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This sounds like I've an electrical issue, but no idea of next steps.

I have a Gazelle Tour Populair with what I believe is a Busch & Müller LYT Lumotec Retro front dynamo-powered light.

When starting to move the light comes on, but quickly cuts off for the rest of the journey.

The shop have already replaced the light with a new one, and same issue.

Could it be a power build-up somehow?

eoinoc
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  • It's unlikely to be a power build up (as you put it), because the only way to store charge is in the light. Replacing the light should have ruled out a lot of potential issues. What type of dynamo do you have (hub, bottle,...)? I suspect that something is losing contact, either mechanical or electrical. – Chris H Oct 26 '18 at 11:44
  • It's a hub dynamo. My one other uneducated idea was that by fiddling with the light I'm earthing it, and then when I let go that something electrical is going wrong. I take your point about it could be a simple connection issue. – eoinoc Oct 26 '18 at 11:50
  • Do you have to do anything to make the light work again at the start of the next journey? – David Richerby Oct 26 '18 at 16:56
  • So my last journey had the dynamo light my full 2km journey. I locked it up. On the back it started to flicker and didn't come back on. Another journey, it did come back on after I opened the light (including the part that holds the bulb) and reattached it while moving slowly, which got it to light for a few more seconds. – eoinoc Oct 30 '18 at 07:45
  • Do you have the model with the "Senso Plus" ? if so, it might be an issue with the sensor. I have the same light with the senso plus, and my light goes out when an on-coming headlight hits my light at just the right angle but my light comes back on after the car passes. I have my light "ON" all the time to avoid that. – Baratier ErebusDuHalm Dec 07 '18 at 21:53
  • @BaratierErebusDuHalm Thanks. No, I don't have "Senso Plus". Interestingly, I did wonder if it's something to do with darkness, as the light works quite consistently now in the dark. Yet I haven't tested it consistently in bright conditions. – eoinoc Dec 08 '18 at 19:40
  • There are basically two possibilities: 1) Something is causing the circuit to be interrupted. 2) There is an overload (short) somewhere. Note that in the case of an overload, an overload protector may allow the device to operate for a few seconds before the overload condition is detected. – Daniel R Hicks Dec 28 '18 at 01:50
  • @BaratierErebusDuHalm Actually, I think you might be right about the Senso Plus. It turns out my Classic model of the light does have a Senso Plus version available. I also noticed that when I went under a bridge that it turned on. There is no mention of this in my bike's spec, but this must be the answer. – eoinoc Jan 03 '19 at 12:33

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You could try measuring the voltage the dynamo outputs when spinning the wheel. Strip two thin wires on both ends , insert them into the dynamo plug (plug which is usually present on the hub to which the wire going to the light Is attached), attach a multimeter and spin the wheel. If the voltage drops to zero/drops considerably after a few seconds whilst still spinning the wheel the problem is not with the dynamo and it's probably a short circuit/ground or bad electrical connection somewhere.

Example of what a dynamo plug/ connector might look like: https://i.stack.imgur.com/fQyr8.png

Maarten -Monica for president
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  • Yes I do have a plug like that. I no longer have the multimeter. I'll try opening up that plug and make sure the wires are clipped into it properly. – eoinoc Oct 30 '18 at 07:49
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    @eoinoc Also make sure to clean the contact points inside the clip and on the connector (using something abrasive like scotchbright or some contact spray and a rag) – Maarten -Monica for president Oct 30 '18 at 08:12