My battery light comes on (i just purchased a new alternator).

Your problem most likely is in the Stator Circuit of the alternator. If the Stator circuit is shorted to open (disconnected), the charge warning indicator will turn on, even if the alternator is operating normally. The charging warning indicator will turn on if the Stator circuit voltage is zero. The warning indicator will turn off if the Stator circuit voltage is greater than 5 volts. dave@apexautoaz.com