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Usually alternators need 2 wires.
1 is the sense wire, and that is purely a 12v feed from the battery (Lets the alternator know what voltage the battery is sitting at). If it can't see this - it assumes the battery is really low and you get the alternator going nuts to try charge it, and will burn the regulator out pretty quickly.
2nd wire is the charge light. This wires comes from the battery light on your dash. Other side of the light is 12v, so when the alternator is NOT charging, it will ground this wire so the charge light will light up. When the alternator IS charging, it will put 12v down the wire which means you have 12v at both side of the light = no illumination. Blown light, or if this wire is not hooked up - the alternator won't charge.
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