Honda CB750 Tail light circuit

One of the simplest circuit to demonstrate how to create a Block Diagram, that came to mind, was the Tail Light circuit off a CB750 wiring diagram (as shown on Lesson 6).  Typically, any technician in need of troubleshooting the circuit would have to either visualize the circuit in his head, or make it on paper.  For a beginner, the piece of paper would be the best option, in order to make sure they are not forgetting anything.

Honda CB750 original

Honda CB750 Service Manual original

As you can see on this Wiring Diagram, the Battery is a 12V 14AH, or 14 Amps per Hour. If the Bike was left with the key ON, and enough components were ON to draw 14A, after an hour, the battery will be flat dead.

CB750 TailWhen the key is turned OFF, the circuit is said to be open (no path), and the electricity is stopped at the Red wire entering the Ignition Switch.  That means that everything prior, is live or powered.  Starting from the battery positive post, through the heavy gauge wire connects to a Relay that contains the Main Fuse of 30 Amps.  It then flows through the fuse; comes out on a Red wire that travels all the way to the Ignition Switch.  Note that before the switch, there is a 2 Prong connector Black in color.  For more recent models, the ignition switch has at least 4-5 wires, and is a multi function switch.

When the key is turned ON and the circuit is powered (or said to be Closed), the electricity will travel (almost instantly) through the light bulbs (the CB has 2 of them), and will return to the battery through the Frame of the motorcycle.  And will keep doing this circuit until the key is turned OFF or something disconnects or opens.  I mentioned the frame of the motorcycle as it is being used on just about all motorcycles as a conductor of electricity (Automotive in general or Aviation).  The engine is also used as a conductor for other circuits.  See the two drawings below.

On all Honda’s you can assume that the Green wires are the negative side.  Every single Load device will use either a Green wire, the frame, or the engine for the return to the negative side of the battery. A few Manufacturer’s Ground wire colors:

  • Honda => Green
  • Harley => Black
  • Kawasaki => Black with a Yellow stripe.
  • Suzuki => Black with a White stripe.
  • Yamaha => Black

To identify what is shown on a diagram, you will need to get familiar with the Symbols used in the industry.

Sample Symbols



Fuses are here to protect the circuits from excessive electrical flow.  They are rated in Amps according to the amount of electricity allowed to flow in each circuit.  Reminder: some fuses are used for multiple circuits (ie: Main Fuse).  Fuses are meant to prevent circuits from melting or catching fire.  When too much electricity flows through them, they are the first to melt or POP, and create an OPEN in the the circuit, rendering inoperable.  You then need to figure out why it happened, and then replace the fuse.  Failing to know the cause and fixing the cause will result in another burned fuse.

Another type of circuit protection is the Circuit Breaker, found on some Suzuki’s and Harley Davidson’s.  Those, you can, and should only reset.

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