Talk Morgan Forums Community Vehicle Talk (non-Morgan) MGB Alternator Light Issues.....

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.

Simplified diagram below (not showing regulator which sits in series with field rotor) does show the diode trio to right of image though.