All of this info is a bit in the past, but, some US vehicles suffered from "ghosting" of the alt light from the difference between battery voltage and alternator output voltage. If the alt put out 14V and the battery was 12V, the light would get 2V and you could see it at night.

I presume that the M/B has diodes to prevent this occurrence, and the diode might be fried. If the alt. puts out 13.5V at 1500 rpm or more, I wouldn't worry about it.