The traditional view of the electrics box on OIF machines is that it was a device that facilitated the removal of the lights (both front and rear) so that a machine might quickly be converted from road to track use. Questioning the merits and validity of this idea should be reserved until you run out of things to talk about. However, there are a few other advantages which are often overlooked:
a. The box is located in a sheltered position, shielded from the elements.
b. It acts as a junction box for four (the main, lighting, and two handlebar) looms.
c. Located above the engine, it should dry quickly if it gets wet.
d. Production is facilitated as the box becomes a sub-assembly that can be built independently from the rest of the bike.
There are a few other tell tales that indicate that the initial concept was altered during the design process (eg the casting marks on the inside of the box), but this is beyond the scope of the scribble.
This write-up assumes original +ve earth system is employed.
The box contains:
1. Ignition switch. A four position switch with three connections on the rear (Feed from the battery, output to the ignition circuit and output to the lighting circuit). The table below tries to show the connections and functions. (Shaded area means connected together in this switch position.)
2. Charging components. These components are always connected, irrespective of the ignition switch position
a. The bridge rectifier which converts the ac from the alternator into dc. Somewhat similar in appearance to Selenium bridges fitted up to the early 60’s and four silicone diodes are mounted on individual metal squares (to sink the heat) to make the bridge. If replaced with the ubiquitous modern potted block, go for the highest current rating you can (at least 50A) as the peak current through this device will be a lot more that you would logically think. They shouldn’t generate much heat if working correctly.
b. Zener diode. This is the only form of regulation and is as simple as it is inefficient, so it requires a heatsink. Bolted to the box with some special paste to aid heat transfer, it should not give any problems. It works by tending to decrease its resistance as the voltage increases. As the alternator spins faster, its output voltage increases. As the voltage increases, the Zener starts to conduct and the current flowing to ground causes the voltage to drop. The current is turned to heat and is wasted. The Zener has two connections; the body which bolts to the chassis and a 9.5mm spade.
c. Capacitor. An electrolytic capacitor is fitted across the supplies. If this capacitor is working correctly, it will provide a very small amount of energy storage enabling the electrical system to work without a battery. If you use the machine without a battery, but with the lights on, it is probably best to start the bike with the lights off. You will have no lights if the engine is not running.
3. Ignition components:
a. Coil. Generated the spark when the points open, should be marked “+” (goes to condenser and points) and “-“ (fed from the ignition circuit via the kill switch)
b. Condenser. Connected in parallel with the points, its function is to absorb the current flowing through the coil as the points open, until such time as the points open sufficiently for the voltage not to be able to jump the gap. Without it the points will arc as they open and soon burn out.
4. Indicator relay. This is a series thermal device that, a bi-metal strip is heated by the current passing through the indicator lamps and breaks the circuit, then cools down and makes the circuit, and so on. If you fit LED indicator lamps, the might not be sufficient current to heat the bi-metal strip and you indicators will not flash. You could put burden resistors in the circuit to make the original relay function properly, but this defeats the advantage of low current lamps, a solid state relay is one answer. Pattern replacements might be too tall to fit in the space provided.
5. Support hardware (mounted using 10.24 UNC set screws 3/4" will do for all, but I think the parts list suggests various lengths).
a. A moulded rubber housing that holds the indicator relay, condenser and capacitor. A drawing of this is available on this site here
, including the necessary files to get one 3D printed if you cannot source a replacement.
b. A bracket to hold the coil.
c. A nine way lighting plug housing. The electric box has multiple lighting wires that pass through the box without being switched, they just go in and out.
6. Wires. BSA used wire colours to identify their function, the table below describes their function, where they travel from/to, their colour and their reference in the parts manual. The standard colours might not be readily available so if your machine has been rewired, alternative colours might have been used.
The wire size used is 2mm2 or 14AWG, capable of carrying about 27A. When choosing wire sizes the following should be considered:
a. The current carrying capacity. 2mm2 is sufficient for most wires, however there are benefits to be had by using thicker wires for power (i.e. battery supply and ground, alternator, headlamp and horn)
b. Robustness. Small wires (say 1mm2) with thin wall insulation are all that is required and will be easier to route, make smaller bundles to tuck under the tank and provide less torsion resistance at the steering head joint.
c. Connectors. Small wires might break if you use the original bullet connections, modern connectors are waterproof, rely on the plug body to provide the mechanical strength and leave the mating surface to provide the electrical connection only. The 6.3mm spade connections in the electrical box are predominantly right angle, hence flag connectors are far more appropriate in most instances, however production costs and availability mean that these are seldom used and straight connections abound. Using all one size wire reduces the variety of connector sizes required. Fewer connectors increase the reliability, so hand made looms with splices within the looms are preferable to multiple wires rammed into a single crimp, but again this is a production cost thing. There should be an earth connection to the main body of the electrics box as it is insulated from the chassis by the AV mounts, the coil, Zener and original rectifier all use the body of the electrics box as a connection.