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UPDATE

Dear All, thank you for replying. Very helpful information in there. I have taken more photos as requested. Some answers to the questions, in order of comments.

  1. I tried to draw a wiring diagram but I am plain rubbish at doing it. The basic wiring is from the AXA headlight one set of wires goes to the Dynamo hub - which works. The other set are connected to another extension wire that runs to the back of the bike.

  2. I can't find a switch anywhere. The bulb is definitely new.

  3. The light might be an older generation, but it is brand new.

The light is an AXA Pico 30T Steady. I have included a screenshot of the spec sheet.

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First timer, learning a new skill. Please forgive any newbie mistakes. I am truly learning step-by-step. I am building my first bike ever from scratch. Well, I found a destroyed bike on the street and broke it down to the frame and built it back step by step to learn how a bike works.

The Dynamo hub in the front wheel is working well, and the headlight I installed is shining nicely.

However, here is where I am getting lost. I have hooked up the rear dynamo to the rear rack, and installed the cable from the front dynamo to the rear.

I can't get the rear light to work for the life of me, and need some advice on how to diagnose the problem.

I am using a multimeter and getting a signal out of one of the leads from the front Dynamo, and the cabling is new (everything is brand new), but the rear cable leads are not registering anything.

I have included a picture of the rear dynamo (Busch + Muller Toplight) in case anyone says "How could you not KNOW you need to do such and such..enter image description here.).

Anyways, I read the posts on Dynamo problems which seems to deal with hub problems. My front light is burning bright, so I'm pounding my head against the wall.

Any advice would be much appreciated.

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    It isn't very clear what the wiring is from your description. You should only have one dynamo. Can you draw out a wiring diagram?
    – Noise
    May 25, 2022 at 18:44
  • the other question is how the electric current is supposed to get from the white square on the left of your image to the connection lower down that might go to the bulb. There is clearly a missing link, which might be a switch or part of the light housing. Please add more angles and component photographs if possible.
    – Noise
    May 25, 2022 at 18:45
  • The system should be very simple, the bulbs should be wired in parallel and to the dynamo. They may or may not be switched.
    – Noise
    May 25, 2022 at 18:46
  • The LED systems are slightly more complex, but you ought to have it easy with the traditional bulbs
    – Noise
    May 25, 2022 at 18:47
  • Is this an older pre-LED busch and muller light? If so the bulb could be the problem
    – Nathan Knutson
    May 25, 2022 at 22:25

1 Answer 1

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In general in modern systems 2-core wire is used, and the rear connects to dedicated terminals on the front light. Is that what you've got? Old systems sometimes used the frame as one of the wires and plenty of new lights still support this with one wire being connected to the mounting screws. It may be as simple as needing to swap the 2 wires at the back light though when I saw that fault it stopped the front from working as well. This could vary with the model of the front light.

I'm suspicious that this is the issue because your multimeter is giving a signal from "one of the wires" implying you're connecting the other multimeter terminal to the hub via the frame.

Still, with the rear light disconnected, your meter should show something between the 2 wires to the rear. Probably AC but the front light could output DC in theory.

But that may not be it. There are a few other things to check:

  • The current Toplight is an LED rear light, but it looks like you've got an old bulb version.
  • Please also give the model of the front light. A few front lights do funny things.
  • LED rear with halogen front may have particular difficulties but plenty of systems work like that. LED front with incandescent rear is a very rare combination - it should work but your front light may be unhelpful.
  • As it's an incandescent bulb, check its resistance (with it out of the holder). It should be in the tens of ohms.
  • See if it works with the rear light not screwed to the frame, as the connection to the frame is usually made through the mounting screws
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    Now I can see it on a big screen it's clear the rear light has a bulb. The description of it as "new" made me assume otherwise
    – Chris H
    May 26, 2022 at 9:23
  • Thanks Chris. This is very helpful. Should I try another rear light? maybe the LED on the front is not compatible with the Toplight (old version) and a LED toplight is the better solve? I haven't soldered the leads from the front to the extension, because I wanted to make sure it all worked before locking it down.
    – daveinberlin
    May 26, 2022 at 14:36
  • I think you should try to get it working with this light first, but an LED rear light with standlight would be a big improvement when it does work. At the moment we can't be sure that simply replacing the light will fix anything - if it does turn out that the rear light wiring is shorted via the frame you'd still have the same problem
    – Chris H
    May 26, 2022 at 15:03
  • How do I fix a short via the frame? I'm sorry, I know it's a sad question but I am a total beginner. PS I did try working it outside the frame and was not having luck.
    – daveinberlin
    May 26, 2022 at 17:12
  • At this point I'd check the bulb, if you can rig a battery across it to make it light, that works well. Or 5V from a USB socket (red and black cores in a USB cable carry 5V and ground) and you could connect those to the whole rear light. Then take everything off the bike except the dynamo wheel, so neither light is connected to the frame, and get it working on the floor.
    – Chris H
    May 26, 2022 at 17:38

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