The warning light illuminating indicates a potential difference between ignition switch fed battery volts to field rotor and diode trio output to same. These diodes, separate from the main output pairs, are responsible for providing a half phase rectified DC feed to the field circuit by taking a bleed from the alternator stators raw three phase output, allowing the alternator to be self energising. One diode may have gone open circuit reducing field current but leaving enough for alternator to provide sufficient charge current hence voltage to battery under light loads.
Is it possible to test the diodes? I don't really want to buy yet another alternator and have exactly the same problem again (the wiper glowing light issue has been the same on both - the sporadic 'full on then off' is a new thing)
