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STEVES CORNER - 6 volts or 12, sir?


This is the first in a short series of articles looking at the electrics of our classic British motorcycles, and it seemed that a quick look at conversion of 6 volt systems to 12 volts would be a good start.


Maybe a little review of history might be helpful…….. Once motorcycles had started to have electric lighting fitted, moving away from acetylene lamps or candles or whatever else was used in those days, the most common practice was to use a 6 volt electrical system. It was a general rule that the sparks for the engine were provided by a magneto and the charging for the battery was by a direct current generator called a dynamo.  Broadly, this configuration continued from the inter-war era right through until the 1960s for most makes of machines. In those days the lighting provided by a 6 volt system was barely adequate for night time riding and, as the sparks were provided by a magneto, the charging of a battery was almost of incidental interest to bike designers.


However, from the mid-1950s to the early `60s a number of things were happening; roads were becoming greatly improved, machines were becoming faster, magnetos and dynamos were being replaced by distributors and alternators (alternating current generators) and Japanese motorcycles using 12 volts were appearing in the market.  As a consequence the British motorcycle industry realised it had to improve bike electrics and move to 12 volt systems. This immediately provided better lighting and greater system reliability. By the late `60s pretty well all British bikes were leaving the factory with 12 volt systems fitted as standard.



So why convert? One reason, as alluded to previously, was the shortcomings of such a low voltage for lighting; basically, you can`t see much beyond your front mudguard!  If you have ever ridden one of our great classic machines with 6 volt lighting on a wet night you will know exactly what I mean….Other reasons were simply system robustness as the change to distributor ignition required a well charged battery for starting, and standardisation of componentry.


What do we have to do to convert? Well, clearly the battery, the coil (for coil ignition machines) and all lamps have to be changed from 6 volt to their 12 volt equivalents.  The fun begins when one considers the generator fitted to the bike, i.e. is it a dynamo or an alternator? For the sake of this article, I will concentrate on alternator conversions (because it`s generally simpler) and I will come back to dynamo conversions in a later article.


An alternator is simply a machine that produces electricity as an alternating current, or a.c. (a sine wave), as opposed to a direct current, or d.c.  The alternator takes the form of a stator (a ring of copper wound coils that is bolted to the engine inside the primary chain case) in which a rotor of magnets cast in aluminium mounted on the engine crankshaft rotates. As the magnets pass the coils an alternating current is generated.


Now, as we can`t use a.c. to directly charge a battery, we have another piece of kit called a rectifier in the system which converts (or rectifies) the a.c. to d.c.; this can then be used for charging the battery.  We`ll come to rectifiers a bit more in a moment…..

All of these alternators, provided the stators and rotors are in good condition, are capable of running a 12 volt system, so no rewinding is needed.


In the 6 volt alternator systems, not all of the available charging is needed for normal daytime running, so the stators are wired such that there are three output cables and the full output required for the lighting load is only switched in through the light switch, when the headlight is turned on.


For conversion to 12 volts, the full output of the alternator is used all the time and the voltage and current are regulated by fitting a combined rectifier/regulator (power module) in place of the original rectifier, these having no capacity to regulate voltage to the battery.  Power modules are readily available from many suppliers who deal in spares for older British bikes.


In order to achieve the full output from the alternator, it is necessary to connect two of the three stator output cables together.  I refer, here, to the Lucas wiring colour codes as the more commonly fitted kit.




Earlier Lucas alternators used light green, dark green & mid green.  Join dark green & mid green for 12V


However, Wipac used white, light green & orange.  Join light green & orange for 12V

(British bikes were normally fitted with either Lucas or Wipac equipment )


At this stage, for those in the know, you might be asking about zener diodes.  Zener diodes were an early form of voltage regulator, bolted to a heat sink ,which were notoriously unreliable and my advice would be don`t bother – use a modern power module.


If you have coil ignition, the only part you will need to change is your coil – the points and the condenser are perfectly fine for 12 volts.


So there you are – you have fitted 12 volt lamps, a 12 volt coil or coils and got a new 12 volt battery.  You have made the necessary wiring changes to the alternator in line with the drawings above and fitted a power module.


Now is the time to test out your work! If all has gone well, check your lights are working and are nice and bright, check that you have got a good spark and start the bike up.  If you have an ammeter fitted you can check that the battery is charging through this instrument; if not shine the headlight on a wall and watch to see that it becomes brighter when you rev the engine.


You should now be able to enjoy the benefits of better lights for both day and night use as well as greater resilience for the ignition and other systems.


Good luck and Happy riding!


Steve