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I'm restoring a 1980 DBS Winner bicycle, and I'm having trouble wiring the dynamo to the headlight and taillight. As a disclaimer, I know very little about electricity and wiring.

The dynamo has two outputs on the bottom, one is labeled S and the other is labeled B. There's a wire between the S and the headlight. From B, however, a wire goes directly to the frame itself (screwed-ish to the head tube). Below the crank is a similar screw-ish contraption, from which a wire goes to the taillight. I'm thinking the current is lead between these contraptions, via the steel frame.

None of the lights work when the dynamo is on. I tested the bulbs with a battery, they work. I've tested the dynamo output with a voltmeter, and it seems to work. I also tried connecting the wire from B to the side of the bulb in the headlight and the wire from S to the bottom of the bulb, which worked when the dynamo was on. Do both the taillight and the headlight need two wires each? I find it strange if that is the case, since the bike came with one wire to each light, and it's weird to remove only one wire from each for whatever reason, isn't it?

Update: Added some images. The dynamo. The Red output is marked B, the white one is mar

The screw-thing on the head tube

The screw-thing under the crank

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  • Normally there's only one wire to each light. The metal frame is 'ground' and replaces the second wire. There are however some requirements: The lights need a metallic connection to the frame and the dynamo must be grounded as well. The dynamo may be grounded through the housing or to that screw on the headtube by a wire from S or B. The screw under the crank is ground for a tail-light with a non-metallic attachment point.
    – Carel
    Jun 9, 2015 at 20:06
  • If you "short out" the two contact points on the frame with a length of wire does the dynamo power the bulb?
    – RobS
    Jun 9, 2015 at 20:07
  • @Carel: Thanks for replying. The dynamo itself is grounded to the fork, that I know. If the screw under the cranks is ground for the tail-light, how would I go about supplying it with power? I thought that was what it was for. The taillight is attached to a metal fender, so grounding it should be easy.
    – Plasma
    Jun 9, 2015 at 23:11
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    If you say the dynamo is earthed via the fork, and you've got a voltmeter, you should first check which of the B and S terminals is earthed and which has a voltage present, and make sure that the 0V lead is the one that's connected to the frame. The screw under the cranks may well be intended to connect a wire to the metal fender as the fender itself may be mounted through painted parts in principal.
    – Chris H
    Jun 10, 2015 at 11:01
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    I got it working! You're right @Carel, the frame itself wasn't negative, but the back fender somehow was. I didn't check if the entire frame was positive, though, so can't say anyrhing about insulated wires or not. Anyhow, the problem seems to have been a missing connection between the positive and the taillight bulb. I DIYed a new socket for the bulb and did some (possibly redundant) new wiring to connect everything properly. Thanks again, guys, I really appreciate your help!
    – Plasma
    Jun 10, 2015 at 21:04

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@Plasma says: I got it working! You're right @Carel, the frame itself wasn't negative, but the back fender somehow was. I didn't check if the entire frame was positive, though, so can't say anyrhing about insulated wires or not. Anyhow, the problem seems to have been a missing connection between the positive and the taillight bulb. I DIYed a new socket for the bulb and did some (possibly redundant) new wiring to connect everything properly.

(because I don't like unanswered questions)

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