B&M dynamo lights

[rolando177]

Old school Raleigh Dover.
Shimano dynamo hub with B&M lights.
Rear Light (Toplight) works only intermittently.
Can measure 20 - 150 mV at rear terminals.

New rear light fitted but no improvement. Have checked connections etc.

I have no real knowledge of electrics so any help will be much appreciated.

Thanks.

 

[vfr400]

I chuckle every time I see dynamos mentioned on electric bikes. What is the sense in adding weight to take electricity from a battery to drive a wheel to generate electricity to work lights, when you can take the electricity straight from the battery without any complication and without any additional weight?

Is the front light working? If it is, the most likely cause is corrosion on the rear bulb contacts, so look what touches the bulb when it's in, and scrape clean if necessary, though I can't see you having that problem if you fitted a complete new light.

The other possibility is that the wire is broken inside its insulation somewhere. Follow all the way along it to see if you can find any sharp kinks or anything like that where the wire could be damaged. You can test for that by setting your meter to continuity (beep) and checking between the bulb holder and dynamo end of the wires, but only when it isn't working.

 

[anotherkiwi]

Is the rear light fed by the front light? Bad contact at the output from the front light in that case would be my first guess.

Second idea would be to put in new wiring to the rear light it has probably seen better days.