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This is a wierd situation and to my friend who rebuilds his mustangs says he has never seen anything like it. He says it is my alternator, but he is stumped to because of the headlight situation. My battery light dims on (not completely lit) at idle when car is warm whether in gear or not, say, at a stop light. When I step on the gas to go it goes out. The wierd part is when I turn my headlights on the battery light shuts off. If the alternator is bad shouldn't the light get brighter? I have got fairly new belts. I also have a brand new optima battery (red top) I installed and still the same problem. Is my alternator going bad? Would thicker gauge battery cables help? Im lost, some help would be greatly appreciated. I HATE ELECTRICAL PROBLEMS!

It is your alternator

Alternator... It sounds like it is definitely on its way out. If the light is not on when your cruising but comes on at idle I would instantly assume alternator.

It sounds to me like your alternator is going bad but just to make sure here is a simple DIY trick to test it.

With the vehicle running, disconnect the negative cable from the battery. If the vehicle keeps running, the battery could be bad. If the vehicle dies, the alternator is probably bad.

good luck I hope this works 4 u

Cool thanks, it has been doing it for about a month or more now. Looks like im getting an alternator.

Maybe it's only the brushes, but then gotta open the Alt.