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Discussion Starter · #1 ·
Hi everyone,

I'm going mad with this issue so hopefully someone has some wisdom for me!

I've finished restoring a Honday CB125s 1973. I'm having an issue with the rear light.

When the light switch is off, the brake light works when I use both the rear and front brake. So the brake light switches are both functioning. However, when I turn the lights on, the front light illuminates but the rear does not. I can't get anything from the rear and so it also doesn't work as a brake light whereas it works fine when the light switch is off.

I've done the following:

- taken the wiring loom out, checked connections and connectivity to the all wires in the rear light - all fine
- checked the light switch - all seems to be working
- checked my earth to the rear light - it's connected between the light casing and the not sure if that's correct. It's had a new paint job so I've filed back to the metal on all surfaces to have a contact.

I've also go an intermittent problem with the neutral light - sometimes this works, sometimes it only works if I touch the bulb casing to the body.

I just don't get why putting the front light on stops the rear light from working.

Any ideas??? I can only ride the bike during the day because I can't use the front light and have no brakes!
 

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Sensei
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OK then...Go to rear of bike (remove/open seat) to where the Green with Yellow stripe, solid Brown, and Dark Green wires are....Verify ALL three are plugged into the same color wires at BOTH ends of the rear fender ( Green to Green, Brown to Brown, Green/Yellow to Green/Yellow )......Also, remove and test the dual filament tail/brake light bulb verifying both filaments work and can work independently......
 

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Discussion Starter · #5 ·
OK then...Go to rear of bike (remove/open seat) to where the Green with Yellow stripe, solid Brown, and Dark Green wires are....Verify ALL three are plugged into the same color wires at BOTH ends of the rear fender ( Green to Green, Brown to Brown, Green/Yellow to Green/Yellow )......Also, remove and test the dual filament tail/brake light bulb verifying both filaments work and can work independently......
They're all connected correctly and I've tested their connectivity with a multimeter.

Do you know how I can test the dual filament? Not sure how to do that!
 

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Sensei
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Test continuity from each bottom solder "button" to the casing shell......Then across the two buttons.....
Also test continuity from harness side of Green wire at each connection point to Battery NEGATIVE terminal.....
 

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Discussion Starter · #7 ·
Test continuity from each bottom solder "button" to the casing shell......Then across the two buttons.....
Also test continuity from harness side of Green wire at each connection point to Battery NEGATIVE terminal.....
Thanks. I'm not sure what you mean about harness side of green wire to battery negative - as in to the point where the battery is connected (with it disconnected)? Sorry, I'm not great with electrics!
 

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Discussion Starter · #8 ·
Sorry and just to be clear, disconnect each earth when testing? So disconnect completely from bike and battery and test between those points?
 

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Sensei
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Nope.... as you follow the Green wire along the fender insert one probe into each Green to Green connection and the other probe on Battery negative....
No need to disconnect anything....

EVERY dark Green wire on the bike should show a direct (low ohms) connection to B- .......
 

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Discussion Starter · #10 ·
Ok I've tested the bulb, seems fine - reading is 0 between the buttons and from each button to the case.

I've tested connectivity (using this setting on the multimeter) for each green wire and it seems fine. Where it's mounted to the bike I've also tested from near this area too to the earth wire to make sure that it's touching bare metal (e.g. the headlight earth bolts through the headlight case to the indicator, so I've tested between the indicator casing and the connected neutral wire to make sure it's grounding.

I really don't know what to do next. There are no breaks, everything is where it should be, I've removed the new paint back to the bare metal for each neutral wire. I don't get it.
 

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Sensei
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With headlight switch turned OFF, Jump B+ to the Brown taillight wire..... Does it light (dimmer filament)?
Jump B+ to the Green/Yellow wire... Does Brake-light (brighter filament) come on?

Headlight switch off, Key switch on, Jump B+ to white headlight wire.....IF headlight low beam lights, while it is lit, step on brake pedal.... Does brakelight work?
Repeat except jump B+ to Blue high beam wire...Does high beam and indicator come on?....While lit, Test brake switch again...... What happened each time?
 

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Discussion Starter · #13 ·
SORTED!! Yes it was a bad earth... the mudguard wasn't going to the frame. I ran a wire between the two and now everything is fine. Thanks for the help!
 

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Sensei
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I KNEW you would find it if I just got you started down the logically correct methodology path..... :D;)...

I feel it is as important for me to guide and teach the logistics of troubleshooting a system as it is to provide a possible problem diagnosis or cure....
(I'd rather be thought of as a "Teacher" than a "consulting Physician").....

Would you like to know specifically how and why it was doing what it did with the faulty ground/earth at the fender?
 
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