Our favorite analogy for the electrical system on vintage Honda motorcycles like the CB175, CB200, CB350, CB360, CB450, CB550, and CB750 families is to compare it to a sandwich. It is easy to get distracted by the multi-color wires that make up the middle toppings but you cannot make a sandwich without the bread. In this article and video above we cover the positive circuit, or the "top piece of bread," to better help you understand what is happening in the middle.
The Positive Circuit
(AKA the top piece of bread)
Think of the positive circuit as two halves, one that runs from the battery to the key switch and another that runs from the key switch to nearly everything on the motorcycle. It is important to make this separation in your mind, as you will often hear the term " switched on 12v positive power " as the key switch plays a highly important role in both being the master disconnect as well as the point where the positive wires change colors from red to black.
Pre-Ignition Switch Positive Circuit
The first half of the 12v positive circuit runs from your battery to the key switch using red or red/white wire. Starting at the battery there will be a thick gauge red wire and a small red/white wire connected to either the positive side of the battery or the first post on the starter solenoid. The thicker gauge wire is solely for the electric starter circuit, so focus on the smaller gauge wire as that is what carries 12v current from the battery, to the fuse holder, and afterwards onto the key switch. Remember, this portion of the harness is always live if the ground is connected.
Good 12v positive connection can be easily tested by connecting a test light lead to any ground and using the test light probe to touch and verify that the red wire both at the fuse and ignition switch are receiving 12v positive power.
Always test your test light before using it!
Its important to remember that current actually flows from negative to positive, making the pre-ignition switch side of the positive circuit essentially the finish line of the electron path. What this means is if the red wire at the key switch has a good 12v positive connection, everywhere from the key switch back to the positive side of the battery is now a verified good connection.
Post-ignition Switch Positive Circuit
The ignition switch marks an important point of being the master disconnect for the electrical system, as well as a color change for the 12v positive wire. 12v positive power enters the ignition switch as a red wire and exits as a black wire. These black wires mark the metaphorical end of the "top piece of bread" as they run throughout the harness, carrying 12v positive current to nearly every part on the motorcycle. It is important to note every solid black wire in the harness is 12v positive and they are all equal or the same. If the key switch is on you should have 12v positive power at any black wire connection in the harness.
1. The positive side can be divided into 2 halves. The battery to key switch is the first half, which uses a red/white wire. The key switch to any accessory or switch is the second half and uses a black wire.
2. If you have 12v positive power at a connection, that means there is a clear path from that particular connection to the positive terminal on the battery. Assuming there are no cut or removed wires from the harness, each connection should test the same.
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