First, how does a light bulb work, I know you know but do you really "know", lol.
So lets say we have a 12volt light bulb
12v battery + ---------Bulb------------12volt battery -
Bulb will be lit up since it has 12v on one terminal and 0v(-, ground) on the other.
What if we did this
Would the bulb light up?
it has 12volts
No, it won't because for a bulb to light up voltage needs to pass thru it, 12v on one terminal and 12v on the other means no voltage flows thru the bulb.
Now the Battery Light part.
When you turn on the key the battery light bulb gets 12v(battery voltage), the other terminal is connected to the alternator, which is 0volts with engine off, so voltage flows thru the bulb and light is on.
When engine starts alternator produces 13.5volts(approx.), so now the one terminal connected to the key has 13.5v and the other terminal connected to the alternator has 13.5v. so..........light bulb is off.
This circuit is also the alternators ON/OFF switch, an alternator is basically an electric motor used in reverse, so if left ON when engine is off it would drain the battery.
Ignition switch-------battery light----------alternator
The alternator connection is part of the voltage regulator circuit, so as said above by mazdaman96, the voltage regulator is probably out or close to going out.
To test voltage regulator make sure all lights and heater fan are off.
Test battery voltage, needs to be above 12.2volts
Test voltage at battery, should be above 14v for a few minutes to recharge battery from starting engine.
The it should drop to 13.5volts, 1.1 volts above battery's voltage, so if battery was 12.5v then you would see 13.6v, that's the maintenance charge, it keeps battery charged but won't "boil off" the chemicals inside the battery.
With stable 13.5 volts turn on the lights, voltage should drop then come back up to 13.5v
Turn on fan to high, voltage should drop then come back up to 13.5v
If voltage drops and doesn't come back up then voltage regulator is going bad or 1 of the alternators fields are bad.
And on a longshot, check the wire from the alternator to the Battery Light it could be shorted to ground.
Unplug the 3 wire connector on the alternator, turn on the key, if battery light is on then you have a shorted wire.
Light green/red stripe wire on that 3 wire connector, runs to Battery Light.
Diagram here: http://www.therangerstation.com/tech...ing_1991_2.JPG