I just bought my first hub dynamo (Shutter Precision PL-8) with dynamo headlight (Busch + Müller Lumotec IQ2 Eyc N Plus). After installation I figured out that light is not working. How to determine what actually is not working?

  • You need some sort of a voltmeter/continuity tester. Commented Mar 31, 2016 at 12:08

2 Answers 2


When checking a Dynamo system, there are 3 basic items which need confirmation:

  1. Power Supply: Is your dynamo producing power? Checking this requires a multimeter, preferably with alligator clips on the wiring, and to spin the hub. Or there are specific tools, as well.
  2. Wiring continuity: Are all connections tight? Are there any breaks/shorts in the circuit? This can also be checked with a multimeter. It is best done by isolating each section of the circuit to check, and then verifying the connections between the sections.
  3. Light Source: Last, check your light source. Change the bulb if possible. Use an alternative 6v power supply to check whether the light head is functional or if there is an issue elsewhere in the loop.

Busch & Muller make a rather expensive tool for troubleshooting a dynamo.

catalog showing Light Diagnostic Tools and Mains power adaptor for B&M Lights

  • 2
    Good points. Actually, I'd recommend your step 3 to be first: Disconnect cables from the dynamo, and connect a power supply instead. That immediately tells you whether light bulbs and wiring are good, which are the parts most likely to fail. While the dynamo itself can also fail, that is fairly rare in my experience.
    – sleske
    Commented Mar 31, 2016 at 13:45
  • 1
    I have made 5.2v DC power supply using four 1.3v batteries in series (i.imgur.com/R3ePfs4.jpg). Contacts sparking and heating up while light connected to it but still not working (i.imgur.com/Pab8RSi.jpg). Seems like it short circuit somewhere inside lights.
    – mrchnk
    Commented Mar 31, 2016 at 20:05
  • 1
    Yes, looks like short circuit. B&M are high quality expensive lights and I think it's unlikely that the problem is inside the light, so I would check the cables and in particular the clips where they connect to the light, perhaps one of the thin copper threads is making contact with the other wire.
    – uUnwY
    Commented Mar 31, 2016 at 21:51
  • 1
    Cable for lights is two separated wires and no way there could be short circuit while I connect it to battery pack just in front of my eyes. I think circuit inside lights could be damaged or something. What do you think?
    – mrchnk
    Commented Apr 1, 2016 at 6:48
  • 1
    I'd say you have an issue with the light. Have you contacted the manufacturer?
    – zenbike
    Commented Apr 2, 2016 at 0:14

Thanks for replies everyone (especially zenbike)

After light disassemble i found out what was the actual problem. There was a extra-solder where wires soldered into board that causes short circuit. After that extra was removed light have come to life (YouTube).


Busch + Müller are high quality expensive lights, but seems like it is not expensive enough for simple quality-control testing

  • I bought one of their lights and the LED was loose inside the headlamp. They're relatively expensive but I am surprised to learn that their reputation for quality is not good. Commented May 16, 2016 at 0:36
  • @YoloPerdiem I have to agree – I am so far very unimpressed with their build quality. Commented Oct 21, 2019 at 16:15

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