'85 bluebird ca20 twin spark alternator prob

my late father and i replaced the motor (ca20 twin spark) in my series 3 '85 bluebird about 12 months ago before this we had probs with the battery going flat so when we replaced the motor we put a new alternator and battery in and it has been fine until now. 2 weeks ago i tested the alternator after flattening the battery with the interior light and it was fine (13.8v output) returning from a trip through the hills the lights went dim and the car died i put a charged battery in it to get home and tested the alternator now only 12v output. i had the battery load tested and it tested fine.

I realised that there is no charge/battery light on the dash... i know that some alternators will die if there is no light is that the case with these alternators.

where can i find a wiring diagram for this car and does anyone has any idea why this would happen after not being a prob for months?

sorry for the long post.
cheers
Jack