Most cars and trucks today have the voltage regulator in the computer. The computer controls and keeps the volts and amps at a steady rate to keep from having voltage drops. The lite will come on if there is one thing out of range, like if you turn on your head lights and pulls to much volts.
I had a Ford Freestar in my shop with the alt lite on, replaced a blown fuse just to find out the alt was bad to. I put a new alt in and after a few seconds the lite came back on. I hooked up my scanner and everything look gook, this thing drove me crazy. I called up a buddy of mine from ford dealer and he brought over there lap top and i was amazed at all the wave patterns that the alt was putting out, a/c, fuel pump, cooling fans etc. I looked at him and said dam I need that lap top as my scanner don't pick up half that, anyways after looking at everything he found nothing wrong but the lite was on. I went back and replaced the battery with a new one and still the lite was on, check the amps and the volts at the bat and at the alt check good. I went back to the fuses and the fuse I had already replaced had blown again so I replaced it and all was good. I am not saying that that's your problem just something to check as there could be a number of things. Good luck Just trying to help.
The cars and trucks today are getting to be harder and harder to fix with all of the elc crap on them and running though the computer.