Diagnosing a fault with a dynamo powered rear light

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GregAC
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Diagnosing a fault with a dynamo powered rear light

Postby GregAC » 14 Oct 2019, 11:09pm

I have Cube Touring 2017 (https://www.cube.eu/uk/2017/trekking/to ... te-2017-z/) parts that are relevant to this post are all the stock parts that came with the bike.

It has a fixed rear light powered by a dynamo that has failed. The front light, powered by the same dynamo, looks to be working fine (nice and bright, doesn't appear to be getting less power than usual).

The rear light has two wires coming out of it. These are fixed to two studs on the rear mud guard. At the other end of the mud guard close the bottom bracket are another two studs. Here two wires are connected which run through the frame to spade connectors that fix on to the front light. I presume the mudguard has some kind of internal conductor that connects up these two sets of studs, there's no obvious visible wire connecting them otherwise.

After some probing with my multimeter it appears the connection through the mudguard is broken on one side. One set of studs show continuity but the other set don't. There seems to be good continuity between both the studs next to the bottom bracket and their matching spade connectors on the wire that connects to the front light.

I was slightly puzzled though because at first both of the studs next to the bottom bracket seemed to have continuity between them, and both had continuity to one of the studs next the light. That stopped happening at some point during my testing. I was wiping the accumulated road muck from the inside of the mudguard away (where the other sides of the studs are) so perhaps that was causing a connection between the two that I removed?

Are connections inside mudguards like this known to fail? Given the mudguard itself is in good condition and hasn't had any major knocks or bends I'm aware of I'm slightly surprised the conductor seems to have broken given it's entirely encapsulated in rigid plastic. I was wondering if there could be something I'm missing here or if this is simply a case of either sourcing a replacement mudguard or running a wire to replace the internal conductor?

KM2
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Joined: 23 Oct 2008, 5:38pm

Re: Diagnosing a fault with a dynamo powered rear light

Postby KM2 » 15 Oct 2019, 12:05pm

Personally, I would hard wire the light in. Either fully from back to front or if it goes through the frame, shrink wrap the connection at that point.
You can always run the wire under pvc tape along the mudguard or just zip tie it.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Diagnosing a fault with a dynamo powered rear light

Postby Brucey » 15 Oct 2019, 12:36pm

those mudguard connectors look clever but in UK conditions they are 'terminally' (groan) unreliable. I don't know any bike mechanic that uses them when they are able to; it is just a recipe for having the bike come back in a few weeks with lights that are not working properly. Nothing you can do to this connection scheme will ever render it properly reliable, so it is always better to fit wires instead.

Note that if the headlamp has an earth return connection and the steered parts of the bike are electrically continuous with the rest of the frameset (they are not always) then you could perhaps get away with an earth return to the rear light, i.e. a single wire might be enough. But twin cables are more reliable for sure; earth return connections tend to corrode and fail over time.

cheers
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andrew_s
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Location: Gloucestershire

Re: Diagnosing a fault with a dynamo powered rear light

Postby andrew_s » 15 Oct 2019, 4:00pm

Brucey wrote:those mudguard connectors look clever but in UK conditions they are 'terminally' (groan) unreliable.

The conducting path between the front and rear studs is via strips of aluminium foil within the thickness of the mudguard, normally set up as a left strip, a right strip, and a central strip that's separated from the left and right strips by a 1 to 1.5 mm gap. The stay mounts are usually riveted through the centre, so that should be used as the ground side. In silver mudguards the strips provide the silver colour as well as being usable as conductors.

I don't see a short between central and side strips as being very likely, but corrosion between the studs and the thin foil is very likely, and might happen in a few days if road salt gets involved, or a few months if it's just clean rain.

The best rear light cable I've come across is that provided by SON for use with an Edelux (one spade terminal, earthed to the mounting point) and a rear light with two spade terminals. Use the black wires for earth, replace the ring terminal by a 2.8 mm spade if your front light doesn't earth to its mounting point.
I tend to run the cable down the front gear cable, across the top of the BB shell, along the non-drive side chainstay, and up a mudguard stay or rack leg to the light. Half a dozen small cable ties keep it in place well.

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mjr
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Re: Diagnosing a fault with a dynamo powered rear light

Postby mjr » 15 Oct 2019, 4:57pm

KM2 wrote:Personally, I would hard wire the light in. Either fully from back to front or if it goes through the frame, shrink wrap the connection at that point.
You can always run the wire under pvc tape along the mudguard or just zip tie it.

Just remember to leave enough slack at both the front for steering (I wind it around a screwdriver into a springy coil) and the back to help cope with bumps/flex (a bit of a loop down before the light is enough). My most frequent cause of light failures is the connector plug to the back light working loose and falling down from bumping over our rough roads. Second is probably me snagging a cable and pulling a plug out while working on the bike.
MJR, mostly pedalling 3-speed roadsters. KL+West Norfolk BUG incl social easy rides http://www.klwnbug.co.uk
All the above is CC-By-SA and no other implied copyright license to Cycle magazine.

GregAC
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Re: Diagnosing a fault with a dynamo powered rear light

Postby GregAC » 17 Oct 2019, 1:41pm

Thanks for the advice everyone.

Think I'll do something quick this weekend with some wire I've got to replace the path that has the failed conductor then look at doing something better with decent cable and appropriate connectors.

I don't see a short between central and side strips as being very likely, but corrosion between the studs and the thin foil is very likely, and might happen in a few days if road salt gets involved, or a few months if it's just clean rain.


Well I've had this system working for almost two years (I commute on the bike and use it to get around town so it's out pretty much every day in all weathers year round), sounds like I just got lucky! Useful to know this is a common fault.

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andrew_s
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Re: Diagnosing a fault with a dynamo powered rear light

Postby andrew_s » 17 Oct 2019, 2:58pm

GregAC wrote:Well I've had this system working for almost two years (I commute on the bike and use it to get around town so it's out pretty much every day in all weathers year round), sounds like I just got lucky!

I'd hazard a guess that the stud was originally well protected - paint, or a rubber grommet stopping water getting at the outside of the stud, and which has since perished.
I've no personal experience - I've always used twin conductor cable, so have only peered on the odd occasion.