The wiring diagrams for our cars are not completely accurate or detailed.

But the theory of how that light works is this:

• One side of the "light" goes to switched ignition 12 volts. The other side goes to the tertiary winding in the Alternator thru a diode. As long as the alternator is healthy and spinning, that winding should have MORE than 12 volts on it, and the diode prevents "reverse current" which would light the light. Now, if that winding is well below 12 volts, the light, thru the diode, sees a 'ground' and lights. This indicates that the alternator is simply not charging.

• The microcontroller in the instrument panel is also capable of providing a "ground" or "low" to the light. Exactly what that microcontroller 'sees' to cause it to light the light is not well documented. I would suppose that the controller is watching, that is, comparing, 12 volt lines in various places so that it can determine if system voltage is low in places even tho the alternator is doing it's job - this leads to 100/80's suggestion of checking battery posts for corrosion and tight, good connections; ground connections, etc.

A fundamental check is Ala_Xfire's suggestion to look at battery voltage with the engine either idling or revved up to, say, 1000-1200 rpm. At those times, I'd be disturbed to see less than 13.5 to 13.8 at idle and I'd expect to see 14.0 or more at 1200 rpm. Not seeing these voltages would, to me, implicate the alternator, most likely.



If this gets REALLY involved, keep in mind there is a chassis-to-engine block ground jumper under the car, if this gets loose, corroded, etc., the car SHOULD have a hard time spinning the starter, but it may actually start - but the charging of the battery by the alternator would be weak at best. ONE GUY on Facebook found this to be the issue in a similar situation. But that's one guy in the seven years I've owned one of these cars.