Replaced Alternator, Alternator light still on
 

So my alt light came on about a week ago. They tested it and said it was don the low end but still charging, but I replaced it anyway thinking it might be going soon. Just finished about 15 mins ago, and start it up and the light is still on. The belts are tight and everything too. Any suggestions as to what it could be? It’s an early 1985 944. Any help or suggestions would be appreciated