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Re: Help Volvo MD5A idiot light wiring
You'll have to post the alternator make and model for anyone to tell you for sure. But no, the black wire cannot be charging your battery, the heavy "pink" one form the alternator should be the charging output. The black might very well be the sense lead, which also goes to the battery positive, assuming there's only one battery and no switch. Which would leave the white for the alarm/alt light, although black/white might be reversed, that's why you need to know what alternator that is or how those terinals are marked.
The pink wire also looks like home wiring, or obsoleted fusible link wiring (now considered a fire hazard) so you might try to see what the printing on it says. And wires should never just run through space, the black and white ones probably should be secured to something and then to the bulkhead, so they don't just vibrate around in the air and then snap off their connection.
I'm not sure how to read the rats nest shown on #12, the original alternator, which seems to have many more connections than what is usual today. (Perhaps for an external regulator mounted on the original alternator, Motorola/Marchall style.) Looks like they took a wire to the "B" connector to feed the alarm system and I'm not sure that what you've got can integrate into that.