my brother has a 90 crx hf w/ a jdm d15b (non vtec) engine. the swap was done YEARS ago at our shop that we work at by me, my brother and out lead tech. the car is 99% stock except for an intake. now about 2 months ago out of nowhere his battery light came on, now sometimes it will come on and stay on steady, sometimes it will come on, go off, come on ect..., and sometimes it wont come on at all. First thing we did, installed a new battery, (same problem), next thing is a new alternator, (same problem) we tripple checked the tension on the belt to make sure its not loose. we have done a complete charging systems test (battery, alternator, starter, volts test, cranking test ect...), we have even left his car sitting engine off but car on, with the radio on headlights on for an hour and his car started right up. also the car drives just perfect, it never dies, or drops below idle, never hesitates, all the lights are brights, everything works, just cannot figure out why the hell this light keeps coming on. anyone have any ideas. thanks.

 

this may sound stupid, but check under the dash above the clutch and brake pedals, maybe out near the steering shaft a little, you will find a group of wires that looks like they end in a flat brown plastic cap. thats actually a jumper plug that connects half of the wires to the others, and seem to be notorious for having corroded connections, i had the charge light flickering on mine becuase of that, and my car would randomly lose ignition power over bumps. its just a flat brown thing with 7 or 8 wires or so going into it. the top comes off and is a jumper. give that a try, ive had plenty of issues with that thing on a few 88-91 needing the terminals cleaned or being corroded to the point they burnt the connectors off.. if you need a picture let me know.
otherwise i might call it a ground problem to the gauge cluster itself or somewhere else causing a backfeed and drawing power through the bulb causing it to light.