Alternator discharging, bad alternator?

Have the mechanic flesh out "not enough". If it keeps the battery charged, it is working fine. To see what the max output is, remove the field terminal (only one on yours) and jumper 12 V straight from BATT+. That should make the alternator put out max current (which varies w/ rpm). Watch your dash ammeter as you rev the engine, or even better install an in-line ammeter. I have a DC clamp on meter for that. Also, verify that the other brush has no voltage drop to the case).

Also, check that the case of the alternator is very close to BATT- voltage. I once swapped one in an auto parts parking lot and the new one still didn't show charge. I then ran jumper cables from the alternator case to BATT- and it charged. I put the old one back (w/ frame attachments sanded) and it then worked fine, so got a quick refund. Aluminum is bad for corrosion and that was in Atlanta. People here in CA have no idea what easterners deal with.