Originally Posted by
wschruba
Remove the wire block (pull firmly up with thumb/forefinger on the black, textured block below the grey piece). Spin the wheel, and hold a key to the exposed metal tabs--if it sparks, there is nothing wrong with the hub, and you can safely re-wire the hub (instructions can be found under "dealer manuals" at
si.shimano.com after you put the model of the hub (DH-XXXX) in the search bar.
This is good information. However... you don't get much of a spark at 6 volts/3 watts unless the key position is just right. It would be easier and more predictable to get a 6-volt flashlight bulb and wire it to the hub.