Been struggling for 3 days to get a 98 Legacy Outback to charge. Apparent dead alternator, but not true after all. The battery light in the dash is wired in series with the L terminal of the alternator. The dash sends current through the light to the alternator to power it up. If the light is either burned out or not installed, the alternator will appear to be dead. Found that out the hard way. Took the dash into a Carquest to find replacement illumination bulbs. In the process of plucking bulbs out, examining them, and putting them back, the battery light got put somewhere else that isn't even used on this car. That means the ignition wiring now fails to send current to the alternator and it just sits there. 3 new alternators later, it charges great. An ammeter and voltmeter in the dash would be great. Or, just look at the battery light as you start the car to see that it lights temporarily in run but not start position. If it doesn't, the light's burned out and will fail to alert you if the alternator does actually fail.
There are 2 terminals on the side of the alternator. One (the big fat white one) senses battery voltage, and the other (smaller black/white stripe) comes from the dash. Both have to be up at battery voltage to trigger the alternator to charge the battery through the big lug on top of the alternator. I could have diagnosed the problem, had I understood what the two terminals do, but all I had was the standard, useless manuals you get at the parts stores. Nothing in them that was very useful to me.
Hope this info helps someone out there.