Changed the alternator on a 1996 E150 and the amp light is still on and now the volt gauge doesn't work. Also tested the wire from the alternator to the starter relay and the wire from the starer relay to the battery and they are both good and it also has a new battery.I even had the replacement alternator bench tested, just in case and it works fine.
With the van running check for voltage on the blk w orange wire and the yellow wire. Also is the alternator charging and just the light is on? Or its not charging and the light is on? Try disconecting just the lt green wire @ the alternator and see if the light goes out.