Have you checked to ensure that the voltage regulator is still servicable? These alternators usually fail only 1 of 2 ways: the voltage regulator is worn out (can be replaced) or the alternator bearings are failing (time for a rebuilt alternator). See www.dieselgiant.com for a procedure to inspect and/or replace the regulator. How old is the battery, and have the techs tested it to determine that it is still serviceable? How often do you drive the vehicle? Several weeks of sitting idle can cause a battery to drain somewhat.