Why is my Battery light on on a good battery?

 

Welcome to the forum

Battery light means the alternator is not charging the battery when engine is running, so battery will drain and engine will stall and not restart because battery is dead
And just a heads up, this can ruin a good battery, car batteries are not made to be fully drained

Could be a blown fuse or alternator has failed, what YEAR Ranger????

You can test the charging system easily with a volt meter, but need the YEAR of the Ranger to tell you what to test

 

2004

 

2004 Ranger uses Fusible links instead of fuses which is actually better since they are less likely to "blow"

Get volt meter out and set it for DC volts
Key OFF
Test battery volts for base line, lets say battery is 12.4volts

On the back of the alternator will be the "B+" terminal, a stud/nut terminal, should have a black/orange stripe wire on it, leave wire connected
Put Volt meters black probe on alternator Metal Case(this is the Ground, battery negative), you will do this for all tests
Put red probe on B+ terminal
Should see Battery Voltage exactly, 12.4v in this example
If no volts then fusible link IS blown/bad

Now unplug the 3 wire connector on alternator, 2 wire if 4cyl engine
Test Yellow/white wire should see Battery Voltage
If not a fusible link is blown/bad

Test Green/red wire, should be 0volt
Turn on the key, engine off
Re-test green wire, should now be battery voltage, .1 or .2 less is OK

If all wires test as OK, then plug back in and start engine
Retest battery voltage, should be 14.2-14.8volts now, alternator is working
If still Battery voltage, or less, then alternator is bad, FOR SURE