The circuit (or schematic) diagram is something that seems to scare most people who are unfamiliar with electronics. They look at something like this schematic from a 1982 GL1100 Aspencade, and it looks like a indecipherable mess:
- 1982 GL1100A Schematic Diagram
What this diagram shows is every wire, component and connector in the bike, and how each thing connects to every other thing. Every black line on the diagram represents a wire. Unless specified on the diagram, the lines don't specify the specific location and layout of wires - for instance, if the diagram shows two wires next to one another, that doesn't necessarily mean that they will be located anywhere near one another on the bike. The diagram is laid out for readability.
Schematic diagrams use very specific symbols to represent components. Here are a few:
- Circuit Components
- Obvious Honda Component Symbols
- Ignition Switch
- Box components
As you may have noticed in your bike, the wires are multicolored. Some of them are solid colors:
Every wire on the diagram will be marked with a letter code to specify its color. At the bottom of every diagram, you'll see a wire color index so you know what those codes represent:
- Wire color code
- GL1500 Cooling Circuit Diagram
Let's say we have a problem - the cooling fans aren't turning on when they are supposed to. First, we figure out how the fans are switched on. Looking at this part of the diagram, we can find the thermostatic switch:
- Cooling Fans Diagnostics 1
From this we can deduce that the thermostatic switch is open when the engine is cool. When the engine gets too hot, the switch closes, connecting the black wire to ground, completing the circuit, and allowing the power to flow through the fans. As a result, the fans turn on, which in turn cools the engine. When the engine cools off, the thermostatic switch opens, and the fans turn off.
There are two things we can test at this point:
1. Is the switch connecting the black wire to ground when the engine is hot?
2. Are the fans receiving power on the blue wires?
If only ONE of the fans was not working, we also have some good information: We can see that the wires connecting the fans to the switch and to power for each fan are connected together - so if one fan is running, the other fan should be as well. If that is the case, we could check the 2P-BLK connector of the inoperable fan and see if it has power on the blue wire, and ground on the black wire (when the engine is hot). If it does, then the fan itself is the problem.
But back to our two-fan problem. Let's check our first item, whether the switch is connecting to ground. We don't have to get the engine hot for this: All we have to do is bypass the switch. At the thermostatic switch, we would disconnect the black wire from the switch, and push it against the engine case, which connects it to ground. If the fans turn on, then we know that the switch is not working correctly, and needs to be replaced.
Let's assume the fans did not turn on, meaning that the switch is probably not the problem. Next, we check to see if there is 12 volts on the blue wire, on the 2P-BLK connector (this is the connector the fans plug into).
We do this using a multimeter, as explained in part two of this series. One lead of the multimeter goes into the blue wire, the other goes against a known good ground. Here's what we see:
That tells us that there is no voltage at the fans. This tells us that the problem is likely not with the fans or the thermostatic switch. We have to move further up the diagram.
- Cooling Fans Diagnostics 2
Open up the fuse box, pull fuse 4, and see if it is blown. In this case, the fuse is blown. We replace it with a new 10 amp fuse, and our fans come back to life.
But how do we find all of these connectors in order to test them? On the earlier four-cylinder Wings, you are left to tracing wires along the wiring harness, or asking the friendly people in a forum. Starting with the GL1200 Limited, Honda produced a separate Electrical Troubleshooting Manual. For the GL1800, this manual was folded into the main Service Manual. This wonderful, helpful book tells us exactly where to find every electrical device in the bike.
To start with, the ETM separates every circuit even more, into very simple block diagrams:
- ETM Fan circuit
- ETM Connector Locations
- ETM Location Image 21
- ETM Location Image 52 and 53
Back to our fans. Where does fuse 4 get its power from? Let's trace backward a bit more and have a look. The power for fuse 4 comes from a Blue/Orange wire that we can follow back to the ignition switch:
- Cooling Fans Diagnostics 3
- Cooling Fans Diagnostics 4
- GL1500 Ignition switch
In the ACC position, the red (which is +12V coming from the battery) is connected to the ACC terminal, which means anything connected to that ACC terminal will have power.
In the ON position, the red is connected to black (which is the main wire that runs the entire bike), ACC (to run accessories), as well as the Blu/Orn wire, which we already know powers fuse 4, and the fans. It also connects TL1 and TL2 together, which are used to illuminate the tail lights.
Lastly, we have the P (park) position, where the red incoming battery power is connected to ACC as wel as the specal "P" circuit which illuminates the taillights.
The last thing we'll talk about are relays:
- Relay
In the diagram above, there is switched power coming into the relay on the white wire, and out the green (because in Honda world green is ground). When the power is applied to the white wire in the relay, it closes the relay, which allows power to flow from Fuse 8, across the relay, and out on the Brown/Blue wire.
Why would we need to do this? A very good example is the starter relay. The wires running to your starter are huge, in order to be able to carry enough current to crank the engine over. Running these huge wires up to your starter button would be crazy, and the starter button itself would have to be huge, in order to handle the current.
Instead, a small starter button is used, and it controls a very small amount of current. This current is used to actuate the starter relay, which then closes and engages the starter.
The GL1500 and GL1800 make very extensive use of relays, and there are a LOT of them. Relays are mechanical devices, and over time they will fail.
Tracing circuits through schematic diagrams is an excellent way to figure out the electrical systems of your bike. I would suggest photocopying circuit diagrams so that you can trace circuits using a highlighter pen. As always, if you have any questions, please feel free to ask!
