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Pat, if you pick up one of the books like "12 volt doctor" or "12 volt bible" you can probably troubleshoot this yourself for the price of the book and a $20 multimeter (Target, Walmart, just as good and half the price of Radio Shack or Home Depot).
The light usually means a charging system "failure".
That can translate into:
-Loose or corroded wire anywhere in the system
-Internal regulator failing in the alternator ($25-100 part)
-Diode failure in the alternature ($25-100 "diode frame" assembly)
-Loose belt, glazed belt, other belt tension problem
And if you have a typical V-belt, the best way to properly set the tension on it is with a tension gauge, pretty hard to fund except used these days since v-belts have been obsolete for over a decade. You can estimate tension with your fingers and a rule, but a gauge will give you better belt, alternator bearing, and sometimes water pump bearing life as well.
If you don't measure at least 13.8 volts between the alternator's charging post and ground (at the alternator mount to the engine) at low idle, there's an internal alternator problem or a belt problem. Better alternators will put out 14.3-14.4 volts at all speeds, including idle. Which is what you should see at "cruising" speeds, too.
You can also unbolt the alternator and take it into most of the auto chains like Advanced or Pep Boys, and they will load test on a machine for free. If it needs a diode set or regulator--I'd spend the money on a good remanufactured (not just rebuilt) replacement, and then take your time getting the repair parts and fix the old one, to keep as a spare.