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!

Wire gauge

So, I have to replace some wires on old Molly. She's a 1981 Honda CX500c. It looks like someone replaced the entire wire harness, because on her model, the flasher relay should be located in the headlamp fixture, but the gray wire to connect it runs to the back like in newer Hondas. Some bloke jimmied in a couple relays in the front and miswired the right blinker so that no power was getting through to the switch.

Thank goodness my daddy, former electrical engineer, passed down his engineering aptitude DNA to me. :)

Oh, so, I was going to share this website: GaugeWire.com. It tells you what gauge wire to use. On this custom, currents can get past 20 amps, so I'm getting 14 gauge. I hope the other wires are 14 also. *sigh*

Bike upgrade!

It's been a while since blogging about Georgia. Well, that's because she's been in the shop. Jason and I decided we'd fix her ourselves. We changed the chain and the rear sprocket, which was bent when hit by a car. The tire has been balanced, and Jason took her for a spin. I haven't ridden her, though, because I was afraid to ride her after an amateur balanced her tire.

The rectifier I installed works. Her battery's been charging just fine. I wrapped it in heat resistant tape (up to 500 degrees F) and bolted/grounded it to the frame where the old rectifier was. I'll put rectifier pics up later.

Anyway, Georgia was too small for me to ride anyway. I couldn't get her over 55 mph over the Manhattan Bridge, the way I ride to get home from the city after work. And I certainly was unable to pass a truck safely, so Jason decided I needed a bigger bike.

Here she is! Her name is Molly. Ain't she a beaut?


We got her yesterday. The owner thought the rectifier was bad. We test it, and it's fine. BUT the wiring is a mess. The battery was probably discharging through all the exposed copper throughout. This is what it looked like behind the headlamp:


I'm gonna work on her today. Clean her up nice. :)