Originally Posted by Greybeard

During these low-voltage points, the bulb is actually feeling some current, but not enough to see with the naked eye.


Depending on what’s happening inside the regulator, the bulb can be “on” while the alternator is putting out correct voltage, or “out” while the battery is getting no charge at all, yet come on with the key turned to “ignition” before start. For example, a cracked diode or dirty brush contacts can result in a low output with dim light and a slowly dying battery.