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I just bought a second-hand bike for my daughter knowing that the dynamo hub was not working and now I would like to try and repair it. I tested that the dynamo does not produce electricity.

I also verified that the positive contact wire is always in short-circuit with the hub body, both before and after disassembling the hub. That to me means there's something seriously wrong inside the dynamo and I will need to replace it, at least the internal magnet. I am just not 100% sure with these new (to me) hubs.

Can anyone confirm that there should NOT be any contact between the wire and the internal assembly ?

Edit: this is a Shimano DH-3N20 Dynhub. There's only one cable coming out of the assembly. I used a multimeter on the cable and on the shaft. I suspect the previous owner attempted some clumsy repairs, perhaps damaging the cable.

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    Why would you need to replace the internal magnet? Is it no longer magnetic? (I assume that this is an "alternator" style unit with a permanent-magnet rotor and the generating coil wholly contained within the stator. The first thing to check is for continuity through the stator.) – Daniel R Hicks Feb 13 '17 at 18:30
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    How are you testing continuity? Note that one of the two cables leading out of the dynamo will often be grounded to the hub body (and bike frame) - but it's not always the one you think. The dynamos themselves produce AC current so there's no "positive" lead - just a neutral/ground and hot wire. – RoboKaren Feb 13 '17 at 18:34
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    p.s. It'd be handy if you told us who made your hub. Include photos if you can! – RoboKaren Feb 13 '17 at 18:35
  • Remember dynamos put put 6 volts AC, so your multimeter must be on a low AC setting to read any voltage coming out. – Criggie Feb 13 '17 at 18:53
  • Thanks for taking the time to answer. I edited the question and provided more information. @Daniel: now that you mentioned, I did notice that the rotor (which in fact does not rotate at all) is not magnetic (anymore ?) – savedario Feb 14 '17 at 8:46
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After some pondering, I ended up buying a new hub, hoping to replace the center part without the need to re-build the whole wheel. I share here some information that might be helpful:

  • even the new Dynamo showed electric contact between the two terminals, so they are NOT isolated from each other and checking that is no proof the dynamo is broken.
  • opening the new dynamo, without the lever of the wheel to help, has proven challenging even with the proper tools. I had to build a custom one with a plank of wood, drill a hole and nail the dynamo to it. This way I could close the dynamo in a vice and use the plank to apply enough force.
  • after I successfully replaced the dynamo, I tried to put back together the old one, just to keep spare the parts. As I did that, I rotated the dynamo and felt current on my fingers. I was able to isolate the little wire properly and now I have 2 working dynamos

It is clear, at least to me now, that a dynamo that is short-circuited is very difficult to turn. When the little wire is not making contact the dynamo spins much more easily.

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I don't know how helpful this is, but here's a picture I took in the process of overhauling a DH-3N71. I believe all the Shimano generator hubs are the same in terms of the layout of the wire and the axle parts.

DH-3N71

The wire in the picture is intact, but the trick with overhauling these hubs is that it seems very easy to damage it. You have to keep the connector (which is holding the wire) from rotating on the axle while you are doing any tightening or loosening of the locknut. You also have to be very gentle with the wire as you take the hub parts off and put them back together, which involves bending it around. It also involves permanently bending (cold-setting) the wire back and forth at a pretty sharp angle where it exits the axle.

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  • I think I've been careful so far, but cannot say anything about the previous owner, hence my question: if I use a multimeter to connect the cable and the axle, should it beep or not ? – savedario Feb 14 '17 at 23:33
  • I'm too electrically unversed to tell you for sure. You might read the section "Grounding" here: peterwhitecycles.com/Shimano3N70.php – Nathan Knutson Feb 15 '17 at 9:14

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