Alternator/generator seems to be working fine now in the warmer weather and it's happy to chuck out a good output so that the PCM internal voltage regulator can do it's job. Not sure why it was so badly affected by sub zero temperatures, but my guess is that the brushes weren't contacting properly due to flood water getting into alternator (it's pretty low down), coupled with freezing temperatures & usual crud build up preventing the brushes from contacting. Had a similar thing last winter on my Henry vacuum cleaner that is kept in the shed. It wouldn't work, because the brushes had seized inside their housings, due to build up of normal crud that had thickened due to moisture & the cold temperature