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CB125s rear light not working when lights are on

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L Plate Warrior



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PostPosted: 21:40 - 15 May 2019    Post subject: CB125s rear light not working when lights are on Reply with quote

Hi everyone,

I'm going mad with this issue so hopefully someone has some wisdom for me!

I've finished restoring a Honday CB125s 1973. I'm having an issue with the rear light.

When the light switch is off, the brake light works when I use both the rear and front brake. So the brake light switches are both functioning. However, when I turn the lights on, the front light illuminates but the rear does not. I can't get anything from the rear and so it also doesn't work as a brake light whereas it works fine when the light switch is off.

I've done the following:

- taken the wiring loom out, checked connections and connectivity to the all wires in the rear light - all fine
- checked the light switch - all seems to be working
- checked my earth to the rear light - it's connected between the light casing and the not sure if that's correct. It's had a new paint job so I've filed back to the metal on all surfaces to have a contact.

I've also go an intermittent problem with the neutral light - sometimes this works, sometimes it only works if I touch the bulb casing to the body.

I just don't get why putting the front light on stops the rear light from working.

Any ideas??? I can only ride the bike during the day because I can't use the front light and have no brakes!
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Nobby the Bastard
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PostPosted: 22:23 - 15 May 2019    Post subject: Reply with quote

The earth is fucked. In both cases. Two different earths.

And wtf is an 1973 Honday?
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L Plate Warrior



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PostPosted: 22:32 - 15 May 2019    Post subject: Reply with quote

Nobby the Bastard wrote:
The earth is fucked. In both cases. Two different earths.

And wtf is an 1973 Honday?


A Honday is a Honda that types too fast.

So. How do I fix this? I don't really understand how they are fucked - there's connectivity throughout the wiring loom and a good contact with the frame in all cases. I'm learning as I go along so sadly I don't have all the knowhow.
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Nobby the Bastard
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PostPosted: 22:48 - 15 May 2019    Post subject: Reply with quote

if you can touch the bulb to the case and it lights up, the bulb is now earthed by the case.

With multiple filament bulbs (like the rear) the earth can be found by the electrons by going through both filiments rather than the bulbs earth.

In both cases, the earth is at fault.

A wankey is someone who is a bit stupid, btw.
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L Plate Warrior



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PostPosted: 23:14 - 15 May 2019    Post subject: Reply with quote

Nobby the Bastard wrote:
if you can touch the bulb to the case and it lights up, the bulb is now earthed by the case.

With multiple filament bulbs (like the rear) the earth can be found by the electrons by going through both filiments rather than the bulbs earth.

In both cases, the earth is at fault.

A wankey is someone who is a bit stupid, btw.


Where is the earth for the neutral light? Is it the one on the inside of the headlight casing?

I was assuming that the rear light is earthed correctly as it works when the brake switches. Am I wrong? Where would that bad earth be?

Could it be that one of the filaments is broken?
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Easy-X
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PostPosted: 02:50 - 16 May 2019    Post subject: Reply with quote

With some neutral switches the gearbox is the earth and as the gear selector rotates a little gold pin contacts a conductive pad so the wire coming off the neutral switch is what they call a "switched earth."

So what you might have is a live feed into the neutral bulb and the wiring going to the neutral switch is its earth. YMMV ofc Smile
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Teflon-Mike
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PostPosted: 09:05 - 16 May 2019    Post subject: Reply with quote

On the tail-lamp bulb, you have two lead pads, one for the volts in, on the tail light filament, one the volts in on the stop lamp filement.
Both filements share a common 'earth' or return through the metal of the bulb base.

Oh kay... the two circuits are joined at the common earth in the bulb.

When the head-lamp is 'off', the volts to the stop-lamp find there way to the stop lamp, light it, but then find they cant get out the return... but there is this back-track, through the tail lamp filement, and another earth for the headlamp.... so the volts go there instead.

When the headlamp is 'on', they try do the same, but theres volts already there, coming out of the tail lamp, so they cant.

Go look at the earth points on the tail lamp assembly, and everything you have painted.

To test, get a long wire on the battery -ve to make a flying earth, and touch that to the back of the bulb holder and other bits of the bike used as earth path to see where the earth path you should have is high resistance or not there.

Neutral lamp... as said, its switched on the earth side of the circuit; if it dont work, then the switch isn't working... could be worn out or crudded up or just missing its tracks, or any comination. Touching the bulb or holder to good earth at the clocks, is then switching the lamp before the switch in the gearbox, lamp doing what it shoule when you give it a good earth, lighting up... so... switch is kerfukkered... go look.

Some times a simple oil change and flush can get rid of crud in the switch contacts, but likely that the contacts are corroded up and need cleaning, which is essentially a motor strip job... but could also be that the switch is fine, but theres no earth opath out of the engine when its made it... cos you painted the frame before fitting the engine perhaps? go look at ALL the earth points, including where engine is supposed to earth through the mount bolts.
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L Plate Warrior



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PostPosted: 09:06 - 16 May 2019    Post subject: Reply with quote

OK. So unlikely to be that?

Still no closer. I've checked connectivity with all of the wires, the bulb seems OK, the earths are all connected to bare frame - I resprayed it so I thought that might be the issue.

Any other ideas as to what it could be? It's the rear light that's the main issue.
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L Plate Warrior



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PostPosted: 09:12 - 16 May 2019    Post subject: Reply with quote

Teflon-Mike wrote:
On the tail-lamp bulb, you have two lead pads, one for the volts in, on the tail light filament, one the volts in on the stop lamp filement.
Both filements share a common 'earth' or return through the metal of the bulb base.

Oh kay... the two circuits are joined at the common earth in the bulb.

When the head-lamp is 'off', the volts to the stop-lamp find there way to the stop lamp, light it, but then find they cant get out the return... but there is this back-track, through the tail lamp filement, and another earth for the headlamp.... so the volts go there instead.

When the headlamp is 'on', they try do the same, but theres volts already there, coming out of the tail lamp, so they cant.

Go look at the earth points on the tail lamp assembly, and everything you have painted.

To test, get a long wire on the battery -ve to make a flying earth, and touch that to the back of the bulb holder and other bits of the bike used as earth path to see where the earth path you should have is high resistance or not there.

Neutral lamp... as said, its switched on the earth side of the circuit; if it dont work, then the switch isn't working... could be worn out or crudded up or just missing its tracks, or any comination. Touching the bulb or holder to good earth at the clocks, is then switching the lamp before the switch in the gearbox, lamp doing what it shoule when you give it a good earth, lighting up... so... switch is kerfukkered... go look.

Some times a simple oil change and flush can get rid of crud in the switch contacts, but likely that the contacts are corroded up and need cleaning, which is essentially a motor strip job... but could also be that the switch is fine, but theres no earth opath out of the engine when its made it... cos you painted the frame before fitting the engine perhaps? go look at ALL the earth points, including where engine is supposed to earth through the mount bolts.


Thanks. I've just finished rebuilding it so the engine has been vapour blasted and the frame blasted and resprayed. As post above I've stripped the paint at all earths but I'll double check. I'm not sure about th engine mounting blocks so I'll try that.

I've literally only ridden it three times yesterday since getting it on the road - the neutral light worked for 2 of those and the middle ride it didn't and only came on when in contact with the casing. So maybe it will work itself out. I was thinking loose connection but it's not that.

I'll try all of this stuff again now. So annoying!
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Teflon-Mike
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PostPosted: 09:44 - 16 May 2019    Post subject: Reply with quote

The CB125S is a 6v bike; it HAS to have decent earths for anything to work, cos with half the volts it don't have the 'weight' to force its way through a bad one.

I would be tempted, on resto, to double-string, and actually provide a return by wire rather than 'frame earth' to be sure volts could get home...

But you have issue there in resto, to start, look at the state of wires; I suspect many are black and corroded inside the insulation, and gone high resistance before you start.

Cruddy old bullet connectors with a few decades of copper oxide corrosion will add some more, and you can spend three times as long as the complete resto, chasing niggling electrical faults as use and vibration shakes down the faults, and corrosion gets rubbed off bullets, that then go intermittent; wires start to break, going thin in their joints or actually breaking and making connection only when they touch, giving more intermittent, as well as high res....

In resto... have you re-wired? Have you even just cleaned all obvious bullet connectors?

I can spend as long and longer on the wires of a resto as on the rest of the build all together.... including a total engine tear..... its the oft overlooked bit of the job, in the rush to hear pikie-pikie noises!

It's murpheys cut time; you put in the hours up-front to save chasing out anomalies, or you do it at the road-side... either way, he get his cut... up to you how you prefer to pay... but there aint NO short-cuts when it comes to the electrics, just patience and diligence.
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Easy-X
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PostPosted: 09:44 - 16 May 2019    Post subject: Reply with quote

maralions wrote:
...I'll try all of this stuff again now. So annoying much fun!


FTFY, positive mental attitude Thumbs Up
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L Plate Warrior



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PostPosted: 09:48 - 16 May 2019    Post subject: Reply with quote

HardlyDavidson wrote:
maralions wrote:
...I'll try all of this stuff again now. So annoying much fun!


FTFY, positive mental attitude Thumbs Up


I know... the rest has been great but this lighting issue is really doing my head in. Three days fannying about with one problem isn't fun!
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L Plate Warrior



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PostPosted: 09:57 - 16 May 2019    Post subject: Reply with quote

Teflon-Mike wrote:

I would be tempted, on resto, to double-string, and actually provide a return by wire rather than 'frame earth' to be sure volts could get home...

But you have issue there in resto, to start, look at the state of wires; I suspect many are black and corroded inside the insulation, and gone high resistance before you start.

Cruddy old bullet connectors with a few decades of copper oxide corrosion will add some more, and you can spend three times as long as the complete resto, chasing niggling electrical faults as use and vibration shakes down the faults, and corrosion gets rubbed off bullets, that then go intermittent; wires start to break, going thin in their joints or actually breaking and making connection only when they touch, giving more intermittent, as well as high res....

In resto... have you re-wired? Have you even just cleaned all obvious bullet connectors?


I'm a beginner with electrics so have a few Qs!

How would you double string? What do you mean? Send a wire from the rear light to the battery negative?

I can replace some of them - might do these back ones. Could you possibly advise me on how to clean them properly? I've been sandpapering the female part and the male part and spraying in some contact cleaner.

about the resistance - how do I check for this and what exactly should it be? I'm new to electrics remember! I've set the multimeter to the connectivity setting and I'm getting readings between 15 and 65. Should I use a different way of testing?

Thanks!
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Teflon-Mike
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PostPosted: 11:18 - 16 May 2019    Post subject: Reply with quote

It's 6v and as old as I am..... And i'm not exactly in great shape, so I wouldn't expect the wires on the bike to be either!

I would give murphey his cut, starting afresh; I would go to Vehicle-Wiring-Products (VWP) and buy a batch load of make and female SOLDER bullet connectors, and insurators; I'd trawl e-bbay for some coloured wire, either on the reel or cut length, probably a couple of meters of each would do... and four times as much 'green'...

And with a nice decent soldering iron, probably also from VWP if I didn't have a rather useful butane one already.... make up a complete new loom, with brand new connectors that didn't need cleaning, and wouldn't snap off if I tries, and had to solder chitty oxidised wire into old corroded plugs... with new should solder up real nice and real easy, and I'd KNOW that the wires was good and the oxidisation of copper inside insulation wasn't making them high res, and that new copper hadn't work hardened over the years etc etc etc...

'Double-String'.. in my new 'loom'... I wwould add an extra wire, that connected ALL the earth bonding points together.... earth usually green hence extra of that colour.... with that connected to the batt neg, then you know that every device that should have an earth has one, and a good one, whatever is going on at the bond point to the frame...

Manufacturers dont do it as standard cos its extra weight and extra cost... but no great loss on a resto, especially if you dont want to be chasing cruddy earth issues on a cantankerouse 6v system, for ever and a day.

There are things like indy earths, where the earth path is through the indy casing, but, the indicator is oft mounted on a rubber grommet cos of vibration, so there should be an earth strap between the indicator stem and the mudguard... oft gone AWOL over time, and even if there, the earth path goes through a now probably corroded mudguard to the mudguard mounting bolts... themselves all now rather rusty and or painted... and the path rasther harder for electrioc to get through.... so... you do first fire.. indies dont work... wiggle some wires, and they start working, then in use vibration ratgles the wires a bit more, and they stop... so you crack out the multi-meter... do your own head in, give up, and start using a flying earth wire on the battery.... you then discover that indy had bad earth, and the book shows this strap between the indy stalk and the mudguard you dont have.... "Ah-Ha!" you exclaim and grab a bit of wire from an old kettle, make a strap, and indy starts working and you THINK you have sorted it.... till the indy stops working again.... when you discover that the connection between the frame and mudguard is rather bad, cos of paint and or rust, as the bolt has shaken a bit loose.... and so you crack out that bit of wire from the old kettle again, make up another bridge, and whey-hey.. indy works.... at least at the back... two days later the front one starts playing up, on the other side, and you are off again! MURPHEY... getting his cut!

On resto, there's normally a common earth point in the headlamp where most wires are gathered; absolutely notoriouse; it goes bad and earth volts instead of trying to find thier way back to battery, cant get down the wire, so the go via the frame... but... headlamp is on the forks, connected to the frame ONLY by the, hopefully nicely greased and hence isolated ball bearings in the head-stock... and the earth path changes with the steering, as the balls turn and they sit in dimples where they rusted for twenty years... and murphey is laughing his cotton off as you chase the intermittent on EVERY DANG THING!

SO.. double string... wire in the loom, offers secure earth for everything, connecting the bond points, including the one in the headlamp shell, and you have an electric by-pass for the electric, that dodges the head-race bearings... no intermittents and no mighty head-scratching.... takes time and effort... but like I said, where and when do you want to give murphey his cut... he gonna get it one way of the other, you will NOT avoid it... he's as remorseless as the tax man!!!

But, that's how to tackle it.... eliminate as many daftr at source with the new loom, and double-string; THEN if you have a niggle it got to be in the switches or the device... so you know where to look... and a little pre-emptive cleaning switches and replacing probably pretty dodgy bulb holders etc, and you are laughing....

Its all patience and diligence, not brain surgery, and even a multi-meter may be a bit high tech for the job! An old bulb and a bit of wire, and you can do most diagnostics. If you want to see if volts... you connect the bulb, via the wire between the two points you want to test; wantr to check conminuity, use just the wire... it REALLY is that simple if you go back to basics... BUT, its all time, patience and diligence to the detail, and this IS where a project starts to bite, seemibgly not making progress because you dont get anyting shiney to look at for your efforts, or pikie-pikie noises or anything else to overtly 'show' for your pains... its all slow go and no show stuff... BUT, it has to be done if you want the MOT cert, and dont want to be scratching head at the side of the road.
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Easy-X
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PostPosted: 13:09 - 16 May 2019    Post subject: Reply with quote

Thinking on all the wiring I've done lately on my cheapo Chink bike there's barely any use of frame earthing...

Thinking even more I can't say I can put my finger on anything on the bike that just has a live feed and earthed to frame... oh yes, just the one: the neutral switch on the gear box Smile That must say something if even the cheap bikes have separate, independent earth cables for most of the bits and bobs!

If you're going down the Tef route (short term pain for long term gain) you might want to start with a circuit diagram and try to break it up into functional units. Start with the essentials: i.e. just getting the engine to run: coils & sparks etc. and then move on to the next system.

Probably something like:

Ignition system, charging, brake light, indicators, head/tail lamps.
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L Plate Warrior



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PostPosted: 14:30 - 16 May 2019    Post subject: Reply with quote

SORTED!! Yes it was a bad earth... the mudguard wasn't going to the frame. I ran a wire between the two and now everything is fine. Thanks for the help!
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Teflon-Mike
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PostPosted: 14:57 - 16 May 2019    Post subject: Reply with quote

On a modern bike, there's often so little metal between a device and the chassis, its cheaper to run a wire-eart than make up gazumpety little earth straps; plus, especially with Chinky out sorurcing, and semi standardisation, component manufacturers are better off making everything two-wire...

But this forty year old 6v anathma....

As for the 'chunks' there probably aren't that many, and what there is should be pretty low tech, and he wouldn't be building from scratch, just copying what he got... and adding an extra earth wire.

On the ignition side; that shoule be pretty stand alone anyway; it'll have a spark coil in the generator, that will be almost hard wired with the points and condensor, and just one wire from that to the coil with the plug hanging on the HT... the added bit for the ignition switch, will actually be an 'earth' that takes a feed from the coil to earth and makes that circuit to short out the sparks when the ignition 'off'.

On the charge side; There's probably two coils in the genny; the head-lamp and tail lamp, will most likely run AC direct, there wont be any sort of reculator or rectifier circuit in there, and again, likely fairly stand-alone.

To charge the battery... there's likely just one feed taken from one charging winding in the genny; that wont on an old 6v bike have much at all by way of regulation... there'll likely be 'just' a zenner diode on that feed, an electric one-way valve, with a leaky seal, essentially... it'll only let the +ve electric pulses through, so semi rectify the current, and if it sees a 'surge' of more than just over 6v, it'll leak, so the battery wont get fried!

Clever stuff, hugh?

There may be a bit of cross-over wiring in the lamp switch, so that when the lamps are turned 'off' the electric made to power them gets sent to the zenner to help charge the battery, but that's about it.

Ways about, really, is to get the original wiring loom off, snip the connectors off the end.. but note what they are, then pull that wire out the loom, and thread in bit of new in its place; terminate with a connector like the one snipped; then do the same with another wire.... and every where you tug a wire out..... push in the extra wire-earth...

Niggle likely to be the state of the loom sleeve, which is probably rather chewed and hard at this stage, and whether you bother tryung to thread new wires into it, or copy with a new sleeve or spiry-wrap, or insulation tape....

Quite often easiest, if remaking, and adding wire for twin-string earth... and almost a given if you are adding accessories ga-frigging-lore, as demanded by a woman like our snowie! (See Pup project post!) to simply lay out the old loom, lay new wires in the same shape, clip them together, but not too tight, with cable ties, add the terminals to suit on the ends, T-together new twin-string earths where you need split them; then flake it out on the bike, make sure the connectors are in the right place and the right ones, and that wiring sits snugly where it should, and any bends bend and dont kink.. then you nip up the cable ties... and keeping the shape of the loom... wrap with insulation.

TRICK is there is one to twin-stringing, is to know where components are frame earthed... like the horn, or the coil, and actually putting in earth straps to the second-string, to go to mounting bolts where the horn or coil and stuff bolt to the frame, and you dont automatically 'see' an earth point to wire your second string to.
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Nobby the Bastard
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PostPosted: 16:34 - 16 May 2019    Post subject: Reply with quote

maralions wrote:
SORTED!! Yes it was a bad earth... the mudguard wasn't going to the frame. I ran a wire between the two and now everything is fine. Thanks for the help!


*bows whilst taking applause*

Shut the fuck up teffers
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