tech15

The most misunderstood part of our vintage motorcycles seems to be the
electrical system. Most of us don't understand what it is, how it
works , or how to make it better. I am an advocate of re-wiring your
vintage treasure, without resorting to a poorly built after market
purchased harnesses, and modernizing the entire electrical system
using components that weren't even available until the last 10 years
or so. For the purposes here I am limiting the article's coverage to
Alternator systems, not available until the 1960's.

History

In the early sixties Lucas came out with an alternator
electrical system meant to be installed within the engine's primary
chain case.

At first they were 6 volt then only a few years later they were 12
volt systems. Since the system's voltage was to be direct current (DC)
and the alternator put out alternating current (AC), additional
components were necessary. For conversion of the AC to DC Lucas chose
a readily available selenium rectifier stack. It was a series of
cooling plates bolted together, utilizing four selenium diodes
connected as a full wave rectifier. This makes a lumpy DC but the
battery could fill in the holes. The major problem was that there was
no regulation of the system voltage except that done by the battery.
The Alternator can be thought of as an ideal lossless generator (of
AC) with a series resistor in its' output path. That resistor is
really the resistance of the wires used to wind the coils of the
Alternator. With the battery connected and the motor running the
battery will absorb current as the RPMs increase (the alternator
voltage also increases with RPM). The current flows through the
generator resistance and the voltage drops across the resistance in
the winding, down to the battery's voltage. If nothing else in the
system is using current, all of the alternators output (full output at
full RPMs) goes to the battery even if it is fully charged. The
battery dissipates the excess energy as heat, which can cause battery
overheating, battery boiling, and battery plate warpage. Lucas tried
to control the alternator output by only connecting all the generating
coils when the lights were on but it just didn't work out. The system
ate batteries.

The next Lucas step was to add a regulator device, the then new zener
diode. This gem does nothing until the threshold voltage is met then
it shorts to ground. The result is the rectified alternator's output
is diverted to the battery for charging, to lights and ignition, and
directly to ground if the voltage gets above the threshold voltage,
usually about 13.5 volts. That threshold voltage was chosen to keep
the battery charged while the motorcycle is underway, but not
necessarily when it is idling or at low speeds with the lights on.

But all was not totally well. The Zener was expected, under some
circumstances, to route the entire alternator's output of 100+ watts
through itself. In the 60's the largest zener available was only able
to handle 50 watts. Well, watts is heat so Lucas started adding finned
heat sinks to help the zener rid itself of the heat. Then they made
efforts to get the zener into the air stream (mounting it under the
lower triple crown of the fork assembly). But at a two to one
disadvantage zeners got the reputation of being delicate.

For the bikes designed to operate off road, for which a leaky battery
was a hazard in a fall down, Lucas added a large (physically)
capacitor to the system. If you wished to operate the bike batteryless
this was possible as the capacitor performed the task of filling in
the lumpy full wave rectified DC output of the alternator/rectifier.
The battery could be removed with no problems. But this capacitor was
fragile (needed spring mounting) and big.

That was the state of the British motorcycle electrical system when
BSA left the market place in the 1970's. Triumph carried on with the
same system for several years after BSA was gone (Norton, too).

Modernization

Over the years several companies have developed new devices using new
technology to replace the original Lucas parts. The selenium rectifier
stack and the zener regulator have been replaced by a small resin
encapsulated module that can be mounted almost anywhere as long as it
gets a stream of cool air passing by it. Additionally, several
manufactures have included in that device an equivalent to the large
Lucas capacitor making the system capable of batteryless operation.
These modules are no bigger than the ones that cannot be operated
batteryless. The two modules that come to mind are the Boyer Power Box
and the MittyMax. The Boyer can be had in a version that has a time
delay for lights-on-all-the-time system implementation. The MittyMax
is an older but workable equivalent without the time delay circuitry.
These modules do away with the selenium stack rectifier, the zener
diode with its' heat sink and the large Lucas capacitor used for
batteryless operation.

While were at it lets get rid of the points and condenser, and the
centrifugal advance mechanism! Well, Boyer makes electronic ignition
modules. They use a rotating magnet and small pickup coils mounted in
the points cavity instead of the points and condenser. The electronic
ignition also has an even smaller resin encapsulated module that can
be mounted anywhere but close to the coils makes the most sense. If
your system has a battery the Boyer can be a digital ignition with
stored multiple selectable ignition advance curves. For batteryless
operation you can only use the old standby Boyer analog electronic
ignition which is physically no bigger than the digital version but
has only one ignition advance curve that is electronically generated.
The Boyer electronic ignitions require two 6 volt (yes, 6 volt) coils
wired in series for twins or a single 12 volt coil for singles. The
twin Boyer implementation actually fires both plugs at once, while one
is approaching TDC on compression and the other cylinder is in the
exhaust cycle. It results in a harmless plug firing at a non-critical
time but greatly reduces the electronic complexities. The coils of
choice seem to be PVL coils available through our friend Keith Moore
of Moore's Cycle Center in Anaheim. Keith claims these work best with
the Boyer.

Implementation Hints

The instruction that come with the Boyer devices are very
comprehensive. The first choice you have to make is the ground
polarity. You get to choose: British positive ground or American
negative ground. The modules support either but, of course, you have
to be consistent for each part of the system. I suggest that you
discard the original wiring harness and rewire the bike completely.
Many problems go away with the 30 year old harness. Draw it all out,
double checking it for correct operation. Plan, plan, plan! Use good
automotive wire 16 or 18 gauge for lights and ignition and 12 gauge
for battery-rectifier/regulator interconnects. Pick a central point on
the bike's frame or motor for a single point ground. Carry all return
paths of the components back to this point rather than using the
actual frame for ground return. Make sure the engine itself has
excellent continuity to the single point ground either with a cleaned
off mechanical connections or with a large diameter wire from the
engine to the single point ground. Connect the grounded side of the
coils with a wire to the single point ground, if possible. Twist and
separate the ignition pickup coil wires from all other wiring (route
on opposite side of bike from the
battery/alternator/rectifier/regulator/coil wiring).

Experiences

My BSA Victor uses the original selenium stack rectifier,
a sealed battery, and the original zener with a Boyer electronic
ignition module. My A65 Hornet uses a rectifier/regulator (for a
battery system), a sealed battery, and a Boyer electronic ignition
module. My Rickman-Triumph uses the Boyer rectifier/regulator/battery
eliminator and a Boyer electronic ignition module. In all three cases
I have used the correct PVL coils. All work very well, all start very
well, and all have proven to be reliable. I must add that Keith Moore
has been instrumental in my success with his excellent
recommendations. He has or can get anything mentioned here. He
advertises in the PA's Classified section if you need contact
information.

Energy Transfer (ET) Systems

The ET system is also a Lucas product of the 1960's. Its'
purpose was to provide a simplified ignition/lighting system for
competition bikes or dual sport bikes. It, too, uses an alternator
(special for the ET system) but the entire system works on alternating
current (AC). That , of course, means no rectifier, no regulator, no
battery or battery eliminator capacitor. The alternator's stator is
timed via the rotor's position on the crankshaft to produce peak
voltage at the point of crankshaft rotation where the plug fires. But
because the voltage peak is so narrow the ignition timing advance has
to be limited too. In the ET system the points are first connected to
ground bypassing the coil until the point in time for firing, where
the points open, allowing the alternator's peak voltage to appear at
the coil, firing the plug. In a battery ignition the points connect
the coil to the battery source, saturating the coil as current flows
through it. At the appropriate firing point the points open, the
coil's electromagnetic field collapses, and the plug fires. So the ET
coils have a limited amount of time that current flows through them,
allowing them to be physically smaller, not requiring oil cooling as
battery coils do. The lighting in the ET system is 6 volt and the
coil's generating winding is used to power the coils and the tail
light. Hence, a shorted tail light circuit and your beauty quits
running. The coils are notoriously fragile and the timing critical.
Limited advance means a compromise between maximum power and good
starting. The coils can be replaced with Japanese dual sport bike "AC
Ignition" coils (I don't know specific models but have both a Suzuki
and Honda one in my possession). The ET system is really a big joke!
If given the choice replace the alternator stator with the latest two
wire resin encapsulated battery ignition stator and convert to a
modern battery ignition system (with or without the battery). I ran my
A65 Hornet with its' stock ET system for several years. The lights
were dim, the idle speed changed depending on whether the rear brake
was applied or not, and the reliability was always questionable even
after the Japanese coil replacement.

Conclusion

I hope this helps you with your bike's electrical system design. If
you need any additional help feel free to contact me.

Addendum:

For coils for twins I have, since writing this article, found a much
better choice: The dual output 12 volt DC coil. The coil resistance
must match Boyer’s requirements. It is more compact and is easier to
mount. My last Brit bike project was a Triumph Trophy Trail TR5T and
this was the choice for that twin.