Lumotech-Sturmey fron hub dynamo problem

[Piscator]

I have a B&M Lumotech Lyt LED N Plus front light (and tail light with stand light) teamed with an S-A X-FDD front dynohub. This is a new set-up.

It all produces light as it should, but the problem is that I can't turn it off with the front light ON/OFF switch (feature signified by letter N in title) i.e. whether on or off, the whole system stays lit.
The wire from light (two spade +- connections) to hub is two wire, one black, one white. I have tried endless permutations, but either the light runs all the time or not at all.

I may be missing something simple here, but I'm beginning to wonder whether the on/off switch on the lamp is faulty.

I should add that I'm hopeless with electrics. All suggestions welcome, as this is infuriating.

 

[mickle]

I think your prognosis is correct - faulty switch.

 

[Piscator]

I think your prognosis is correct - faulty switch.

Thought so, but now I'm not sure!

The headlight has two groups of 2 (+-) spade connectors, one side marked "In" and the other "Out". The In is for power into the system from hub dynamo ("to dynamo"), and the Out carries two wires to the rear light ("to rear light", per makers diagram).

Now, if I connect the 2 wires In from the hub (plus 2 connections at the hub), connect the Rear light Out negative but disconnect the rear light Out positive, this happens:-
1) the front light on/off switch works to turn the front light on and off
2) the rear light runs constantly i.e. won't turn off. I don't know whether it's supposed to be always on as a safety feature.

I begin to think that it's a grounding or earthing problem.

There is some stuff on the B&M website which is in FAQs but in German. Via Google Translate, I get this:-
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"Why will not switch from my headlights?

Detailed error description:

The switch for headlamps designed for hub dynamo is constantly lit without function and headlight and tail light. Primarily upon LUMOTEC Fly models are affected. Thus, it appears, must meet four points:

Headlights without grounding
Rear light with ground terminal
Double the headlight cable polarity (black to earth sign of the dynamo)
Crossed wires from headlight to taillight, i.e. the positive pole of the Rüli is also grounded

In this constellation, the switch of the spotlight completely taken out of the circuit and the light stays on.

As a remedy, it is not enough just the wire on taillight turn stoppers. This creates a short circuit. You must set both cables are connected correctly or the ground connection to the rear light must be isolated.
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Seems to me that unplugging the + rear light is what is referred to in the last sentence.

 

[Piscator]

Update.

I have now got it all working as it should, but how in technical terms I can't quite work out.

The problem seems to be one of multiple earthing, as I think the rear light is earthed and there is a frame earth on the dynamo connection. What I ended up with is:-

Dynamo - light connection terminal attached, frame connection terminal not attached
Power IN side - both + and - attached
Power OUT side - negative - attached, positive disconnected

This gives me no light when headlight switch is off, front and rear light when on, and I think the standlight front light feature works, although a bit tricky to say in broad daylight.

I'd still welcome some explanation, hints or tips from anyone who knows about these things.