Voltmeter broken?

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Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

………….pity the guys at my garage were also absent from Car Mechanics School when electricity (in relation to LEDs at least), was discussed.

I shall take great delight next week in going down to tell 'em what I have learned on here about electricity. After all, every day's a school day, eh?
oliver90owner
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Re: Voltmeter broken?

Post by oliver90owner »

Don’t try to educate the mechanics too much. Actually filaments in incandescent bulbs normally get nowhere near melting point - probably only around 1500 to 2000+ Celsius. Melting point is much closer to 3500 degrees. The inert gas will not react with the filament like oxygen would. Fuses melt (note the term - fuse means heating until close to or at melting point).

Bulbs most often fail at turn-on. They current at start-up is far higher than during operation and may cause overheating of a thinned part of the filament (the filament will very slowly ‘evaporate’ in use - more at higher temperatures), or the very rapid expansion, on heating, fractures the filament. Another mode of failure is at the glass /metal seal - the gas in the envelope obviously gets very hot and expands and on cooling contracts so if any gas is lost when hot, air will be drawn in on cooling, which is bad news next time the filament glows red hot and is basically oxidised.

Halogen bulbs generate more light (light output increases with filament temperature) as the filament will run much hotter and the halogens reduce the sublimation damage by not allowing loss of evaporated metal by deposition on the glass envelope.

Incandescent bulbs convert approx 5% of the supplied energy to light; LEDs are greatly more efficient!
Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

Wow! I never knew bulbs could be so interesting. I thought you just switched them on and they worked...…..ONLY JOKING!! I didn't know that start-up current was higher which presumably explains why bulbs often blow as soon as they are switched on.

I've just come back from the garage who fitted the alternator etc and let them know, in no uncertain terms, that I'm not a happy bunny. I know that LEDs were not exactly original equipment on a Moggy but I shouldn't have to find out from an Owners Club forum about reversing the terminals and then have to impart that info to a mechanic who thought they had just 'blown'.

I was met with a somewhat glazed expression when I mentioned that the temperature gauge now has to be fitted with a neg earth voltage stabiliser…...which I've ordered and will fit myself, incidentally.

What has really naffed me off, of course, is that I'm convinced my voltmeter was 'fried' by some incorrect wiring up of the alternator in the first place. I've no way of proving anything but one of other mechanics, not with whom I was remonstrating, did look decidedly sheepish when he looked and realised that there was a 'bit of an issue' kicking off in the office!!

I had a further on-line chat with the original voltmeter supplier and agreed that, cost-wise, there would be diddly squat difference in the cost of a repair compared to just buying a new voltmeter, which is what I have done.

If anyone on here thinks they can do something with (i.e. repair) an otherwise pristine voltmeter I've now got spare, you can have it.
StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

I would be interested in looking at it and posting what I find on here.
It would have to be posted to me though, I will pay for the postage costs.
Regards John
StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Just thought, Have you checked they have reversed the Ignition coil connections. The terminal marked with SW or – sign now goes to the wire from the distributor and CB or + is the supply from the ignition switch. It might be a good idea to change the points as well, they are known to fail shortly after a polarity change. I know don't ask, the points arc in the opposite direction!

Regards John
Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

Just checked - the coil connections are the correct way round......at least that's something they got right :D

My car is fitted with Accuspark electronic ignition so....no point(s) in pursuing this any further (See what I did there? :D :D :D )

I did have to buy a negative earth replacement gubbins for the distributor, which has been fitted.

John, I've PM'd you (I think!) about sending the voltmeter to you for forensic examination.
StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Thanks Owlsman, Got your PM thanks, I PM'ed you and somehow managed to PM myself regarding a late night Typo, new eyes and brain required :roll:
Regards John
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geoberni
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Re: Voltmeter broken?

Post by geoberni »

Owlsman wrote: Tue Oct 01, 2019 10:25 am Just checked - the coil connections are the correct way round......at least that's something they got right :D

My car is fitted with Accuspark electronic ignition so....no point(s) in pursuing this any further (See what I did there? :D :D :D )

I did have to buy a negative earth replacement gubbins for the distributor, which has been fitted.

John, I've PM'd you (I think!) about sending the voltmeter to you for forensic examination.
The Coil Connection thing is also important for the life of Spark Plugs and the strength of the spark. When I got Basil 2 years ago, he'd been previously fitted with an Accuspark system, although I don't know if that was at the same time as the conversion to -Earth.
But they had not changed the Coil connections and the spark was going in reverse.
I posted the info here in the forum earlier this year : viewtopic.php?f=12&t=70183&hilit=spark
Basil the 1955 series II

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StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Look what the postman bought me today. Thanks Owlsman
I think it is 1970's vintage.
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First test, get my trusty multimeter out.
2k range so that's 537ohms, far too high it should be around 120ohms. With a high resistance the meter will draw less current and therefore read low.
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Lets open her up and investigate, the element has lost its colour due to the heat and has a fluffy appearance due to the insulation breaking down pretty much what I expected to see with the age of the gauge. The element is a single core not stranded as I said earlier in the thread, that is just my poor my memory.
The ends of the wire element are spot welded to the posts and are secure.
It's difficult to say if it has seen abuse, the resistance of the element is too high, it could have been on its way out and perhaps the alternator output voltage is slightly higher than what the dynamo regulator was set at and has finished it off.
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It has had some water ingress, although not ideal I don't think that was the cause
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It is either just bad luck with the alternator voltage slightly higher damaging the 40 plus year old element or the garage did manage somehow kid the alternators regulator to give an over voltage output, there are some very talented people out there.
Sorry I can't be more conclusive.

Regards John
Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

Thanks for posting the results of your 'post mortem', John. I have to admit that your report has completely 'whooshed' me...…. but only because anything to do with electricity is like alchemy to me :D

I'm sort of relieved that you suggest that the voltmeter was probably not in the best of health anyway.

Thanks again.
oliver90owner
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Re: Voltmeter broken?

Post by oliver90owner »

An increase in resistance of four fold, or more, would lead me to check the resistance across the coil windings - as that extra resistance can only be due to two things - a thinning of the wire or a high resistance connection. The latter is more likely?
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geoberni
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Re: Voltmeter broken?

Post by geoberni »

I suspect the 'fluffy' appearance is probably not much different from when it was made, it's basically cotton wound around a thin wire. The fact that the cotton has gone so blue/green makes me believe that the single strand copper core has oxidised a lot and stained the cotton.

The blue/green indication Copper Carbonate with Copper Hydroxide if I recall.

If you think of a copper roof on a building, that goes green from the Carbonate, when it's bluey green, it's got Hydroxide there too.

From the colour of it, I wouldn't be surprised if the core near the post connection has thinned considerably, thus increasing the resistance.
I bet if you try to unwrap it, it'll break easily.
You might be lucky and find it's damaged right by the solder joint to the post and the majority of it is OK.
Basil the 1955 series II

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StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

The ends are OK, I'm sure they used to be soldered, these are spot welded.
I thought I was going to find a section unwound and twisted together. I have tried and failed to do this in the past.
For the wire length it is a high resistance, thinking with Vapes being wide spread now might give more options for resistance wire, I was looking the other day and the highest resistance is around 500 ohms per metre. I haven't measured this one but it's probably 6 inches max.
I'm not sure what the insulation is made of, I thought it was glass fibre similar to the heat resistant rope used for sealing doors and flues on wood burning stoves, OK until you move it then it falls apart.
It dosen't matter, I wanted to see what was wrong with it, I will keep it as it is for now, I took a chance on it having the same bezel as the pitted one on my oil pressure gauge and it is. I will keep my eye open for suitable resistance wire though.
John
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