED through the ammeter and to the black and out to the alternator.

THE BATTERY IS NOW THE ONLY THING flowing actually through the ammeter, so the ammeter now shows a charge.