Introduction: Basic Introduction to Motorcycle Electrical Systems

While cars have increasingly complex electrical systems that continually reduce the feasibility of DIY-repairs or diagnoses, motorcycle electronics, although also including more and more advanced electronics, are still far simpler; almost all diagrams you'll find will fit within 1-2 pages. While some basic understanding of electronics is recommended especially if you are trying to currently diagnose an issue yourself, I will assume a nearly-blank foundation to build off of. Covering the basics is a great way to tie everything together and show some of the fascinating engineering and problem solving behind general automobile electronics systems. I’ll use a diagram from my current project, a 1972 Suzuki GT250 motorcycle to help explain. This instructable intends to act as a very, very basic coverage of these systems and variation between motorcycles should be assumed at any point. However, a heightened sense of intuition for a less-technical individual trying to address an issue on their motorcycle may be something that can be taken away from this.


As this Instructable acts as an introduction to reading these diagrams, nothing is inherently required to continue.

However, in actual practice with resolving motorcycle issues, the following is recommended.

Motorcycle wiring diagram (required)

Multimeter (required)

Generic Wire (may be optional)

As with most electronics troubleshooting activities, reasonable Google search ability is a great asset that can often make or break projects.

Step 1: Electronics Review

Here are some of the more useful electronics symbols pulled from the diagram of the GT250. For the most part, you'll notice that almost all of the components are labeled in the diagram itself, and those that aren't tend to be very basic ones that have relatively universal symbols in diagrams or are able to be intuitively understood (such as the lines across the pages being wires, and the dots in them wire junctions).

Connectors: Disconnect points for wires.

Ground: Connection to motorcycle frame.

Bulbs: Generic "load" (power-consuming component).

Switch: Opens or closes circuit.

Diode: Prevents current opposite direction of arrow.

Fuse: Opens circuit in case of excessive current.

One essential formula that should be known is Ohm's Law, represented as


where V is voltage, I is current, and R is resistance. For us, this is saying that given the constant voltage of our battery, the current will be inversely correlated with the resistance we induce. Resistance is created by loads, which are bulbs, fuel pumps, sensors, etc. An example would be if we assume our standard battery of 12V to have a load of 3 Ohms (unit of resistance) bulb, the current would be equivalent to 4 amps. Were we to add a second bulb of the same kind, totaling to 6 Ohms, the current would be equivalent to 2 amps.

This is a very simplified explanation of Ohm's Law and I highly suggest learning further about it if you plan to do anything with electricity, but this will suffice for the remainder of this tutorial.

Step 2: From the "Ground" Up

As we know, electrical systems follow the fundamental rule that the flow of electricity will only occur if a circuit is complete. A circuit is complete (or closed) when there is a path from the positive end of the source (in this case, the battery's red terminal) to the negative end of the source (the battery's black terminal). For every electrical piece in the motorcycle, it will, in some way or another, be between the positive and negative terminals of the motorcycle battery.

In most electronics, ground represents a connection back to the negative terminal, not necessarily actual earth. One fundamental concept derived from this is the usage of "ground" for motorcycles; it is almost guaranteed that in motorcycles(and even most cars), the metal frame will essentially act as one massive "wire" connecting to the negative terminal. As such, ground symbols will very commonly be actualized as a wire or terminal clip bolted down to the frame, and so the frame can be pictured as the negative battery terminal itself. There also tends to be a singular wire color that represents the ground wire, which in this case is black and white (B/W).

Continuing on to the positive end, you can see there is one wire that comes from the red terminal which will supply power to every other component in the system. It is almost always a red wire, which stands to be correct in this case as well. Although this may not seem to be the case at first, if you trace the wires you will notice that every line that is not B/W will be on the power "delivery" side with B/W on the ground side. A colored line will never connect directly straight to B/W or the frame without going through a component first as this would create a short circuit. This concept will be elaborated on in the next step about fuses.

Understanding this concept is critical as now tackling many electronic issues is a simple as this: If there is a load (such as a bulb), there will always be at least one "live" line (traces back to the positive terminal) and one "neutral" line (which will connect to the frame). When we get to testing later, this will establish a degree of certainty so that during diagnoses of problems, there will be no second thought nor misstep to frustrate the process.

Step 3: Fuses

Before the positive red line can provide current to anything else, you'll notice the fuse is the first thing that current will be able to flow through (as the rectifier diodes prevent flow in their direction). This is our short-circuit protection.

A short circuit is a huge flow of current beyond what the system is specified for.

Fuses are metal wires or strips that melt in the case there is too much current, therefore protecting the rest of the circuit. We will need Ohm's law again to describe how a short circuit would occur.

The greatest electrical concern comes from the current; the battery can heat and explode under high current, releasing toxic gas, and wires will melt and destroy entire wiring harnesses (bundle of wires). If every path for the electricity to travel has enough resistance in it, the current can never reach a dangerous value. Little resistance will do the opposite, however. Take the situation above; if I were to place a wire where the blue line is and connect R to B/W, I create a path where the only resistance is the little amount in the wires, say .001 Ohms for example. Using V=IR where V is 12V from our battery, if R is .001 Ohms, then the current, I, will be 12,000 amps! In realistic, circumstances, this situation with the blue wire would be any case where a live wire touches the frame or B/W.

While the dangers stated above would occur in those situations, the fuse prevents this from happening. In the GT250, the fuse is rated to 15 amps, which provides a relatively high margin for a regular running system but is still nowhere near the amp flow in a short circuit. As soon as current which surpasses that 15 amp rating of the fuse, the metal piece in the middle melts and opens the circuit before any damage can be done to other components.

Although the GT250 has just one fuse, in more complex vehicles like cars and the GSX-R we will look at later, there is an entire fuse box that has fuses for every group of electronics separately.

Step 4: Complex Switches

While most of the switches in electrical diagrams are consistent in how they are notated, more complex switches involving various wire colors and states are often drawn in a different manner. Although many can be understood with some intuition, it is nice to outline the nuances of them when other electrical components are at stake.

Using the ignition switch for the first example, we can see it has three states which are represented as the columns; on, off, and park (P, which does not permit ignition but turns on passive lighting). Each of the rows represents the line attached to it coming in from the right, so the top row is correlated with the top wire, the second from top row is connected to the second from top wire, and so on.

A circle in a box represents a line that is engaged in that switch setting, and lines indicate which rows are connected. To explain, if we look at the "P" column, we see the there are circles in the top and bottom row, and the line connecting the two of them show the circuit in this switch position is through those two rows. For the "ON" position, we can see there are circles in each box, so every row is somehow engaged in this setting. However, the line connection shows that the top two are connected but are separate from the bottom two which are connected. Since there are no circles in the "OFF" position, there is no engagement (and therefore no connection) of any of the rows, which makes sense for the ignition.

You can see this is the same for the lighting switch and dimmer switch, but this consistency may end with other switches. For example, the horn and passing (or turn signals) switch look a bit different from traditional switches. Although you can probably understand how they work with a bit of intuition, you can do some deductive reasoning using the switch's mechanical operation if you are unable to understand its diagram. Motorcycle turn signals are used by pushing the piece left and right, so knowing that we can imagine the center piece on the diagram to activate either side of the switch. The horn signal is pressed in to activate the horn, so in the diagram if we imagine the button pressing into and making contact with the two wires, we know that is how the circuit will be completed.

Step 5: AC/Charging System

Arguably the most demanding system in terms of electrical knowledge is the alternator and associated components. The system is what keeps your battery charged while running as well as supplies power alongside it.

We'll start with the alternator/stator, or as it's labeled, the "AC generator". This has two primary components; a spinning ring aligned with magnets (that is generally attached to the camshaft and/or crankshaft) and a stationary array of wire windings. This is represented in the diagram as the two sets of coiled lines. In the image shown you can see the epoxy-covered windings, but the magnets are hidden behind the center, closer to the center of the circular cover. Without intending to dive into the physics behind it, essentially, as the magnets create a moving magnetic field during their spin, this changing field creates a current within the wire windings as useful to us as the battery electricity but with one small catch. The battery outputs direct current (DC), in which the current flows in one direction, but the alternator/stator outputs alternating current (AC), where the current swaps directions. This issue will bring us to our next component, which is the rectifier.

The small, hash-marked square is the rectifier. The diagram represents it as four diodes, which it truly is; just four diodes. What this does is apply an electrical concept called full-wave rectification, in which all of the current is applied through a diode "tree" so that it all moves in one direction for the larger system. If you trace the wires leaving the alternator and imagine the flow switching back and forth, you'll notice that regardless of the direction, the current will always pass through the rectifier and move in one direction afterward. While the GT250's simpler electrical system uses what is called a single-phase rectifier, most newer bikes have a three-phase rectifier which is a bit more complex.

Although we are a step closer to mirroring the direct current of the battery, we still have these ups and downs, not the desired flat line. The solution to this is the regulator, which is included in the rectifier (and therefore often referred to as the regulator/rectifier) in most modern motorcycles.

The regulator is a component that simply smooths out the very bumpy full-wave rectified curve we have above and turns it into the direct current flat line identical to that of the battery. We essentially take the excess voltage spikes that are above our desired number (usually 14V) and redistribute it to the low points through the use of capacitors. This is important as heavily fluctuating voltage can be dangerous to many components, particularly the battery since this newly smoothed current will also act as our charger for the battery.

The final component that is less engaged with the actual electronic operation of the motorcycle itself but is mechanically intertwined with this system is the ignition. The cylindrical components seen on the outside of the alternator are condensers, which are just larger capacitors to store charge. As the center camshaft spins, the two timing switches mechanically engage and disengage, with their contact opening the condensers to send current to the spark plugs and firing them off in the cylinder. Since the camshaft/crankshaft is inherently tied to the speed of the engine's revolutions, it is an excellent way to time the spark plugs. More modern motorcycles use what is called a pickup coil and a CDI box, however. These older timing switches wear out to become another maintenance item and their simplicity tends to isolate their interconnectivity with more complex computer systems present nowadays.

Step 6: Putting Into Practice

The primary purpose of this instructable was to give a broad overview of simple motorcycle electrical systems as something of a one-stop crash course. From this, I highly suggest looking further for more in-depth information on concepts that you feel weren't covered enough, particularly since the amount of variation between models and manufacturers is significant enough to necessitate this in any instance. The most critical piece to have when doing any work on a motorcycle (not just electrical) is to have all available manuals for it on hand. Whenever there is a moment of doubt, consulting the manufacturer documentation should be the first step in trying to resolve the issue.

Many individual problems that will occur on your motorcycle are often very heavily documented online by others who have run into a similar, if not the same issue. Further, there are so many resources online that go further in-depth on every single component on the motorcycle. It is absolutely within your best interest to pursue such forums, blogs, and any other assets that can delve deeper into whatever you are trying to understand.