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  • On 27th May 2015

Miles of wires.  Literally.  Some say that modern cars may have as many of 40 miles of wires in them.  Add to this hundreds of individual electrical components (such as relays, circuit breakers, switches, sensors, instruments, etc., not to mention heated steering wheels, computers, video cameras, folding mirrors at the push of a button and many more) and building a new harness can become a daunting task for the do-it-yourself mechanic.  I doubt it can actually be done “at home”.

Luckily, the vintage bikes from the 1970’s, such as the HONDA CB550, have a much simpler electrical system, consisting of approximately 40 components altogether, making it possible for anyone with some basic skills and tools to build their own electrical harness.

Here are all of HONDA CB550’s electrical components:


All things considered, it’s not that bad.

Even though such a set-up may be as basic as they come, to be able to work on any electrical system and/or to make a new harness, a clear understanding of the essence of the electrical system is a must.  For me, anyway.  I am not very good at simply following instructions like purchase these components and then do this and that.

I like to know how things work.

So, here is my take on what is going on in any electrical system that is employed in any motorized vehicle.

The heart of the electrical system is the battery.  It powers everything.  But its power diminishes with use and so it must be replenished.  Exactly like the battery in your phone or the battery in any of your power tools. To recharge a battery, we need a source of electricity.  At home, it is easy.  Plug your phone into the wall outlet and it all happens automatically.  In a vehicle, a different source of “battery recharging power” is needed.

Enter the alternator.  The alternator (and all electricity generating devices that send power to your home) operates on the same principle as the one first, reportedly, employed by Michael Faraday nearly 200 years ago in the 1820’s when he observed that rotating a magnet inside a stationary coil of wires (or moving a “magnetized” coil in and out of a stationary coil to be precise) produces pulses of electricity.  The electrical current so induced alternates from + to – and for this reason it is called alternating current, and hence the name of the device that produces it.

The take home lesson here is “a magnetic field rotating inside a coil induces electricity in the coil”.

In the HONDA CB550 engine, the “rotating magnet” is a combination of a rotor bolted to the crankshaft and a coil called a“field coil”.  The stationary coil is called a “stator coil”.

Neither the rotor nor the field coil are magnets themselves and so they cannot generate a magnetic field.  In order for the alternator to work, the field coil must be magnetized.  When electricity from the battery is sent to the field coil, a magnetic field is generated, which, through the rotor, is effectively rotating inside the stator coil.  This induces electricity.  (Like the old adage “you have to spend money to make money”. ) Exactly the same kind of electricity that Michael Faraday observed in his experiments mentioned above.  The electricity produced in this way is then used to recharge the battery.

Fair enough.  But, there is a problem.  An alternating current cannot charge a battery.   We need direct (a constant + and a constant –) current for that.  So, a way must be found to turn the alternating current (AC) into direct current (DC).

Enter the rectifier. The rectifier is essentially a bunch of diodes inside a housing.  Diodes are “filters”.  They allow only positive or negative charges to go through.  So, the AC goes into the rectifier, the positive charges go through one set of diodes, the negative charges go through another set of diodes and as a result DC comes out.  Job done.

But, now, there is another problem. What happens when the load on the battery is low (you are riding on the highway in daylight and the only electrical current consumed from the battery is going to the coils, which fire the spark plugs and that is not much at all)?  The DC current coming from the rectifier will charge the battery completely and if that current is uncontrolled, the battery will eventually become overcharged.  That’s not good.

Enter the voltage regulator.  This gizmo is placed between the battery and the field coil.  When the battery is low, the voltage regulator sends plenty of the battery’s electrical current to the filed coil, which produces a strong magnetic field and consequently a strong current is induced in the stator coil, which then goes through the rectifier into the battery.  When the battery is fully charged, the voltage regulator reduces the electrical current going to the field coil consequently reducing the current that goes to the battery.

Lets’ put this into numbers to make it a little clearer.  When the battery’s voltage is below 12V, the voltage regulator sends plenty of the battery’s power to the field coil.  When the battery’s voltage is around 14.5V or so, the voltage regulator sends very little power to the field coil and the induced electricity is essentially like you battery charger at home going into battery maintenance mode.

Here is a very basic schematics showing how the alternator, voltage regulator and rectifier are connected:

Basic Electrical Schematics

Looking at that, a seemingly obvious question is, “Why are there three yellow wires going from the alternator to the rectifier?” That is because the alternator is designed to produce three separate “pulses” of alternating current called phases.  All 3 go into the rectifier and all 3 are turned into DC.

The two thick black vertical lines to the right of the alternator represent a connector.

Now, let’s find out what these things actually look like.  Once the left side cover is removed, here is what we are going to see:

Electricla box with rectifier and starter relay identified

The labeled connector in the photo is where the harness coming out of the engine is connected to (i.e. the connector mentioned above).  The three yellow wires coming out of that connector carry AC from the alternator.  That AC goes into the rectifier and DC comes out via the red (+) and green (-) wires.  The red wire is directly connected to the battery.

Taking a look at the back of the electrical components box, we can see the voltage regulator.

Voltage regulator identified


Power to it comes down the black wire once the ignition is switched on.  Power to the field coil is sent via the white wire.  The green is ground.

That’s all there is to it.

Let’s recap: When the ignition switch is turned on, power from the battery arrives at the ignition coils and the voltage regulator.  The ignition coils are energized.  The field coil is magnetized. Kick the kick start and the engine fires, the alternator starts producing electricity (AC), the rectifier turns it into DC, which goes to charge the battery.  Once the battery is fully charged, the voltage regulator makes the alternator produce less electricity so the battery is always optimally charged.  Cycle complete.  And we are back to where we started when we said, “The battery is the heart of the electrical system.  It powers everything”.

What this teaches us is that all that is needed for the machine to run is: ignition switch, battery, voltage regulator, alternator, rectifier, ignition coils and points.  None of these components can be eliminated without negative impact on performance.

Everything else is, more or less, optional.  A fuse or fuses are a prudent addition.  A headlight, tail light(s), brake light(s) and turn signals (as well as their associated switches and relay) may be required by law. If the electric start is to be retained, then the starter relay and a start switch will be needed.

And then come the “conveniences and luxuries” such as neutral gear switch, neutral gear light, oil pressure sensor and associated light, etc.

With this in mind, a brand new harness can easily be put together.  If the original components are not available or desired, many quality aftermarket parts can be used.  These days the rectifier and voltage regulator come in a single unit, which is the preferred solution for café racer and custom builders since it saves a little bit of space.

For information on how to test the electrical system’s components, SEE WHY IS MY BATTERY NOT CHARGING?