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I wish to repair a faulty dynamo light system however I am a complete newcomer to these systems.

I bought an old bicycle with a bottle dynamo ("Vitalux C 6V") connected to the rear wheel. There is a single wire connected from the dynamo to the front light. There is also a single wire leading to where the rear light should be, but the light has been removed, with the wiring just loose in the air where it should connect normally.

When I connect the dynamo to the tyre and spin the wheel the globe does not light up.

Is it possible to have this system functioning without connecting the rear wiring to anything? To what and how should I connect this wire if I need to complete the circuit?

The wiring at the from the dynamo to the front globe had been cut so to reattach it I stripped some of the coating and twisted about 1cm of the internal copper wiring together. Could this be the problem and would using a length of unbroken wire potentially resolve this problem?

Thanks for your help.

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    It would be unusual for the dynamo to refuse to work due to the absence of the rear light. However, one needs to be sure that the wire is not shorting to the frame anywhere. "Splicing" a wire is OK, if you take care to get a good connection and properly insulate it, and if you separately splice all conductors in a multi-conductor cable. – Daniel R Hicks Oct 30 '17 at 17:28
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    (Given how cheap battery powered LED lamps are anymore, it's hardly worth messing with a dynamo.) – Daniel R Hicks Oct 30 '17 at 17:29
  • @DanielRHicks I guess the primary advantage of dynamo lights is that they're fairly securely attached to the bike, so they're always there when you need them. LED lights are easily detached, so you need to take them off the bike to stop them getting stolen which, in turn, means you have to remember to bring the lights with you if you leave during daylight but will be returning after dark. Also, many cheap LED lights are simply atrocious. – David Richerby Oct 30 '17 at 18:13
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    On older dynamo systems, not having a working rear light could put more power through the front light, causing that to fail in turn. But that won't be a problem just spinning the wheel by hand - if the problem is that your front bulb has broken, it might explain it, and it might need fixing in the longer term. (I'd partially agree with @daniel-r-hicks though - it's not worth messing with an old bottle dynamo light set, unless your aim is restoring a vintage bike authentically.) – armb Oct 30 '17 at 18:32
  • (Also, I have hub dynamos with modern LEDs on some bikes, but haven't bothered with rear dynamo lights, because batteries in a rear light bolted to a rack will last for literally months of commuting. Front lights need regular charging, so a dynamo light is easier, as @DavidRicherby says. The dynamo can also optionally be used in daylight to charge a phone or GPS. On the other hand even reasonable battery front lights are cheaper, can be left off altogether when definitely not needed, and have no drag when off.) – armb Oct 30 '17 at 18:40
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On old bikes usually the (steel) frame was used as second wire for closing the circuit. The old dynamo have one wire going out and some sort of built in electrical continuity with the frame.

First of all check that the bulb works, by using a trusted electricity source. Then:

  • If you bought a modern dynamo and a modern light, just lay an additional wire between the two.

  • If you instead want to use a new dynamo with the old light, make sure you connect one of the two outputs from the dynamo to the frame (a washer in the right place will do the trick)

connection dynamo to frame

Not requested by your question: The rear light usually had connection to the frame via the mud shield (either it was made of metal, or had a metallic strip in the plastic body.

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