Newbie: 1980 XS650 US Taillight Wiring Questions

[anomie]

Hello all. I am in the process of replacing the stock taillight on my 1980 XS650 with a non-DOT approved single bulb light. I picked up the taillight that I am going to use and the wiring is very simple, a red and black lead connect directly to the bulb socket.

On the US 1980 XS650, the taillight is connected to the main wiring harness with a six prong plug, from this plug three wires connect to each bulb socket. It appears that the blue and yellow wires are positive and negative. I am not sure of the black wire, is it a ground? (See photo below)

The bulbs are Stanley 12v32/3CPBsr, I think that these are stock? And I think that the more common name for this bulb is #1157 dual filament.

I disassembled the bulb socket assembly and the bulb prongs are wired to the yellow and blue wires as expected. The black wire is attached to the bulb housing by way of spot weld.

The bulbs on stock have two filaments, the smaller filament is lit all the time when the bike is running. When the rear brake circuit is closed, the larger filament is lit in the bulb making the brake light brighter. This makes the light legal for night riding.

If I connect my new taillight to the yellow and blue wires on the stock wiring harness, when the rear brake circuit is closed the bulb filament lights up, this is great but I don't get the "always on" bulb filament to allow me to drive legally at night.

I want to modify the taillight that I bought to add several bright LEDs that will provide the constant light for night riding.

So here are my newbie questions to you knowledgeable folks:
1. What is this black lead? Is it related to lighting the larger filament in these bulbs or is it just a ground lead?
2. Is it possible that the 2 wire circuit has an increased voltage that is used to light the two filaments? It seems like that would be a clean way to design the light.
2. If it is not possible to use the 2 wires (blue/yellow) to have a constant and bright light, then do you have any ideas on how to modify stock 6 prong cable to provide constant and bright light modes?

Thank you all so much for your time and help. Hopefully these "newbie" questions will help somebody else in the community down the line.

 

[SangreDeDios]

sorry i can't help but im also trying to figure what exactly lights the "always on" exterior light since mine only wants to light up when the brake is pressed. so i'll be watching this post, hopefully someone can explain it.

 

[lopejd01]

I'm going to follow this. In same boat.

 

[anomie]

So like my other posts it turned out that this was really easy after a little investigation. I'm sorry for wasting peoples time. I put together some photos on how I converted the old 2 bulb wiring harness to a single double filament bulb.

First I disassembled the housing for the new taillight. I attached a green 16 (Or 18, the pics show 18, but I increased the gauge) gauge wire to the taillight housing (There might be a ground screw if you are lucky, or you can drill the housing and solder the wire to the housing if you are unlucky), and replaced the single filament bulb with a dual filament bulb from the old taillight.

I passed the new green wire through the rubber grommit and crimped simple socket connectors to the ends.

The taillight is now ready to be used but I need a new single bulb harness. So I cut the plug from the original taillight (see photo 1 in the original post), and taped off the second set of black/blue/yellow wires (In case I ever want to go back to the old light).

I soldered a length of wire (blue, yellow, black) on the original socket to match the wires in the my taillight, and taped the joins (I later fixed up this mess with shrink wrap and connectors to make it tidy, the tape was so that I could quickly test.)

The mapping is heinous:
Blue -> Red
Yellow -> Black
Black -> Green

It would of course be better to match colors for wires but I had to use what I have laying around because I am on a super low budget. I finally put connectors on the end of the cable and connected it to my new taillight.

Everything works now, running light is on (small filament), brake light works when brake circuit is closed (larger filament). I know this is really easy stuff but I want to try to contribute where I can, I hope this post helps new people like me.

 

[gggGary]

This should go in the tech section.

 

[lopejd01]

Thanks, saved me some head scratching.

 

[jayel]

it's real simple.. stock wiring = Blue is tail light(s), yellow is brake light(s), black is ground those after market two wire tail lights with a dual filament bulbs use the light body as ground (bolted to the fender/frame)