How are hub dynamo lights wired and why does a rear light cause the front to come on?

Question

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.

Answer 1

Shimano hub generators have an electrically connected axle (stupid but fact, it comes from the time when bicycle lights were connected unreliably using a single cable closing the circuit using the frame). Even modern rear lights have this electrical frame connection, too (B&M Cyo is clean in that matter). This means your circuit is built of two cables and one frame ... which may cause interesting effects.

Make sure that the marked cable (on cable pairs one is usually marked with a white line) connects to the connector marked with the ground symbol ON EVERY COMPONENT (generator, front and rear light).

Another solution is opening the rear light and cut the electrical connection of the screws used to mount the light. This voids warranty but you do not need to pay attention on cable ordering no longer.

Answer 2

Do you have two wires going to your front and rear lights or only one?

If you have two wires going to your rear and only one going to your front, then my guess is that your dynamo is not correctly grounded to your frame -- and is only grounded when the two wires to the rear (ground and +) are hooked up, and the rear light's internal grounding then grounds the frame which allows the front to work.

Or, alternately, that your rear light is hooked up backwards and when connected, is shorting itself to the frame ground which causes the front light to not work. You can try switching the polarity of the rear light to see if this fixes things.

Your LBS should be able to diagnose this easily with a multimeter.