How do I read a wiring diagram?
It is said that a picture is worth a thousand words. In the case of wiring diagrams, this is definitely true.
Wiring diagrams are an effort to put down on paper how the wiring is routed within your motorcycle's electrical systems. A good diagram will include all of the electrical components (including switching and sometimes even the connectors) and all of the wiring runs.
Each of the wiring runs is done in a different color wire in order to make it easier to trace the flow of electricity, and each of the major motorcycle manufacturers will often use the same color wire for the same circuit across many models of bikes. This last part is important to remember, because if you know what a certain color of wire is for on a CB125, you can have a pretty good idea of its purpose on a CB350.
As mentioned in the Electrical 101 post, current flows from the positive terminal of the battery and back to the negative terminal. This flow of current must be completely uninterrupted or no electrical work is being done. It's not enough to hook up a light to the positive or negative terminal alone; it only works when you have a complete circuit. In order to prevent circuits from being complete, we add mechanical devices called switches into the circuit. Turn the switch off to break the circuit and stop the flow of electricity or turn the switch on to complete the circuit and allow the electricity to flow.
So how does this apply to reading a wiring diagram? Well, in order to make the best use of a diagram, it's important to first identify the wires that are carrying current to a component from the positive battery terminal and then the wires that are carrying current away from the component and back to the negative terminal. Then we can also identify any switches that are used to control this current flow.
To begin, take a look at the negative terminal of the battery in your diagram. There will usually be a thick black wire coming from it and going to ground, but there will also be a colored wire joining in at the terminal. In the case of the attached diagrams (and for Hondas as a rule), this is a green wire. 99% of green wires on a Honda will be a ground wire. As mentioned above, all electrical components need a connection to ground or they won't function. So we know that everything we want to run needs to eventually connect to either the frame of the bike or to a green wire.
Next, eye up the positive terminal of the battery. This one is a bit more complicated as we usually have several connections here. You'll see wires running to the rectifier, starter motor, and the ignition switch. Ignore the first two for now and focus on the switch. We know that switches are used to control the flow of current and when they're off, so are our components. So what happens when we turn it on? Where is the current going now? Looking at the switch's schematic, we can see that in position one, we're bridging the gap between the red and black wires. With the ignition switch on, the black wire now has current. Continue tracing the black wire and you see see it feeds most of the electrical systems on the bike. This black wire is known as the "switched hot" wire. It can be thought of as the "trunk" from which all the individual circuits will branch.
By now, you've probably noticed a couple of other diagrams attached to this post. Aside from the "official" diagram, I've also attached a diagram I put into the Simplified Wiring Diagrams post and then one I've made just for this post. The two diagrams I've made are functionally equivalent (with the exception of the omission of the neutral and turn signal indicator lights aka idiot lights), but they're drawn in different manners to help illustrate the concepts.
The difference between the official diagram and the original simplified diagram are as follows:
*Removal of the horn
*Gauge backlights and running lights are on from the ignition switch, not the headlight switch
*Combination regulator/rectifier replaces separate stock units
*Connections from coils to points are implied and are not shown in my diagram
*Removal of electric starter, solenoid, and wiring to the starter button
*No high beam light
*Ground wires are implied for all components
The differences between my original diagram and the one we're using for teaching purposes are these:
*No idiot lights shown at all (so no wiring for neutral or indicator lights)
*All ground wires shown
*Some slight differences in wiring colors
So with those differences in mind... lets follow the wires on the simplified diagram...
Tracing back from the negative terminal of the battery we can see all the components have a ground. Perfect.
Now tracing forward from the positive terminal we can see that when we get to the ignition switch, the wire colors change from red to black (wire colors will almost always change at the junction to a switch). The black wires past the ignition switch are switched hot. Following the black wire we can see that it feeds the tail light, the speedo and tach backlights, the flasher relay, the kill switch, both the brake switches, and the high/low switch.
Each of these switches then controls the flow of electricity to an electrical component. Because each component has it's own switch, individual (parallel) circuits can be controlled without affecting other systems.
Try a few mental exercises and see if you can figure out what changes to make for the following modifications:
*Add a single switch that controls the headlight whether the high beam is on or not
*Remove the kill switch so that the coils will have power so long as the ignition switch is on
*Remove the front brake switch so that the brake light still works if the rear brake is pressed
*The the turn signal indicator light back into the circuit
*Run a secondary fuse for the headlight circuit
*Add the starter motor, solenoid, and starter switch back into the circuit without affecting the operations of the other devices
*Add an AC relay that controls power to the headlight such that the headlight will not turn on until after the engine is running
Hopefully, that's enough information to get you started. If you run into anything or have any questions, go ahead and PM me and I'll help as I am able.