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My experience is with the 240s, but an alternator is an alternator, methinks.
In the 240s with internal voltage regulator, three wires are necessary:
One large gauge to carry the charging current to the battery, usually via the starter.
One to ground the alternator frame to the block. One the 240 it is black.
One to bring an exitation voltage to the rotor to get charging started. On the 240 this one is red, and is the usual suspect for no-charge situations and warning light problems.
As Mapleleafer said, check the ground wire, as the rubber mount bushings isolate the alternator frame from the block.
Also the belt, an easy check and embarassing when your shop finds it.
The then red wire. It serves several functions.
When the key is on, voltage travels to the instrument cluster warning lights, then to the alternator where it goes through one brush to the rotor windings, then out the other brush to ground. So the warning lights come on, and there is a small current through the rotor windings.
As the alternator begins turning, it begins charging and the charge causes the red wire to get positive polarity. It is the negative polarity that allows the warning lights to come on, so when it is positive the lights go out.
The alt or batt light comes on when the red wire has negative polarity, usually because the alternator has stopped turning.
BUT it can go negative if the red wire is grounded some other way. Such as damaged insulation somewhere, or the attaching terminal falling off. In the case of this illegal grounding, you get a red light that stays on after engine starting, and NO charging.
If the drive belts are OK, trace that red wire. With key on and engine stopped, check for 12 volts at the alternator end. If yes, check the brushes. If no, look for the illegal ground.
Good Luck,
Bob
:>)
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