I have a bike with a Shimano hub dynamo, a B&M Cyo switched LED front (LED) light, and a B&M D-Toplight rear (LED) light.
With the rear light disconnected, things work as I expect:
- When switched off, nothing happens when I spin the front wheel
- When switched on, the front light comes on when I spin the wheel, and the standlight stays on when the wheel stops.
But when the rear light is connected:
- When switched off, the lights still come on when the wheel spins, but not the standlight
- When switched on, the lights and standlight come on.
In fact only one of the two wires needs to be connected to the rear light for the above to happen.
So what's happening? I thought I knew how dynamo lights were wired, but clearly I'm missing something. I assume conduction through the frame is involved somehow.
This has happened on two bikes (with the same type of lights and dynamo); on one the guy in the LBS said the back light was faulty, and fitted a different one which fixed the problem.
Some other information: On one of these bikes, until recently there were different symptoms when the dynamo plug was wired the other way around (I swapped the wires at the dynamo). It stayed off when switched off, but the lights were significantly dimmer than it should be.