- Sat Mar 28, 2020 9:46 am
#88701
Further to the above, a 12 volt conversion on old British bikes with 6 volt crank mounted Lucas alternators retained the original alternator and was a wiring exercise rather than a replace exercise.
The alternators have six coils and on the 6 volt systems four coils were used for general running and other two were switched in when the lights were being used. This was because old voltage regulation was poor and unable to cope with all six coils when there was no demand, preventing blown bulbs and boiled batteries.
When modern voltage regulation came along they used the same alternators but used all six coils all the time to supply 12 volts. The alternators will actually put out 20 volts-ish, but the regulator/rectifier tames this down to 12 volts.
These older alternators have three wires, some modern replacements for these have two wires, but are effectively the same.
The 3 wire alternator should have three wires coming from it, Green/Black-Green/Yellow-White/Green.
The Green/Black wire and the Green/Yellow wires are connected together and are connected to one of the yellow wires on the Regulator/Rectifier. (The two wire alternators effectively have the Green/Black wire and the Green/Yellow wires connected.) The White/Green wire goes to the other yellow wire on the Regulator/Rectifier.
The two yellow wires are where the (AC) power goes into the Regulator/Rectifier and the red and black wires are where the rectified (DC) regulated to 12 volt comes out, red + black -.
I'm not sure how the 4 wire alternators would be connected, other than the fourth extra wire is, as said above, is an AC feed for the headlamp, and doesn't go via the regulator/rectifier.