One of the best conversions you can do to your C15 is to change it from 6v
to 12v electric’s. This results in better lights, giving a 30 to 50
watt headlamp and an improved charging rate control at low revs. A further advantage
is an ignition and direct lighting system without a battery for competition
& trials machines, which is easy to set-up the ignition timing and you can have luxuries like electronic
ignition, indicators, etc.
Before describing the how to do it, let us have a brief comparison of the two
systems. Both use a generator which has six output coils. In the 6v
configuration the output is divided so that when the machine is running on ignition alone
one third of the output is used and when the lights are switched on the
rest of the power is brought into use.
The 12v system differs in that the full output from all six coils is
used continually. To prevent overcharging and damage to the battery a device
called a Zener Diode is fitted into the system, which absorbs the
surplus current and converts it into heat which is dissipated through an alloy
heat sink. The result is a much simpler wiring harness and cheaper, but more
reliable switches. This is the type of system fitted to later models of BSA. You
can even change the ignition switch to a standard automotive part with a proper
Making the change
Conversion from 6v to 12v is quite straight forward and well within the scope
of the average person using normal home workshop equipment.
The first thing to consider is the alternator. The 6v which has three output wires
has to be changed to 12v which has only two. Note that the
colours of the three output wires are
Stripe (WG), Green/Yellow (GY),
a double bullet
connector, join the
together and you now have
a 12v unit. On
early models the wire colours are
green, in which case join the
The next thing is the rectifier. If it is a black
three-bladed type, Lucas 49072, then it is OK for 6v or 12v use. There is also a
small square solid state component which is not of Lucas manufacture which is
also suitable for 12v use. (See tips below)
You will need to obtain a Lucas Zener Diode, part number 49345 and a BSA type heat-sink, part
number 68-9428. When in use the diode produces quite a bit of heat and the heat-sink
which should be mounted somewhere in the air stream helps to dissipate the heat. If you can not get the
correct heat-sink, you can make one using several pieces of alloy sheet
sandwiched together with washers and a bolt. Two popular positions for the
heat-sink are under the front fork yoke or
the petrol tank. Make sure the mating
surfaces between the diode and the heat-sink are clean and be careful not to over tighten
the nut on the diode. A separate earth wire should be attached to the heat sink
which should be Red 28/0.30mm automotive cable.
A new 12v battery will be required, which are easy enough to get hold of
nowadays, and you will also have to obtain a 12v coil and horn, along with new bulbs. The ignition points
and condenser can be re-used, or you could fit electronic ignition which will
increase reliability and reduce points maintenance.
Next the cabling has to be dealt with. You can modify the existing loom or
this may be the right time to make a complete new harness. To modify the loom,
remove the two wires from the
rectifier and make up two new leads
White/Green (GY & WG)
to connect your two alternator
wires to the outer connections on the rectifier using 14/0.30mm automotive
cable. It does not matter which one goes onto
which connection. Any other green/yellow, green/white and green/black wires that go
into the harness or connects to the switches can now be cut and insulated, as they are
no longer required. Next a
wire needs to be connected into the ignition
side of the wiring. This can be done at the ammeter were the
wire is connected to it. If in doubt disconnect the wires from
the ammeter and check for power, as the battery lead will have power, but the
rectifier side does not.
You can now fit the new battery and due to the extra current concerned a
35amp in-line fuse should be fitted. Having changed the bulbs and horn, you are
now ready to go.
For competition a Lucas capacitor 54170009 2MC can be fitted. It should be
mounted with some sort of absorption for vibration and preferably with the terminals pointing downwards.
It has two terminals, the small 3/6"
blade has a red spot near it and is
connected to earth. The other ¼" blade is connected to the centre
connection on the diode, which is also connected to the coil, and lights if
direct lighting is fitted.
Study the wiring diagrams. It is not as difficult as it
appears when you read it.
Click on diagrams to expand.
Simplified Wiring Diagram
How much does it cost (Spring 2001)
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