Fault finding challenge!

[fv1609]

Before you read this I have resolved the problem, so I am not stuck (any more!) but if you want a diagnostic challenge & want to learn what the real fault turned out to be, so that you never get caught out, then read on!

 

Recently I was just about to leave a show ground, having reconnected the trailer I tested the lights. It was annoying to see that one turn light on the trailer was intermittent, although they were working fine on the towing vehicle.

 

When I wiggled the bulb it started flashing & continued to do so until it was turned off. But it would not start flashing again until I wiggled it in the socket or wiggled the holder.

 

The bulb itself was obviously working. It looked to be intact, the glass still sealed to the base, the main metal collar clean, the soldered end contact clean & prominent.

 

 

Passers-by offered two strands of advice. Either it was an earth problem from the bulb holder or it was a poor contact between the metal collar of the bulb and the socket. I had just witnessed a field repair on another vehicle were a sliver of plastic was wedged in to press the bulb more effectively against the holder & this cured his problem to get him home.

 

I was uncertain which category my fault fell into. All the terminals on the holder were clean & the spring for the central contact firm & clean. The earth cable was quite new & looked to make a good contact to the chassis as it was underneath the earth terminal that was supplying the sidelight that was working.

 

I disconnected the turn light earth wire & pulled the bulb holder out of its mounting I was able to use a screwdriver to earth it & the turn light started working. So it was clearly an earth problem, so I cut the turn light earth lead from its terminal. I stripped the insulation of the known-to-be-working earth lead of the sidelight & wrapped the strands of the turn light earth lead into it.

 

A slight wiggle of the bulb & it started working again. Ready to move off, no turn light. So I cleaned the metalwork of the bulb & the holder with an emery cloth. I blew out the dust & bingo it worked.

 

To reassure myself I ran it for a minute & it even continued to work after turning it off & on again. So I set off home.

 

I only went a little way, turned onto the main road & noticed that the turn light repeater light on the dash showed the trailer had failed again. Eventually I found a lay-by & all the nonsense started again.

 

I got my meter out & measured the correct 24 volts on the contacts. I went through all the procedures that I did at the show ground. I put another bulb in that worked but quickly blew as I didn’t realise it was a 12 volt one. I hadn’t got another 24 volt bulb but at least I knew the one I had was working.

 

Anyway I did eventually solve the problem, very annoyed that I wasted so much time but at least I will be prepared if it happens again & will be ready to offer show ground advice whether it is initially wanted or not!

[Degsy]

What was the fault Clive, was it a dodgy bulb?

[gritineye]

Did you forget give the bulb the usual flick with your finger to see if the filament was loose?

[N.O.S.]

Strange how soldering the trailer cable wires into the trailer plug pin terminals seems to cure all manner of wierd problems

[Swill1952xs]

Bit of a long shot, and a guess. As you were able to get the light working until you moved the truck; I suspect the lights were earthing through the drawbar of the trailer, rather than through the trailer plug, which would explain why everything worked until you started moving again. A rusty hitch would give an intermittent earth.

Just a guess though, as you didn't say whether the remaining lights were affected in any way.

[fv1609]

Well the filament looks pretty intact as you can see, I did flick the bulb several times & even felt like stamping on it, the nearest I got to that was accidentally dropping it on the roadside. It survived this harsh treatment & continued to work in the holder when it chose to. All the other lights on the trailer were working.

[Runflat]

turn light on the trailer was intermittent

 

Reminds me of the old joke about the [insert put upon race/creed of choosing] vehicle inspector who when asked whether the indicators were working replied: "yes... no... yes... no... "

[radiomike7]

Reminds me of the old joke about the [insert put upon race/creed of choosing] vehicle inspector who when asked whether the indicators were working replied: "yes... no... yes... no... "

 

Common problem - indicators on the blink:)

[gritineye]

Well the filament looks pretty intact as you can see,

 

The bayonet pin looks a bit dodgy though.

[fv1609]

The bayonet pin looks a bit dodgy though.

 

That was all quite rigid & clean.

[Dougiebarder]

It wasn't meant to be a twin filament bulb was it?:idea:

[fv1609]

It wasn't meant to be a twin filament bulb was it?:idea:

 

That's a thought, but nope just single.

[Adam Elsdon]

Dry solder joint in the bulb, causing intermittent fault?

[gritineye]

It was a 'on off' bulb it should have been an 'off on' type.........:coffee:

[Degsy]

Broken wire making intermittent contact?

[fv1609]

Right time to spill the beans!

 

I have the bulb in front of me now & it looks alright! But most of you are right to be suspicious of the bulb.

 

I need to set the scene. I am in the Wolf having dragged the 1-Ton Cipher trailer up a long & steep hill, now the temp gauge has moved over to red on a very hot day. I am more than an hour behind schedule with over 100 miles yet to do & this damn flasher has packed up again. I pull in & wiggle the bulb in the socket & it starts again. I am in no mood for cr@pping around.

 

I hunt for the bulb circuit tester, each of the vehicles has one. But no it's nowhere to be found, so that means one of the vehicles has two! I find a military Lucas bulb holder with long leads that doubles as an inspection lamp & circuit tester. No good its bulb has burnt out.

 

Somewhere I have a box of spare bulbs. I should have a mixture of 12v & 24v. For although both tow vehicles were 24v the Hornet had 12v turn lights. Having totally rummaged through everything in the trailer I find the box of bulbs. But of course I find I have large numbers of 12v bulbs but no 24v ones!

 

As Degsy & GE would expect me to, I really wanted to just try another bulb to rule out a problem with the present bulb although it appears intact & ‘does work’. Substituting with the 12v bulbs was short lived a few flashes & they had popped. One could identify the 24v bulb from the 12v ones by the more sturdy filament. It the struck me to put on my strongest glasses on hold the bulb up to the Sun & rotate to see if perhaps the wire had burnt out & there it was, but only just discernable.

 

 

The filament was broken, but it was sturdy enough not to have collapsed. The vibration of fiddling with it in its socket each time caused the broken ends to touch. The heat caused the ends to expand & touch, so the rate of flash prevented it from cooling & breaking the circuit.

 

I found a spare 24v dual filament stop/tail bulb. I was able to insert it in such a way that the ‘stop’ filament was connected & I got home much relieved.

 

Now of course it is easy realise I should have changed the bulb to something else straightaway. But the filament did look intact & it ‘worked’. In the heat of a breakdown, behind schedule, a hundred miles from home it is easy not to think straight.

 

Not only am I annoyed with myself for wasting time & being stupid, but I have had another thought. The earth wire on this light socket was quite new, then I remembered a couple of years ago I was about to move off to the GDSF & this turn light had packed up. It was cured by a new earth lead. I always check the lights on a trailer at the start of any journey. I just wonder if it was this same bulb or did this bulb end up in the spares box because the filament looked ok?

 

PS Just re-checked before posting this & Degsy was there with the answer.

[Ed Batchelor]

Swapping the LH & RH bulbs would have eliminated whether it was the bulb or not, however one normally expects a bulb to fail fully as you describe with a collapsed fillament an "intermittent" failure is less common.

If towing a trailer and you have no spare bulb I guess you could take a bulb from the rear of the towing vehicle and swap it to the trailer as this is the point nearest the back.

Interesting story and something to be learned!

Regards,

Ed

[Chrisg]

When your on the side of the road broken down logic goes out of the window

[Tony B]

Yeah like spending an hour striping bulb holders out before someone says 'Have you checked the fuse'. :red:

[fv1609]

Swapping the LH & RH bulbs would have eliminated whether it was the bulb or not, however one normally expects a bulb to fail fully as you describe with a collapsed fillament an "intermittent" failure is less common.

If towing a trailer and you have no spare bulb I guess you could take a bulb from the rear of the towing vehicle and swap it to the trailer as this is the point nearest the back.

Interesting story and something to be learned!

Regards,

Ed

 

I thought of swapping with the other side.

 

But I didn't want to do anything at all to interfere with the off side lights as they were working. For all I know I might have broken the bulb if it was stuck in or dropped it etc Although if I had done that diagnostic test, I would still have been a bulb down. I think the off side flasher is the most important of the two, I just didn't want to end up having no rear flashers.

 

Yes swapping with the tow vehicle would have been a good idea, but this was a Wolf which has horrible plastic light fittings & different bulbs.

[Degsy]

Clive, when you first posted and being a dozy old sod I hadn't realised you were challenging us hence the tone of my first post,I just thought you'd missed the fault off accidentally:embarrassed:

I've seen similar faults many times and have long ago given up messing about, changing the bulb cures the problem almost every time even though the bulb appears ok.

[Chrisg]

Like spending 1/2 hour checking wiring to find the bulbs fallen out due to a c**p repro buld holder :argh::argh:

Edited August 16, 2009 by Chrisg
typo error

[fv1609]

Clive, when you first posted and being a dozy old sod I hadn't realised you were challenging us hence the tone of my first post,I just thought you'd missed the fault off accidentally:embarrassed:

I've seen similar faults many times and have long ago given up messing about, changing the bulb cures the problem almost every time even though the bulb appears ok.

 

 

After my exploits there I'm not sure who was the "dozy old sod".

 

I posted it as form of public humiliation for my stupidity, but also as an exercise in diagnostics, I was interested what others would think. But I didn't want people to feel annoyed that they where wasting their time if I only revealed at the end that I had sorted it. Sort quiz + diagostic exercise. You were there at the outset. But I can still feel how I was so convinced that the bulb "worked" & the filament "looked alright".

 

Would you think that the filament is more likely to hold up even after brakage in a 24v bulb than in a 12v bulb?

 

I went to return the bulb prodder/tester to the Wolf where it should live. But on checking I see that has a 12v bulb in it, so that would have popped even if I did have it with me! So a few lessons learned!

[Richard Farrant]

Clive,

 

I have come across bulbs that fail but with filament seemingly intact, trick here is to hold up to light and flick your finger against the glass, you may just see one end of the filament tremble.

 

Now many years ago, I had strange one, I bought a brand new 1971 BSA motorcycle, I was asked by people on several occasions why I was flashing my headlights.........which I was not. But....I worked out that whenever this happened, lights were off and I was under braking. The problem was a short within the stop/tail light bulb which only happened when moving and not when stationary. A small piece of solder was loose within the bayonet cap of the bulb shorting brake circuit through to light circuit.

[Degsy]

It's always easy to be wise after the event and in the circumstances you found yourself in I totally agree that all logic goes out of the window and you tend to go into a flat spin, been there etc:sweat::-D Another thing I have found is that when it's somebody else's vehicle that is broken I will quite confidentally get stuck in to repair it but when it comes to my own vehicle I tend to get paranoid:shake:

Interesting point regarding the 24v bulb, I think perhaps because of the thicker filament the fault wouldn't show as easily as with the 12v bulb.

When we operated tippers, we used to use 28v bulbs in the indicators and stop/tail lights as 24v bulbs used to break filaments all the time, presumably due to the hammering they got when running empty. As trailers tend to rattle and bounce around it might be worth the extra cost involved in using the 28v bulbs.

[fv1609]

I have come across bulbs that fail but with filament seemingly intact, trick here is to hold up to light and flick your finger against the glass, you may just see one end of the filament tremble.

 

Richard, I've just tried that but can't really detect a vibration as the bulb moves a bit as I tap it. But of course you are looking with the eyes of a younger man.

 

But if I hold the bulb to my ear & tap it I can hear a high pitched clanking, like those annoying oriental wind chimes. So that'll be another diagnostic test up my sleeve.