UPDATED - Rear SON dynamo light wiring issue

[kwkirby01]

I have an Edelux front light powered by a Shimano DH-2N70 dynohub fixed to the bike with a metal bracket.

I thought I had carefully then carefully followed the instructions for connecting my SON rear light, but when I connect the the rear light neither front nor rear work. Disconnect the rear and the front works.

Can anyone on the forum give me some guidance as to where I'm going wrong?

Thanks, Kevin

[PH]

Oh dear, I've wired up several rear lights, to a few front ones, including a couple of Edelux and as you'd expect they've been straightforward, though I haven't had a SON rear. 
I'm pretty sure polarity isn't relevant, all the same if it's simple enough, I'd try swapping the wires round to start with.
Knocking the front light out sounds like a short, is there anywhere the wires could be touching? if not it's a process of elimination, the need to establish if the problem is the front connection, the rear connection or the wire in between... a swap test with a different setup would be ideal, or a multi meter or just a different bit of wire between the lights. If you don't have access to any of those, a circuit tester is only a couple of quid and a handy thing to have.   

https://www.toolstation.com/circuit-tester/p53383

Good luck

[leftpoole]

Somewhere along it is earthing out. In my opinion. Based on 'expert' experience.
John

[kwkirby01]

Thanks for the suggestions and apologies for not responding sooner.
Turns out I'd wired the rear light earth to front live and visa versa!
Once I'd switched the wiring (a fiddly job once the cables have been cut to length...) everything worked just fine.
I've made a quite a few other changes to my Sherpa over the holidays and will post in the 'Member's Gallery' soon.
Thanks once again.
Kevin
 

[PH]

Thanks for the suggestions and apologies for not responding sooner.
Turns out I'd wired the rear light earth to front live and visa versa!

Glad it's sorted, swapping the wires was my first suggestion, though I'm still surprised polarity matters.
Yes, lets see some photos!

[steve216c]

I am not familiar with your rear light, but many rear lights wire themselves to the negative circuit the bike through one of the bolts. On German bikes where it was a legal requirement for all almost bikes to have a dynamo fitted (until a recent update where battery lights are now an allowed alternative) this is common practice so that only the live wire is required between dynamo and lamps- which saves the bike fitters costs and materials.

So although the lamp might burn brightly with cabling going attached to either pole but when not attached to the bike, and if your dynamo is also sending the negative through the frame and the lamp also has a 'pick up bolt' for negative- then that might be reason for lamp creating short circuit when fitted- and therefore not working.

Main thing is that you have it sorted. It can be most frustrating trying to find where an electrical fault might be, even more so when intermittent.