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Discussion Starter #1
I bought an 85 Honda Nighthawk cb450 the other day. Most of the bike seems to be good but there's a very odd issue with the tail light. With the ignition in the Run position the headlight works but the tail light is out. If I apply either brake then the tail and brake light bulbs light up. If the ignition is in the Park position then the tail light is on properly.

I checked the connector under the seat and it's getting power properly in the Park position but nothing in the Run position.

I checked the grounds and they seem fine. The ignition switch is in good repair for being almost 30. I have no idea what could be causing the issue and I'm not that great at electrical.
 

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Have you checked each pole on the ignition switch to make sure they are all energized correctly based on switch position? It sounds like you may have a bad switch contact or a short in the tail light circuit. A wiring diagram for your bike would be a good thing to have to help you trace the problem.
 

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Discussion Starter #3
I've got the diagram, it's been a little bit of help but like I said I'm an electrical noob. I've spent the last three days just trying to figure out what the damn thing says.

I'll check out the switch more carefully when I get home. Hopefully I can make sense of the diagram enough to let me check the switch properly.

I'm so much better with bits I can see moving LOL.
 

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It's always easier to fix stuff when you can see what's broken. :)

Most wiring diagrams will have a small drawing, or chart, showing which switch terminals are active with the switch in each position. You can use that to check the switch function in all positions.
 

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Sorry but I have to ask. Did you check the bulb? This should be a 2 filament bulb and one could be burnt out being that you have brake lights. Might be a matter of $2.50 and not a headache.
 

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Discussion Starter #6
That was actually the first thing I checked. It's actually two single filament bulbs. Both light up nice and bright when either brake is applied and the tail light bulb lights up when the ignition is in the Park Position.

I've spent some time in the YouTube classroom learning about auto electricals and I've been color coding my wiring diagram. Turns out there's a frame-ground on the Dark Green wire from the tail light that I haven't checked and for the life of me I can't find the damn thing.
 

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Discussion Starter #7
Solution

Okay, so I solved it totally by accident but I'm almost positive I've found the fault... even though I can't seem to recreate it.

I was trying to test the wires with my multimeter but I ran into the issue that my wiring diagram showed six wires at the appropriate connector and my bike only had five, and only the brown and dark-green wires matched colors. I was poking at the connector trying to figure out which one was which and what the hell was missing when I noticed that the brown wire on the tail-light end of the connector seemed loose. I pushed it back into place, reconnected, turned it on, and ,voila, let there be tail-light.

Please, someone who knows more about wiring than I do look at diagram for the 85 cb450sc at oldmanhonda.com (I'm not allowed to post pics yet) and tell me if I'm crazy. The Br/W wire takes 7amps from the bottom fuse, runs it through the ignition switch in the run position to the Br wire, which delivers power to the tail-light, the ignition switch and the tail-lights are grounded on the Dg wire.
Also, are there supposed to be six wires at the connector: brown/white, black, black/white, dark-green, red, brown (top to bottom at the head-light side of the connector)?
 
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