You say the light comes ON when you rev the engine. The light indicates the difference between battery voltage and generator voltage. It is normally on when the engine is idling and goes off when engine speed increases. Generator output voltage is dependant on engine speed and is not enough to charge the battery at idle. Its not like an alternator where output voltage is high enough to charge the battery at idle. Positive or negative earth makes no difference to this.
Some possibilities
Loose fan belt
Wiring fault in light circuit
Faulty connections in generator wiring/earth connections etc
Faulty regulator.
Faulty generator - it is almost 40 years old.
The designation C40 means it had a service life of approx 40K miles before needing some maintenance. It needs a few drops of oil in the hole in the brush end cover every now and then or the end bearing bush wears. Check brushes and/or commutator are not worn and brushes are free to move in the brush holders. The brush end of the generator is open to the elements and it doesn't take long for road muck etc to build up around the brushes.
Most likely possibilities are:
Loose fan belt. Only starts slipping at higher RPM.
Worn or sticking brushes which are OK at low speed but can't cut it at higher engine speeds.
Poor connections
If this doesn't fix it then check and clean regulator contacts. Manual required for settings and test procedure.
If this doesn't work then more serious surgery required along with a visit to the auto electrician.
This should give you a start
RonR
