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hmm....that box you made, is it supposed to be grounded? and are you using it as a ground. my earlier comments were predicated on the idea that it shouldn't have been grounded and it was, but based on what TR said I am thinking you have a circuit in search of a ground.
 

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Discussion Starter #22
The box wasn't designed to be grounded. That kind of design is way beyond my pay grade. I'm thinking of unwrapping the harness along where the brake light wire runs and take a look see. I've inspected the wrap on the harness throughout that section of wire, and there's nothing obvious, but this problem seems to be less obvious than I'd hoped for.
 

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Do what you have to, but the whole idea of meter testing is to reduce the need to rip things apart :|
Meter test/trouble-shooting is a learned skill, I also have a dual trace oscilloscope, that's like the next level of electronics, just fancy tools unless you learn to use them well. Meter test everything! every bulb while it is in the socket, every connection and component and relate it back to the wiring diagram, if you don't understand what something does ask.

The reason motorcycles have lots of ground wires is because lots of things need to be rubber mounted, like headlights, tail lights and signals.
 

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Discussion Starter #24
Can't argue with that logic. Does the fact that my brake light wire showed some resistance against the box mean anything?
 

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Connecting wires and switches should not show resistance, only bulbs and coils and solid state electronic assemblies. Your bike is pretty bone simple as electronics go, doesn't even have many relays.
http://manuals.sohc4.net/cb750k/technical_reference/WD750UK.jpg

... that site won't let me link it as a picture, can you follow the diagram and understand what we're looking at?
wire colour code is rather critical, everywhere there is a big black rectangle painted over the wire that indicates a pull apart connector.

 

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See the thing called "position light" in english that would be a parking light, that and the tail light come straight off your ignition switch. If those don't work and they have good ground and working bulbs, there is a problem in or before the switch.

... your switch is highly suspect, unplug it and meter test the hell out of it, zero resistance between each of the posts that are suppose to be connected at each switch position. the "off" "I" and "II" indicate the key switch position, the diagrams show which posts and when they should read zero resistance.
 

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Discussion Starter #27
Sooo, Over a few cocktails yesterday afternoon an electrician buddy and I were chatting. He asked if the taillight was grounded. I assumed yes, but went home and checked... It wasn't. The wiring diagram shows three wires coming out of the brake taillight assembly. I only have two, and must have connected the wrong wires. The brakelight is now grounded and in proper working order. The taillight, however, is still dark.
 

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...
Ground wires and parts on the other hand should have zero resistance between the battery ground and almost anywhere on the motorcycle you can touch the multi-meter lead to without pealing back hot wires..
;) we covered that, don't you have it fixed yet?
I can look at it tomorrow if you can get it here :|
 

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thanks for posting up trials-rasher i am still learning and as much as i hate electrical work its always good to hate with knowledge
i did work on and off over as period of years in a friends starter/alternator auto-electric shop
as a mechanic mainly removing and replacing components..
2 things i learned that are very good to know and apply is to use a simple 12 volt testlight whenever possible instead of a meter ,of course a meter is important for functions that cannot be performed with a light
but the most help[full bit was to sharpen the probe tips to perfect hypodermic needle sharpness..
actually the angle and shape needs to be like a heavy duty sewing needle ,so that it can easily plunge thru any wire insulating covering,make a sure contact with inner strands, without causing damage
i have never seen a new out of the package test light have the right kind of dangerous sharpness
 

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You know those machines that the eye doctor uses on you to instantly determine your eye glass prescription (objective auto-refractor)
and the ones that blow a puff of air into your eye to read your intra-ocular pressure (non-contact tonometer)
... for many years :I on-site serviced those.
 

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Sooo, Over a few cocktails yesterday afternoon an electrician buddy and I were chatting. He asked if the taillight was grounded. I assumed yes, but went home and checked... It wasn't. The wiring diagram shows three wires coming out of the brake taillight assembly. I only have two, and must have connected the wrong wires. The brakelight is now grounded and in proper working order. The taillight, however, is still dark.
The reason you only have two is that they are likely both power wires and the taillight uses the body of the light as a ground, which means you should have the taillight hard mounted to the frame of the motorcycle and then send power to the two wires. Or...you bought a light that is only a tallight and not a brake light as well (i.e it's single stage and not dual stage). Some modern bikes only have two wires because instead of having a two stage bulb, they vary the output to the light from the signal box - my wife's audi does it this way too.
 

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Discussion Starter #32
When you say single stage and dual stage you're not talking about dual filament are you? It's a dual filament bulb. I can only get either the taillight or brakelight to work when I connect the brake/taillight live wire to the corresponding wire on the bike and connect the second wire to ground on the ground wire on the bike.
 

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When you say single stage and dual stage you're not talking about dual filament are you? It's a dual filament bulb. I can only get either the taillight or brakelight to work when I connect the brake/taillight live wire to the corresponding wire on the bike and connect the second wire to ground on the ground wire on the bike.
Re-read the first part of Geeto’s last post.

Are you getting the brake light and taillight to work the way you are connecting it there? Or are you really just getting the tail light both ways you hook it up? Look at the bulb and see if it is the same element that lights up both times. Sounds to me like you don’t have the tail light assembly grounded properly.

Is there a ground terminal hiding somewhere on the taillight assembly?
 

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Typically the 2 solder terminals are the incoming leads, the brass socket part is ground for both filaments and the fat one is the brake light because it's brighter (lower resistance one)
The socket needs to have zero measurable resistance between it and your battery ground terminal. (other wise you are missing a ground wire on your rubber mounted taillight assembly.)
The pins on this type bulb are staggered (because that way you hopefully can't install it wrong or jam a single pin bulb into the socket and short out your tail lights)
 

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Discussion Starter #35
I understand the brass socket part is the ground for the filaments, and the socket that receives it must be grounded. This is where I'm running awry. I don't see an obvious ground for the socket. Can I jerry-rig one? There is a female ground terminal on the bike's harness that I'm sure if it gets a ground from the socket will make everything work as designed.
 

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Adding a ground wire is a totally acceptable solution, easy test it with a jumper wire first to confirm that's a problem.
 
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