A maze with no exit (brake light wiring)

[misterprofessionality]

Let me lay out some circumstances for those of you who may or may not have ever attempted to put an aftermarket rear brake light on your cb1100:

You have a Green/yellow wire, a Black/brown wire, and a Green wire.

The brake light has a Red wire and a Black wire.

So, the goal is to figure out which two wires attach to the Black and Red wires and which one is left unattached (the ground). Of course, we don't live in a world where wire colors make any sense or live by any form of consistency, so i thought "trial and error right?" wrong. I've now blown two fuses, i think i've tried every possible color combination in the universe, and I still can't figure out why this won't work.

The best result i've gotten and the only thing close to success: When attaching the green wire to the black and the Green/yellow wire to the Red, the brake switch activates and the light comes on. I've taken the red cover off so I can see the actual bulb filament ignite.

When I swap these wires to their opposite positions, the brake flasher switch STILL works in the exact same manner, and the same bulb filament ignites.

I have never succeeded in getting the bulb to light up when the black/brown wire is attached at any position.

The bulb, upon visual inspection, appears to be in perfect working order, both filaments.

CB1100 forum: WHAT DO I DO?

 

[Cormanus]

I'd start here, although I have no idea whether it will make any sense.

For what it's worth, it looks to me like the green/yellow wire should control the brake light. The black/brown light looks to control the tail light and the green is the earth.

Are you sure the aftermarket light allows both functions? I've got all sorts of theories, but Lord Popgun or max or peterbaron or one of our other wizards will solve it for you.

 

[misterprofessionality]

The aftermarket light does have both functions, based on the fact that the bulb has two separately actuated filaments.

I've seen all the wiring diagrams, but i can't make any heads or tales of it other than to say i think you should be correct that the green/yellow wire delivers power from the brake switch.

If the black/brown wire delivers power to the running tail light, then why can i get no operation at all whenever it's hooked up to either wire?

 

[pekingduck]

If you have only two wires from the new taillight, a red and a black, the red goes to the green/yellow for the brake. The taillight black goes to the brown/black for running light/taillight. The body of the taillight grounds through the metal fender.

When you hooked the red to the green/yellow, and the black to the green, and hit the brake pedal, both filaments were lit, the brake light brighter so you didn't notice the taillight filament.

The power was coming in through the green/yellow, through the brake light filament to the bulb's shared metal casing, into the taillight filament, through the black wire and into the green ground wire.

 

[misterprofessionality]

Thanks for the input Peking. Everything you said makes sense save for one thing.

"When you hooked the red to the green/yellow, and the black to the green, and hit the brake pedal, both filaments were lit, the brake light brighter so you didn't notice the taillight filament."

see, i have the bulb exposed for this reason, because i wanted to see which filaments are lit up. in fact, only one filament is lighting up in this case. You said i just didn't notice because the other one was brighter but i can confirm that the lower filament is not lighting up at all when i hook the Green/yellow and the Green up.

Does this change your assesment?

"the red goes to the green/yellow for the brake. The taillight black goes to the brown/black for running light/taillight."


Can you offer any explanation as to why when I hook the Green/yellow to the red wire, and the Black/brown to the Black wire, no filaments light up at all?

 

[Ben70_imp]

misterp, don't want to assume anything. You are grounding the body of the new light, right? it isn't being insulated from the fender somehow?

Ben

 

[pekingduck]

My assessment changes slightly regarding the taillight coming on. I went to verify it, and the tail light does not come on, but power is running across it. Basically when you have the black and red hooked to the green/yellow and the green, both filaments are in series. The brake light filament has 1/3rd the resistance of the running light, (so it is much brighter). The taillight filament just acts like a wire.

But on a headlight bulb, where high and low beams are closer in resistance, both filaments will light. I just verified that.

When you hook the Green/yellow to the red wire, and the Black/brown to the Black wire, no filaments will light because they are both hot wires coming in, and neither filament has a ground.

 

[Cormanus]

So, in this new arrangement, what do you do with the green wire on the bike end of the plug? According to the wiring diagram, it heads off to the regulator / rectifier.

 

[pekingduck]

So, in this new arrangement, what do you do with the green wire on the bike end of the plug? According to the wiring diagram, it heads off to the regulator / rectifier.

The green on the harness side can be disregarded - it doesn't ground the reg/rectifier, it just taps into the same common ground wire.

 

[Cormanus]

Thanks, pekingduck, that makes sense. Except that with the Honda OEM light, surely it completes both circuits? How is that supposed to happen with the new arrangement?

 

[misterprofessionality]

misterp, don't want to assume anything. You are grounding the body of the new light, right? it isn't being insulated from the fender somehow?

Ben

I'm not dreadfully familiar with these things, but since you're asking, the fender has a powder coated paint job on it. would that serve to insulate it?

does it fail to work at all without a proper ground connection?

 

[pekingduck]

Thanks, pekingduck, that makes sense. Except that with the Honda OEM light, surely it completes both circuits? How is that supposed to happen with the new arrangement?

Yes, the green ground wire is necessary to complete both circuits on the stock taillight because it has a plastic taillight bracket rubber-mounted to the stock steel fender. (The chrome plating on the plastic is actually conductive).

But the aftermarket light gets its ground, assuming the base is metal, when it is bolted solidly to the steel rear fender, in turn grounding to the frame.

 

[Ben70_imp]

Ground is as important as power in a circuit. Try grounding the unit to a bare metal engine part with a jumper wire just to see what happens. Or to battery ground.

Ben

 

[Cormanus]

Yes, misterp. If it isn't grounded, the circuit can be completed and will not work. I confess, I'm still struggling to figure out the wiring diagram, but then I know little about these things. I can't tell you whether powder coating would insulate the fender.

Apologies Ben70 and pekingduck. Seems we posted simultaneously. I think I'm beginning to get it now.

 

[Ben70_imp]

Great minds...

Ben

 

[misterprofessionality]

Ground is as important as power in a circuit. Try grounding the unit to a bare metal engine part with a jumper wire just to see what happens. Or to battery ground.

Ben

This has yielded no results. i tried to ground it all over the bike using a jumper cable i had laying about. i clipped the green wire to it and tried to hook the cable in various spots on the bike to see if it would produce a result. no dice.

 

[Ben70_imp]

The black/brn wire is powered by the ignition, for you running light. the green/yellow is to the brake light switch. The green is your ground wire. it won't power anything. However, if you can connect it to your light via the mounting bracket screw it may provide your ground for your new light.

Ben

 

[pekingduck]

Ground is as important as power in a circuit. Try grounding the unit to a bare metal engine part with a jumper wire just to see what happens. Or to battery ground.

Ben

This has yielded no results. i tried to ground it all over the bike using a jumper cable i had laying about. i clipped the green wire to it and tried to hook the cable in various spots on the bike to see if it would produce a result. no dice.

This has yielded no results. i tried to ground it all over the bike using a jumper cable i had laying about. i clipped the green wire to it and tried to hook the cable in various spots on the bike to see if it would produce a result. no dice.
You are grounding the harness ground to ground.

With the red and black wires connected to the new taillight, run the jumper between the base of the new taillight and the green wire.

 

[Cormanus]

Would powder coating limit conductivity? Yes, I know it's irrelevant, but I'm interested.

 

[Lord Popgun]

Cormanus,

It will. And regular paint too. A good clean metal to metal contact is required.