Author Topic: Rewiring a 1973 CB500 Four  (Read 121 times)

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Offline kingdom

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Rewiring a 1973 CB500 Four
« on: January 21, 2020, 01:06:20 pm »
This is my first electrical project. I recently adopted this 1973 CB500 Four that needed a good home. When I picked it up it fired up with a fresh battery and all of the electrical seemed to work (ignition, headlight, brake light, etc). The bike died a short while later with a drained battery so clearly something is wrong with the charging system or the bike wiring is messed up. I opened up the seat to find a rat's nest of wires and have done my best to sort untangle them without unplugging too much. I'm not totally sure what I'm looking at here but it sure looks like it might need to be rewired. What are the first steps I should take to diagnose the charging issue? What do you experts out there recommend I do to get started given this is my first time working with this much electrical?

Offline Tomshep

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Re: Rewiring a 1973 CB500 Four
« Reply #1 on: January 23, 2020, 12:52:54 pm »
One: don't be intimidated.
Two: don't go pulling stuff apart willy-nilly.
There are ten million wires in there and you don't know what any of them do. Without the right guidance, they will multiply in your head at night and become venomous so  listen up.
Everything works. Leave everything else alone.  Any wire you pull out makes something else stop working.
The battery is going flat. Work the problem
Make a test lamp. 12 Volt low wattage bulb with a crocodile clip on each end of a foot of wire each side. Make a jumper, that is a lead a foot long with a croc clip at each end. Charge the battery fully.
Disconnect the positive lead from it and put the test lamp between the battery positive terminal and the wire you just disconnected. Key off, does it light up?
Yes? change the rectifier. Repeat the test. Should be fixed.
No. Then the battery is either going flat due to old age and irregular use or it isn't getting recharged when the engine is running.
As the battery is fresh (your description,) we need to assume the latter.
You need to be putting the test lamp in series with the field coil (Green and white from the generator usually but check the 500 diagram.) It will be the coil on its own in the generator.
jumper one wire to ground, the other to the test lamp and the other end of the test lamp to positive of the battery. Light? good. It isn't that.
No light? You are looking for a broken wire.
You got light so next check.
Unplug the rectifier. Jumper one of the yellow wires from the generator to ground. Test lamp from positive of battery to each of the other yellow wires in turn. Each should light up. If not, the stator is busted, but if both light then the stator is 99.9% certain good and you need to investigate the voltage regulator which is involved but quite easy.
To avoid blowing your head's fuses, start with the checks I have suggested and let us know how you get on.
« Last Edit: January 23, 2020, 12:58:45 pm by Tomshep »