Tail light wiring issue

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Good day everyone! So I've stumbled into another (hopefully) minor problem. I just got my new tail light housing from OldBritts, which is absolutely beautiful. I was installing it on my 1974 commando and have a wiring issue that I didn't have before. When everything is hooked up, the tail light doesn't light up, but the brake light lights up fine when the levers are pulled. Both turn indicators work fine as well. I have the red wire from the harness grounded to the frame. I tested continuity on all the leads from the harness and they all check out fine. I suspect a grounding issue, but would think if that was the case the brake light wouldn't work. Any ideas from the pros?
 
davecox2 said:
I have the red wire from the harness grounded to the frame. I tested continuity on all the leads from the harness and they all check out fine. I suspect a grounding issue, but would think if that was the case the brake light wouldn't work.

Yes, probably a grounding problem and I'd guess it's probably due to the wires being connected incorrectly.

If the wrong bulb wire is connected to ground then one bulb filament may still work as the working filament's circuit will ground through the other bulb filament and grounded wire.

What are the three wire colours?

I'm guessing they may be Black, Brown, Red?

If so, then I suggest you try the following:

Red = Brake light
Brown = Tail light
Black = Ground.

If in any doubt, follow the wires back to the bulb holder if possible.
 
When you are asking about the color of the three wires, are you referring to the wires on the actual brake light? If so, there are only two; black and red. Thanks for the quick reply!
 
davecox2 said:
When you are asking about the color of the three wires, are you referring to the wires on the actual brake light? If so, there are only two; black and red.

If you mean there are only two wires in total, then logically, neither of them can be a ground wire if there is a brake light and a tail light?

If so, then the ground is probably through the lamp body itself.
 
Yep, that's exactly how it is set up, which is why I'm baffled that the brake light works but the tail light doesn't!
 
davecox2 said:
Yep, that's exactly how it is set up, which is why I'm baffled that the brake light works but the tail light doesn't!

If the lamp or bulb isn't grounded properly then the brake light circuit could be using the tail lamp circuit wiring as it's ground.

Try running a wire from a known good ground point to the housing or bulb holder and see if the tail lamp works.

Edit: Are you sure it isn't just a faulty bulb or bulb holder?
 
Tried running a wire from a good ground point to the housing, bulb holder, etc. and got nothing.

Tried running a wire directly from the battery to the bulb lead and it lights up, so I know it's working.

I have good current at the wire coming from the harness, so I am convinced its a ground issue, but just can't figure it out.
 
I'll bet if you put your volt meter on the ground terminal of the bad lamp, and try to turn it on, you'll read 12V. You'll just have to figure out where the ground issue is for that circuit. Is there a ground strap that's insulated or not hooked up?
 
davecox2 said:
Tried running a wire from a good ground point to the housing, bulb holder, etc. and got nothing.

Tried running a wire directly from the battery to the bulb lead and it lights up, so I know it's working.

I have good current at the wire coming from the harness, so I am convinced its a ground issue, but just can't figure it out.

Did you try another bulb?

Have you tried swapping the two wires over (or are you sure the tail lamp circuit is connected to the bulb tail filament and brake circuit to brake filament)?
 
As usual, I missed something. I apparently do NOT have current at the brown/green wire at the end of the harness. I must have had a few to many IPAs in me the first 6 times I checked it. So, does anyone know the best place to check for current next? Where is the next place the brown/green wire is accessable on the harness?
 
davecox2 said:
So, does anyone know the best place to check for current next? Where is the next place the brown/green wire is accessable on the harness?

The most logical place to make the check would be at the ignition (master) switch (Brown/Green, Sw.Terminal 3).

Also:
What about the position/parking bulb (if fitted) and the instrument bulbs? If they are also not working that would suggest a fault at, or close to, the ignition switch, because those bulbs are fed from the same (Brown/Green) circuit as the tail lamp.
 
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