1987 Ford Tempo Battery light came on

Disconnect the voltage regulator and jumper A to F terminal on the regulator connector if the lights turns off replace the voltage regulator if not replace the alternator-this is what you called full fielding the alternator.

2 reasons for light to come on. Alternator has stopped generating power or it is generating over voltage. Since lights seem dim then it has likely stopped generating usually attributed to worn out brushes on bolt on regulator. You can take off car and have tested at local auto store. For the price of a new regulator you can get a whole alternator from used parts yard/dealer