So, I got a DLumotec Oval LED dynamo light (w/ standlight but no senso) from Peter White Cycles, and it is powered by the Shimano DH3N72 hub.

My problem is...the light won't turn off. When I switch the toggle to the off position the light goes a bit dimmer, but it never turns off all the way. I don't really mind when I'm on the road, but it seems to be a bit distracting to other cyclists when I hit the wooded paths here in the daylight. Any thoughts on the why and how to fix this. When I switched the wires I just got nothing... Puzzled.

I don't mean to insult your intelligence, but is there any chance you're just observing the standlight? If I recall correctly, the power switch will not kill the standlight on my IQ Fly. However, the capacitor does not charge when the light is switched off, so even during additional riding, the light will go out.

I suggest a phone call to Peter White. IIRC there is something about some B&M lights that has to do with grounding. If used with a Shimano dynamo hub which grounds itself through the hub axle then SOME B&M lights require that a wire be moved or removed for proper operation. This is not required for use with Schmidt SON dynamo hubs.

I looked on the B&M and Peter White web sites and could not find the info covering this so am possibly mistaken. Try running the light while it is not bolted to the bike, held in your hand with only the wiring attached to the hub. If it works correctly then you may have a grounding problem. If not then you may have a defective light.

I don't think this applies to the LED lights. I had an incandescent Lumotec Oval Plus for which this was an issue, but it is easy to remedy. Here's a post I made about this several years ago.

Peter White's site incorrectly says that those lights are only compatible with a Schmidt SON hub. But, that's not true: the user simply needs to back out a screw as described in the post to which I provided a link above. (The necessity of doing this is even described in the light's manual, but it is poorly translated from German.)

Hmmm. I tried removing the screw like Derailed suggested, no dice. I tried moving the wires to all possible combinations, no dice. The dang thing just stays on. Oh well, I guess there are worse things in life than a running headlight...

OP, are you turning the switch off after you've been riding a while? How long does the standlight circuit keep the LED lit?

Barrack, I sometimes start with the switch in the off position but the light is still on (though slightly dimmer). You might be thinking I'm seeing the standlight, but I don't think that's it. Also, when I spin the wheel on a stand it gives the same resistance on either setting. The light might just be faulty. I called Peter White and was told they'd called back, but I've heard nothing yet...

Just looked at mine (I have the same light plus the Senso option and the same hub) --

The headlight's ground wire -- with the white stripe -- is in the ground receptacle on the hub's plug. That's the hole towards the front since the plug is on the starboard side of the bike.

I still have the screw installed; I thought I might need to remove it, but it works anyway. I have both taillight wires attached to the tabs on the left side -- striped (ground) on back/bottom, and plain on front/top.

When the switch is off, the system is off. The Senso circuit works, but I end up leaving it all On anyway since I think the headlight helps me stand out from the visual clutter and shade in my neighborhood.