I have a 91 si hb with an b18a1. It had a bad alternator so I went and picked up a new one. Everything is converted over to obd1. I put the new alternator in and now the battery light in the guage cluster is on. I still have the stock obd0 plug if that makes a difference.

 

Sounds like you didn't get something connected back up. Did you have a battery light on prior to the alternator swap??

Make sure your connections are good, and that the alternator belt is properly tightened.

 

parts store alts are known to be bad out of the box.. return it and get another or go denso to be sure to get a working reman alt

 

I had a bad ground once that left my lights and electrical crazy...I replaced my entire starting system, from starter to alternator to battery to distributor...finally I changed all my grounds and solved my electrical issues for the most part...