1981 Honda CX500c wiring

She works! Two nights ago, after getting home from the shop, I looked at the Honda manual’s electrical diagram thinking about why the front brake, headlight, and running lights work, but not the rear brake, turn signals, oil and clutch lights.

The problem with figuring it out was that the harness did not match the model bike I worked on, and it wasn’t the same as any of the 3 diagrams I used. So, I had to really think about how power is routed. Fortunately, once I matched the connectors that go in the headlamp fixture to the ones in the diagram, I could see that only 4 or 5 wires were different. (When we bought the bike, the wires were in a tangled ball outside the fixture.) Maybe there were more mismatched wires, but there were only a few that mattered.

Power for the lights must come through the ignition switch. The lights I mentioned– front brake, headlight, and running lights– did. The rest were powered by…? That was the problem. Those were connected directly to the voltage regulator attached to the rectifier. But power doesn’t come through there unless the engine is running. The oil and clutch lights must come on before the engine is started. So where was the power going to come from? It also had to come from the ignition switch.

The original Honda plan has a black wire that appears to end on a connector. That connector was obviously different from what I had on the bike. Sure enough, that black wire on the plan was on the same circuit as all those non-working lights. The original connector probably had connection to power in it. But my connector is different; it doesn’t have that connection.

I wanted to put these lights on a different circuit from the front brake so that in case one circuit went bad the other circuit would still have a brake light. This second circuit also had to come from a 10amp sub-fuse. The headlamp needs its own fuse since it uses the most power at about 40 watts on low beam. The brake light is 27 watts, each turn signal, 23W. The lights on the indicator panel use 3-4 watts each.

I had to find a battery-powered wire to splice into. Fortunately, on this harness, there was a spare brown wire routed via the ignition switch from a non-headlamp fuse. The only other thing this circuit powered was the speedometer/tachometer night lights. I connected this brown wire to a spliced-in black wire from the turn signal circuit.

Unfortunately, with everything wired, too much power was needed to make everything work through one sub-fuse. I ended up disconnecting the front running lights (turn signal lights that stay on when not indicating a turn) in order to have enough power to run the essentials– brake light, running tail light, oil and clutch indicators, and turn signals.

I have to say, I really enjoyed working on this project. Moreso than other kinds of projects. Writing my book wasn’t even as fun.

Now that I can get the bike inspected, next electrical step is to replace the turn signals for LEDs. They will draw less power. Then I’ll be able to reconnect the front running lights. (I’ll swap those, too.) I also want to add 2 more brake lights to make a braking triangle. This could potentially prevent being rear-ended by absent-minded cagers.

Can’t wait to ride! Snow, go away!

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