Crazy thing about the way the alternator light works is that if the brushes wear out and that causes the alternator not to work, the indicator light will not illuminate. Therefore if you determine that the alternator isn't generating electrical current and the alternator light isn't illuminated, it is probably bad brushes.

This is a design error in my opinion. When my brushes wore out I was on the middle of Alaska with a dead battery. I didn't know the alternator wasn't working for the reason above. Luckily someone came along and gave me a jump start to get going. Being a Diesel, it would run with a dead battery, just wouldn't start by itself. I made it all the way home to PA without the alternator working by charging the battery every night and just driving during the day.

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