I own a 1993 Nissan Sunny 2.0L Diesel.
My battery light stays on while driving. I checked the following:
- drive belt ok
- voltage over battery while running = 12V

At this point I assumed my alternator was dead so I replaced it with a second hand. However the yielded no improvement.
What are the odds that two alternators are dead? very little huh, so I dug some more.

Could it be fusible link related? I'm not sure what the purpose is of this fuse is and where it is located.
According to my voltmeter, the cable going from B+ to Alternator voltage out seems to be intact.

Any ideas/suggestions?

 

I would replace all major electrical wiring. You may also need to replace your belt.

Maybe she justs need a good tune-up. But i would also suspect the light on becuase she's just old.