Tandem Tom

My wife's bike has a SP dyno hub and a B&M light as does my bike. Recently her light stopped working. I swapped wheels and her hub powers my lights so that is OK.
Wondering how to proceed to determine the problem.
Thanks!

2_i 

Did you try powering her light with your dynohub? What B&M light is that? Some can be tested with batteries and some cannot. (Yes, in instructions they will always say that it is a dynamo only light.) Is there a rear light connected to the front light? Did you check the cables?

Tourist in MSN

The 2020 B&M catalog on page 52 shows a light tester and zooming in on the photo it is labeled as 6V DC. I have tested B&M lights with a Li Ion battery that is 3.7 to 4.1V DC. Worked well for testing but it was not very bright.

What light model is it. When it was last used, was it raining out? Some of the B&M headlamps are poorly sealed at the bottom or back sides, was her light located where it could collect tire spray when riding on wet pavement?

If your front light was out, does she also have a taillight connected to the headlamp, and if so, did the taillight work or not?

I would first check and make sure that the connector to the hub does not have any short between the two wires and that both wires appear to be wired correctly to the hub.

Photos would help.

Tourist in MSN

Should we assume the light is way past warranty? If I recall she had a bad SP hub several years ago, so should we assume her light is several years old?

Tandem Tom

A few weeks ago everything was working fine. It just stopped. Not taillight either. I did pull the light and checked continuity on the wires from the hub to the light. Basically 0.
This hub is newer than the one that was replaced 5 years ago. In fact that one the bearing went and I got a replacement installed last winter.
So do the lights just go bad?

downtube42

A good general troubleshooting rule is, if there are wires involved it's likely the wires. The end of the wires are highly suspect, but the middles are also known to have issues.

In my business the wiring guys always point to the software as first choice, but they are wrong more often than not. No software on lights; I'm calling it wires.

unterhausen

I'm not sure there's no software in lights, especially with supernova. I had one fail, and it had disassembled itself. I have no idea what all those components are in there, it didn't surprise me that it failed.

ThermionicScott 

I would hook up a voltmeter to the hub and confirm that it's putting out juice, then see if the light works when hooked up to a battery. (I have one of those 6V lantern batteries hanging around for odd jobs.)

znomit

Which B&M light is it? Have you inadvertently switched it off?

Remove the light itself and connect it to your hub. Still not working, bin it.

Tandem Tom

So got the headlight working but ONLY if the taillight is disconnected.
Advice?

downtube42

​​​​​​Disconnect tail light wires at the tail light. Test.

Tandem Tom

Working!
I disconnected and then reconnected everything and all systems are go!

Tourist in MSN

If there is a loose connection somewhere in one of the lights, it may quit working again. If you do not bring a battery powered taillight on tours, you may want to do so in the future.

downtube42

That's a little troubling. If it worked with taillight unplugged but not plugged, that suggests a short circuit somewhere. Unplugging and replugging resolved the short circuit. Hmm. I kind of hate problems that appear to fix themselves; they tend to unfix themselves at the most inopportune time. I would want to know why that happened, before trusting the lighting system at night.

unterhausen

I feel like the taillight wires were swapped and also there is a connection to ground through the frame. So the power was being shorted out. Also, disconnecting the wires resulted in it being fixed. Or else maybe it will just come back.
Probably if there are two wires to the taillight, it would be wise to pop the taillight open and break the internal connection between the mount and ground. There are directions on Peter J. White's website on how to do this for some B&M taillights.

Tourist in MSN

I was thinking that too, but then I remembered that it is an SP hub, their hubs are not grounded to the fork. So, if the taillight is grounded to the frame, would it matter? I think only if the headlamp is also grounded to the fork. Both Znomit and I asked which model headlamp it is, but the OP has not told us that. I think the vast majority of B&M headlamps are not grounded to the fork.

Tandem Tom

The taillight is grounded to the frame. That connection which is on the underside of the fender covered with foil tape looked a bit wonky. So I think that may have been the source of the problem. This is on a sport touri g frameset that I built. Our full touring bikes are 2 wire systems.

unterhausen

Just losing the taillight ground shouldn't cause both lights to stop working.

steelbikeguy

If I understand the situation correctly, the inadvertent swapping of the tail light wires grounds the power from the headlight. Not only does this not provide power to the tail light, it also drains all of the power from the headlight.

I've heard of this problem a number of times, and I don't understand why B&M designs their lights in a way that allows users to incorrectly connect the wires.

Steve in Peoria

unterhausen

I think they just assume everyone has a level of electrical knowledge that would keep grounding problems from stumping anyone for very long. It's a big assumption. I know some people disagree, but I wish they would stop using the frame as ground. It's not a good ground, and causes more problems than it solves. Sorry if some people are going to have to update their 50 year old dynos. It's true that spotting which terminal is ground can be a vision test on B&M lights sometimes.

Someone noted that SP hubs are not grounded to the frame. However, if both headlight and taillight are grounded to the frame, swapping the taillight power lines would cause power to be shorted to ground even though the hub isn't involved.

Tourist in MSN

Yes, my SP hubs are not grounded to the fork. I had not thought of it that way, but I think you are correct.

OP was asked which headlight he had but he would not say. Only one of my B&M headlamps was grounded to the light mounting. It was an old Lumotec that was intended to be used on bottle generators, there is no switch on it and the mounting is grounded. My other B&M headlamps are not grounded to the fork.

I have a couple used B&M taillights that I picked up at a swap meet that are not in use, not sure if they are grounded or not. I have Spanninga Pixeo taillights on a couple bikes, but those are mounted to plastic fenders. And a Secula that is attached with a zip tie, that is not grounded.