Altenator or regulator

The “battery” light on my 96 Windstar came on. Car made it home. Took out altenator and brought it to Advance Auto. At first they said, “the needle is bouncing all over the place”. Next day, (after altenator cooled down), I brought it back and it was registering 15 volts. The regulator on this van is attached to the altenator, but it is removable. Does anybody know how I can tell which one it is because the price difference is large.

I think a defective regulator would more likely to be sensitive to heat changes than a defective alternator. The regulator contains transistors that are more prone to heat damage than the diodes in the alternator.

Over-voltage is a voltage regulator problem.

It is undervoltage not overvoltage, but only when the altenator has been running 20 minutes or so.

You said "I brought it back and it was registering 15 volts."
That’s over-voltage.
Under-voltage when hot would the the brushes (part of the alternator).

Alternators have brushes that pass current to the rotating field winding. If these wear down you will get intermittant contact and a jumping output voltage. One quick way to check is to turn the radio to an unused part of the AM dial. If you hear a buzzing that changes with engine speed there is a possiblity that what you are hearing is the arcing of the brushes as they try to maintain contact with the slip rings.

Hope that helps.