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!

[fv1609]

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.

 

Degsy my annoyance is compounded by the fact that I thought I was well equipped for breakdowns, especially electrical ones. I was carrying:

 

Multimeter

DC Clamp meter

Inductance/Capacitance/Resistance meter

Megger

HT meter measuring up to 30Kv

Soldering iron + solder

Large number of fuses

 

Yet I hadn't realised I hadn't spare bulbs of the right voltage! Quite why the 24v Hornet was made to have 12v turn lights I don't know.

 

My panic/lack of thinking level was severely effected by seeing the temperature rise & rise on the Wolf. I couldn't stop on the hill, the Wolf has never done that before. It was a very hot day but that rather scared me especially being so far from home. The bulb issue was an annoyance I could have done without.

 

I see what I have in the main bulb box, I have some 28v ones although I think those are 5 watts. Anyway one of those is going to go into my inspection lamp/tester.

[gritineye]

All useful stuff Clive, as Degsy says trailer electrics have a hard life, hate to think of the hours I've wasted on them. I'll let you know if I ever get all the lights on all of mine working all the time!

[antarmike]

I was rewiring the lights back onto Andi's Scammell rear mudguards (just replaced on Sunday) and to check which contact of the Stop/tail unit was stop and which was tail, I used a multimeter to measure the resistanceof each.

 

One lamp was fine with a sensible resistance for the stop filament and a slightly higher resistance for the Tail filament.

 

When I checked the other side lamp, the Stop lamp filament resistance matched almost precisely the one prevoius one, but the Tail filament was up at about 170 Ohms.

 

I took the bulb out of the holder , cleaned the contacts, and tried the bulb directly on the meter. Almost identical readings to when it was in the bulb holder, far too much rsistance.

 

I could see absolutely nothing wrong with the filament, holding it to the light, flicking etc. All I can guess is that by chance I coaught it just before it failed, and the very high resistance reading is because, although the filament is still in one piece, it must be at some point I cannot see, burnt down to almost zero diameter.

 

Why else such a high resistance on a visually Okay bulb?

 

No I didn't even bother to power it up, that is what bins were made for.

Edited August 17, 2009 by antarmike