Converting an Alternator system from 6 to 12V


If your bike has a Lucas 6V 3 wire alternator system, with a simple wiring change it will be able to power a 12V system.

Disconnect the Green/yellow alternator wire from the loom and join it to the Green/black alternator wire. That's it. The job's done.

The 3 wire alternator is only 100W max. so will be OK for points ignition but won't run a Boyer ignition with your lights on for long, the battery will go flat.

The existing 6V rectifier will be OK, but you need some form of Voltage control. Fit either a Zenor diode or remove the rectifier and fit a regulator/rectifier unit (Power module) (£29.95) These are more reliable, more efficient and easy to fit.

Fit 12V coils (£15.50) unless you're using electronic ign. then 2 x 6V coils or a 12V twin output coil £34.95

A 6V horn will be OK, but will sound louder.

Obviously you will need bulbs. If you've fitted a new alternator a 60/55W headlight bulb will be OK and give you a light as bright as a cars. If you're sticking with the old alternator a 60/55W will probably be OK, but a 45/40W is guaranteed. £8.25 for British Pre-Focus.

Tail light, standard 0.60P. Much Brighter Quartz Halogen £9.95. Brighter with low current LED £12.95

Speedo light 0.75P

For the battery, measure up for the largest battery that will fit.

Click here for the batteries page.

Earlier Lucas alternators used Light Green, Dark Green & Mid Green. Join DarkGreen & Mid Green for 12V

Wipac used White, Light Green & Orange. Join Light Green & Orange for 12V


This is a diagram of a basic 6V system converted to 12V using a Zenor diode for Voltage control. The wiring details of the more modern regulator/rectifier conversion are above.
Courtesy of the most marvellous old bike magazine in the whole wide world at They're modest as well.


What's new page
LED Bulb replacements for car & motorcycles 6V/12V
Brighter Quartz Halogen car & motorcycle bulbs
Brighter headlamp bulbs for modern vehicles
Standard replacement car/motorcycle bulbs
Motorcycle batteries
Motorcycle & Car lighting
Motorcycle electrics
Pazon electronic ignitions for classic bikes
Alton generators, dynamo replacements
Motorcycle accessories
Motorcycle Oil Filter Kits
Speedometers and rev counters
Handlebar levers for Classic Bikes
Filter Plus oil filter magnet
Replica Girling motorcycle shock absorbers
BSA fork upgrade kit
Stainless Steel Silencer Clamps
Order Page
Goffys Norbsa 850
Goffys A10