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 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 (A Reg One) These are more reliable, more efficient and easy to fit.

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

A 6V horn will be OK, but will sound louder & higher pitched. Cheap foreign horns may not last long

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.

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

Click here for the batteries page.

An article written for Classic Bike guide April 2015 on 12V electrics

" My mate said that if I converted my bike to 12V my dynamo would catch fire & my feet'll go flat & he should know he used to race a whippet"

Six Volts or Twelve, to convert or not to convert? + odds & ends

If you never go out at night, you may as well stay with 6V until your dynamo or alternator dies of old age, but one trip out at night on our crowded roads away from streetlights with 6V standard lighting will be enough, it's frightening & you won't want to do it again.

Why is 12V better? Even if you used the same Wattage bulbs, a 12V system will give brighter lights than 6V, and you can generally safely increase the Wattage of the headlight bulb as well. Something to do with Ohms & Volts. Most dynamo bikes from the mid 50s on had a 60W Lucas E3L dynamo. (You can tell these from the earlier E3N as the pole piece is held in place by two countersunk screws in the body, not one, & it's longer - 4 5/8"). These will happily run a 12V 45/40W Quartz Halogen headlight bulb all day, & night. Or if you only ever do short journeys a 60/55W. I ran my A10 like that for years with no problems.

Apart from the latest models with LED or HID headlights that's what most cars use, so you really will have modern lighting. The earlier E3N will run a 35/35W or 45/40W. Miller dynamos are OK with 35/35W or 45/40W for the later 50W versions. Don't over do it. If you put a 60/55W bulb onto a bike with a pre - war 30W dynamo, it will try, & try so hard to keep your battery charged that it will burn itself out.

What's involved? On dynamo bikes it's simply a matter of replacing the old mechanical regulator with a modern 12V device. It can usually be fitted inside the old Voltage Regulator with the innards removed if it's visable. There are several on the market including a couple of horrors, so if you see one for less than around 45.00 I'd avoid it. I like the V Reg, use it on my bikes & sell it with few problems. This uses the same four connections labelled F A D & E as the old regulator so wiring it up is straightforward. Pre-war 3 brush dynamos can be easily converted to 2 brush & 12V as well. Apart from that, it's battery & bulbs. Some people will tell you that you have to fit a 12V armature & field coil, but if your dynamo is charging OK on 6V it will also charge OK on 12V without any mods. A good 6V dynamo WILL run a 12V system without any drama. But if your dynamo wasn't keeping your battery charged on 6V there's something wrong & you need to establish if it's the regulator or dynamo at fault, as fitting a new regulator won't fix a dud dynamo.

How do I check my dynamo? Disconnect the two wires, noting which way they are fitted, & using another piece of wire, join the two terminals together and connect the pair to one side of a Voltmeter or a 12V 21W indicator bulb. The other side of that goes to earth. Start the bike & give it a good rev. The bulb should blow or the meter read 15 - 20V or so. Much less or the bulb just glows & there's something wrong with the dynamo. Check the brushes & clean all the black much off the commutator for a start. Some digital meters can't cope with the varying Voltage produced by a dynamo (or alternator) & will give nonsensical readings, best to use an old meter with a moving needle.

My Ammeter doesn't read any charge. If your battery is fully charged, it doesn't need any more charge so the regulator won't let the dynamo give it any & the needle will be at zero, in the middle, regardless of how fast you go. This applies if the lights are on as well, at 35 - 40MPH that's where it will be, even on 12V. Slow down to 20 MPH and it will be in the negative, that's what we have a battery for, to feed the lights when the dynamo can't. Speed up again and back it will go again to the middle, or if the battery Voltage is down a little the Ammeter will read to the plus side as it is charging the battery. This happens on 6V as well, & on either system as long as the Ammeter spends more time in the plus than the negative all will be well. If your battery is knackered you dynamo will try to revive it and your Ammeter will read way into the positive all the time with the lights off. This will damage the dynamo, cheaper to fit a new battery. Don't hit the kerb while you're watching the Ammeter. What about alternators? Converting 6V alternator bikes is equally straightforward. The rectifier is replaced with a 12V Regulator/Rectifier, & apart from one simple wiring mod covered in the instructions & with 12V battery & bulbs, it's done. In countless Motorcycle Mechanics articles in the 1960s & 70s was written how to use a Zener diode & the old rectifier for 12V Voltage control, well it still works but modern Regulator/Rectifiers are better.

As with dynamos, there can be a power issue with alternators. The last Lucas 3 wire alternators, mid 1960s, would give as much power as the 120W 12V versions that followed, so no problems there, but earlier alternators & ones fitted to smaller bikes had a lower output, so if your headlight beam gets brighter as you rev it but never gives a really good beam & your battery isn't ever fully charged, your chosen headlight bulb is consuming too many Watts & you need to fit a smaller one. LED tail lights are also good here to save power. If you have a three wire stator you probably don't have a Three Phase stator but an earlier 6V one. The only safe way to tell is to count the pole pieces showing on the inside of the stator, six for Single Phase & 9 for Three Phase. If your alternator doesn't have all the windings encapsulated in resin it's at least 50 years old so thank it very much & throw it away. High output 200W alternator stators are cheaply available, cure all undercharging issues & fit most bikes form the late fifties on. If you insist on running without a battery the only Lucas type stator that will start the bike is the 12V 120W version & a Capacitor is needed, known to many as 'that funny blue thing'. They're black now.

Modern stators suite a 74MM dia. Rotor which was fitted to most bikes from the late Fifties on & if you have a smaller 70MM dia rotor everything will fit together remarkably easily but you will only get a minute charge, so have a look first. They're usually interchangeable with the new ones though some packing out with washers under the mounting studs or on the crank is sometimes needed to get them roughly in line. Most important is making sure that you can slide an 0.008" (8 thou".) feeler gauge between the rotor & stator all the way round. Any less & the two will touch as it heats up & bye-bye stator - or worse. The two may seize & rip the studs out of the crank case. If your bike starts slowing down & there's smoke coming from the chaincase, it isn't going to get better by itself . It tells about this required gap in all the manuals so if you return a charred stator with internal score marks to the chap that you only bought it off last week " .& what about the warranty" - don't expect any sympathy. Three phase stators are available that give more power at low revs, so are better for low revving bikes or if you do mostly town work but are more expensive. So there you are, easy wasn't it?


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


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