Lucas C39 C40 C40L Charge problems

The dynamo is easy to check:
1) Bearings - slop in pulley-end ball-bearing or commutator-end bush? Rumble
from bearing? Note that the classic way to burn out a Lucas dynamo is by
failure to oil the commutator-end bushing. On an Imp the hole with the word
"OIL" is invisible; on Minx etc it's stuck too close in front of the
exhaust-pipe! When the bush wears, it eventually allows the armature to rub
on the field pole-shoes resulting in heat from the friction. If you then
drive on a wet night, with maximum electrical load from headlights, wipers
and blower, the armature-winding fries.
2) Brushes - enough length left; free to move?
3) Windings - sniff test!
4) Output voltage when disconnected, and with 12 V applied to the Field
terminal only.
If it passes these tests, it ain't the dynamo that's at fault...
RB340 Control Box:
Clean all 3 contact pairs with the edge of a business-card or similar, then
apply a drop of WD40 to each. And then ask it 'what seems to be the
trouble?' With the cover removed, on increasing revs the Voltage contacts
should 'buzz' and the Cutout contacts should close. If the battery is fairly
flat, the Current-limit contacts should also become active, but otherwise
they'll just sit there.