Voltmeter broken?

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Owlsman
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Voltmeter broken?

Post by Owlsman »

Sorry, firstly, for the slightly enigmatic subject title. Really, this thread is a bit of a follow-up of one I started 2 weeks ago about 'positive v negative earth' and the merits of an alternator but somehow that topic veered off into sash windows and alloy wheels :D.

After some deliberation I have today had a Dynamator (alternator in a dynamo casing) fitted and also the car converted to negative earth. I chose what I thought would be a suitable 'old time' garage i.e. familiar with classics......not so sure I made the right choice (of garage) now! Please read on.....

When I picked the car up I was assured it was all done - albeit, the Dynamator installation instructions were apparently useless - but that the voltmeter didn't work as it was broken :(. I did query that as it was virtually a brand new Smiths meter and it was working perfectly well when I took it in. The garage said they even had it independently tested by an electrician who was working next door and he confirmed it was kaput. Could happen, I suppose! He also told me that the LED lights that illuminate the voltmeter and the ammeter didn't work but, again...…. they did before! The back lights still work on the oil pressure and water temp gauges....and they are all wired through the same electrical connector into the lighting circuit.

I was also told that a couple of blown fuses had been replaced. I thought there were only 2 fuses anyway and if they had blown then I wouldn't have been able to get the car to the garage in the first place.

On the 4 mile journey home in ''rush hour' traffic I noticed the needle on the voltmeter did move over to the right but to just about '11'. Previously, with the dynamo, it usually went to '13/14' before settling down at between 12 & 13. The battery is just a few months old. Surely the voltmeter can't be just a bit broken? It either works or it doesn't.

I was also concerned to see the temperature gauge moving over to nearly 180 when there was no sign of overheating. This was the problem I had experienced previously. then I fitted an electronic voltage stabiliser. Literally, just had a thought now.....are these gizmos polarity sensitive? By changing from + to - earth has it broken it? Do I need a - earth stabiliser?

I've already spoken to the garage to tell 'em about the temp gauge and they've offered to have a look at the car next Thursday. I've not mentioned the 'half-working' voltmeter yet. Before I do so - just in case he tries to fob me off or blind me with science - I'd appreciate some advice please about why the voltmeter doesn't appear to be working properly. Why would it work perfectly well with a dynamo and then not with an alternator. If it affects anything, the Dynamator does seem to be working i.e. when I blip the throttle the ammeter needle whizzes over to +15 or so before immediately settling down to just into + territory.

Any ideas why the back lights on the voltmeter and ammeter would stop working?

As ever, many thanks in advance for any advice/information. Forewarned is forearmed and all that!
StillGotMy1stCar
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

You will need a new voltage stabiliser for Negative earth.
The back lights in the gauges are LED so just need the connections to each one reversing.
If you want a laugh you will see that I mentioned Smiths voltage gauges here viewtopic.php?f=11&t=70786
And got told off by Berni :D
If you have a multimeter I would check the battery voltage yourself, your looking for 14.2 to 14.7 volts when charging.
I would have thought it would be charging at idle but that may be right for a Dynamator.
Regards John
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Just to add to the above, it is odd that the voltmeter was fine before, but I have known them fail and have known them read low before dying completely.
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Re: Voltmeter broken?

Post by oliver90owner »

StillGotMy1stCar wrote: Fri Sep 27, 2019 8:31 pm Just to add to the above, it is odd that the voltmeter was fine before, but I have known them fail and have known them read low before dying completely.
At the same time it may not have appreciated being driven backwards by nearly fsd!

Previous post re voltage stabiliser - not if it is simply a bi-metal strip device they work on heating and that is the same whichever way the electrons flow.
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Re: Voltmeter broken?

Post by Owlsman »

Thanks for you prompt and helpful response John.

I had actually answered one of my queries before I read yours. The electronic voltage stabilisers are indeed polarity sensitive. I checked with the eBay vendor I bought it from and I've already a new negative earth jobbie.

I don't have multimeter but I can probably borrow one. Do I need one to check if the voltmeter is faulty? Is there any other way I could do it? I just find a bit of a coincidence that this morning a near new voltmeter was working/recording perfectly with Dynamo and this afternoon when someone fitted an alternator, it decided to 'go slow'! Hmmm?
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Another way of testing the meter would be to take it out of your moggy and make a couple of leads and try it across your modern car battery, assuming you have another car!
Unless they have changed the design recently the voltmeter isn't polarity sensitive.
If it is polarity sensitive it will be marked on the back with a + and - and may have been damaged by reverse connection.

oliver90owner wrote: Fri Sep 27, 2019 8:41 pm

At the same time it may not have appreciated being driven backwards by nearly fsd!

It can't be driven backwards, works with a Bi-metal strip, unless recent design change.
oliver90owner wrote: Fri Sep 27, 2019 8:41 pm

Previous post re voltage stabiliser - not if it is simply a bi-metal strip device they work on heating and that is the same whichever way the electrons flow.
The OP has an electronic stabiliser.

Regards John
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Re: Voltmeter broken?

Post by Owlsman »

I have got another car (Honda Jazz) so I can try what you suggest. If the voltmeter is Ok, what should it read?
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

On the Jazz I would expect 14.2V with the engine running.
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Re: Voltmeter broken?

Post by Owlsman »

Thanks a lot. I'll give a whirl over the weekend.

Watch this space......
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Re: Voltmeter broken?

Post by Owlsman »

I've taken the voltmeter off the Moggy and put it across the jazz battery, with the engine running...….and the dial struggled to get to just about 11v which was exactly where it reached in the Moggy when I drove it home from the garage.

Does that mean the voltmeter is indeed kaput?

Not that I would ever be able to prove anything but is it likely that the garage is responsible its sudden demise? Could they have 'missed' something or literally got their wires crossed and damaged it? :D I seem to remember when I first asked for advice on here how to wire up a voltmeter, I was asked if my car still had a dynamo or did it have an alternator as the wiring was different...…...but I could have just made that up!!

Rather less of an issue but I still haven't resolved the LED lights on the ammeter and the voltmeter. I haven't tried reversing the connections yet as I'm puzzled why the back lights on the other gauges still work. I don't think these 2 are LEDs but they are 'modern' insofar as they are in a black plastic sheath, with 2 wires (1 earth) and a glass bulb that looks like the end of a very small test-tube. They are certainly not original equipment with screw in bulbs with casings that earth via the meter itself.

Is it possible for an LED 'bulb' to blow?

If you haven't already gathered 'electrics' is not my forte! Not sure I've got a forte at all, to be honest. :D
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

The voltmeter is indeed kaput and I don't think they damaged it. It makes no difference if the have a dynamo, alternator, Pos or Neg earth with this design of meter.
It is best described is a heater element wrapped around a Bi-metal strip.
The heater wire appears to be stranded and my thought is that as a strand breaks perhaps through expansion and contraction the meter drops each time. If you take the meter apart you will see what I mean, they run rather warm and the element will look burnt.

I'm confused as to whether you have a mixture of LED and ordinary bulbs, needs more investigating. If it has LED's they only work one way and will need the connections reversing, they won't be damaged.
If ordinary bulbs, then the plot thickens!!
Regards John

Edit:-Just re read your last post and it doesn't sound like you have LED back lights, so they have possibly pulled a wire off.
The garage blowing two fuses is worrying though, does make you wonder what they got up to!
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Re: Voltmeter broken?

Post by geoberni »

Owlsman wrote: Sat Sep 28, 2019 11:35 am
Rather less of an issue but I still haven't resolved the LED lights on the ammeter and the voltmeter. I haven't tried reversing the connections yet as I'm puzzled why the back lights on the other gauges still work. I don't think these 2 are LEDs but they are 'modern' insofar as they are in a black plastic sheath, with 2 wires (1 earth) and a glass bulb that looks like the end of a very small test-tube. They are certainly not original equipment with screw in bulbs with casings that earth via the meter itself.

Is it possible for an LED 'bulb' to blow?
Yes an LED can 'Blow', quite literally if excessive voltage is applied. This guy applies 12v to a 3v LED:
https://youtu.be/vRtg09j0PcE


Are your lamps
in a black plastic sheath, with 2 wires (1 earth) and a glass bulb that looks like the end of a very small test-tube.
like this? https://i.ebayimg.com/images/g/SJwAAOSw ... -l1600.jpg
if so, they are LEDs
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

geoberni wrote: Sat Sep 28, 2019 2:36 pm
Yes an LED can 'Blow', quite literally if excessive voltage is applied. This guy applies 12v to a 3v LED:
https://youtu.be/vRtg09j0PcE
Yes true but Owlsman's LED's, will have a series resistor fitted to each LED to limit the current to a safe level so his LED's won't be damaged, they are designed for 12V.
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geoberni
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Re: Voltmeter broken?

Post by geoberni »

StillGotMy1stCar wrote: Sat Sep 28, 2019 3:45 pm
geoberni wrote: Sat Sep 28, 2019 2:36 pm
Yes an LED can 'Blow', quite literally if excessive voltage is applied. This guy applies 12v to a 3v LED:
https://youtu.be/vRtg09j0PcE
Yes true but Owlsman's LED's, will have a series resistor fitted to each LED to limit the current to a safe level so his LED's won't be damaged, they are designed for 12V.
Yes, I was just answering as a general principle and I clearly indicated that the guy on the video was subjecting the LED to 4x it's designed spec.

I do suspect he has some LEDs fitted to the meters, since he describes them as looking like little test tube ends, and that they are now being subject to reverse polarity after the work that was done. With a bit of luck swapping connections around will sort it out.
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Re: Voltmeter broken?

Post by StillGotMy1stCar »

Berni:-I've got confused as to what is fitted, but glass like a test tube does suggest LED's.
And I know you were showing LED's are easily destroyed, just didn't want to worry Owlsman :D
I try to run them at half their maximum rated current for a longer life span.
Regards John
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geoberni
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Re: Voltmeter broken?

Post by geoberni »

StillGotMy1stCar wrote: Sat Sep 28, 2019 5:17 pm Berni:-I've got confused as to what is fitted, but glass like a test tube does suggest LED's.
And I know you were showing LED's are easily destroyed, just didn't want to worry Owlsman :D
I try to run them at half their maximum rated current for a longer life span.
Regards John
No problems John.
I have an aircraft electrical background and I try not to confuse people who are like a 'Bunny in the headlights', when it comes to basic electrics like the Moggie. I'll often try to simplify an explanation so as not to confuse them even more; but sometimes that doesn't work. :lol: Some people around here give advice about the colours of the wiring as though that's the answer to everything, and sometimes it is, but after 50-70 years you don't know what's been done, especially if someone is exploring their new purchase.
I know from looking that there are some very interesting colour changes along cable runs behind my dash..... :roll:
My first thoughts are often "just get a multimeter and see what voltage (or not) is lurking where"! :lol: :lol:
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Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

Thanks for your help and advice everyone. Much progress since my last post:-

a) I contacted the eBay supplier of the voltmeter and after a few exchanges this was his penultimate reply : ''It sounds like it has been fried for want of a better phrase . I would suggest that it has been overloaded possibly with the alternator being wired in not quite correctly and then started up , typically they are very reliable instruments , unfortunately we don’t have any refurbished voltmeters currently available for sale''

He did previously confirm the diagnosis that the v/meter was now faulty and has agreed I can send it back to see if it can be repaired.

b) I switched the connections round on the 2 LED back lights and Hey Presto, they both work!!

c) Attached is a photo of one of the 'test-tube' lights/bulbs which I thought were also LEDs.....but presumably not. The terminals have not been reversed on these as they were in the 2 dials that still illuminated.

JUST AS A POINT OF INTEREST - why did the LED bulbs need to have the connections reversed and not the others? :-?

Thanks again, all.
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geoberni
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Re: Voltmeter broken?

Post by geoberni »

Owlsman wrote: Sun Sep 29, 2019 3:41 pm
JUST AS A POINT OF INTEREST - why did the LED bulbs need to have the connections reversed and not the others? :-?

Thanks again, all.
The clue is in the name, Light Emitting Diode.
A Diode only allows the flow of electricity in one direction, hence why it's symbol on a wiring/circuit diagram is like an arrow head with a bar across it. An LE Diode emits light when the electricity is flowing through it, so if it's not in the circuit in the right direction, there's no electricity flowing, so it doesn't light up.
A regular filament lamp is simply a bit of thin wire that is so thin it glows almost to the point of melting (only prevented from doing so by having inert gas in the glass container) so it doesn't matter which way around it is connected.

That lamp in the photo is most likely a Capless 2W lamp.
Like these https://www.ebay.co.uk/itm/T5-286-284-1 ... gapoPT9R9g
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Owlsman
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Re: Voltmeter broken?

Post by Owlsman »

I knew somebody on here would know :D .

I think I must have been off school that day and missed the physics lesson when this was discussed. Not sure that LEDs were even around then to be fair...….and before anyone asks - ELECTRICITY HAD ALREADY BEEN INVENTED when I was at school!! :D

Thanks again for the info and help. As ever, top drawer.
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Re: Voltmeter broken?

Post by myoldjalopy »

Well......electricity was never 'invented' - its just a force of nature that was investigated and eventually understood by us. After all, it has been around for billions of years in the form of lightning. And it even occurs in some creatures, like the electric eel.
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