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Dashboard voltmeter variation


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Hi everyone,

On my way back from Donington yesterday I noticed that from time to time the voltmeter would drop to around 11V, most of the time it runs at around 13V. It would then rise again to its normal running position for some time and then drop again etc etc. Most of the time it runs at the higher reading.

There didn’t seem to be any reason for the fall or subsequent rise, I mean I had my headlights on and windscreen wipers running the whole time. Car runs fine and battery, which is only about 4 years old, charges fine and holds its charge.

I am not too concerned as there doesn’t seem to be a problem, but is this sign that something is starting to fail……thinking particularly the alternator which I think is the original.

The car is a CR ‘73 model.

I look forward to hearing your thoughts.

Cheers, Rob

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It could be the alternator beginning to fail. If it is the original and has never received any attention, the slip-ring and brushes could require replacement as they may be well worn. 

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Hi Rob,

Is this a straightforward job, are repair kits available? If so do you know a reputable source. I saw some old posts with eBay supplier mentioned but the old links don’t work any more.

If it can be restored/repaired I’d rather do that than replace it as it seems to work fine apart from this low voltage reading.

Cheers, Rob

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Hi Rob Y,

RobH is probably right (he always is:D) but you could first check the voltage with a multi meter (on the battery poles), at the time the dash board instrument reads low (keep engine running).

Cheers,
Waldi

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As Waldi suggests, do check system voltage with a decent meter.  If it agrees with the dash gauge, it indicates a probable alternator issue.

Alternator's can be rebuilt, but It's not a job for the inexperienced.  Replacement is fairly simple if you get one with the same electrical terminals.

Ed

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Yes -  Waldi and Ed are right of course,  you should first check the meter in case it is lying to you. Given the intermittent nature of the fault that probably means connecting another meter up and comparing the readings while driving. 

Your alternator  should be an  ACR type and repair kits are fairly easily available but most seem to include bits you may not really need like a new regulator and diode stack, though if you are going to do a refurb maybe its worth replacing the lot anyway.  Some include new bearings too.

e.g.

https://www.rotatingelectrics.co.uk/collections/alternator-parts-repair-kits/products/lucas-alternator-acr-large-repair-kit-15161718-mob-lra100

 

Changing the brushes and slip ring isn't difficult but does need some soldering.  See these links for photos:

https://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?t=4513

https://www.classiccars4sale.net/classic-car-how-to-guides/feature-articles/alternators

 

Edited by RobH
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Thanks for the quick replies gents. I also found this;

http://bullfire.net/TR6/TR6-7/TR6-7.html

It looks do-able, so I’ll probably have a go. Interestingly I had also ended up at the rotating electrics website for the spares kit.

 I did wonder about the voltmeter in the car but as RobH says this would need to be checked whilst driving, so thinking about Rob’s first comments that if it’s an original unit it could have worn brushes and rings then doing an overhaul might be a good thing to do anyway. So I’ll give it a go and see how it goes.

If I can do this then I’ll have new bearings etc and it should last for many more years.

Thanks again for the quick replies.

Cheers, Rob.

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Rob 

If you want a professional job then I can recommend Eurolec of Birmingham - http://www.eurolec-components.co.uk/cms/ - they manufacture Powerlite Alternators and Starter Motors, and offer a full refurbishing service for your Lucas original alternator, or a swap to an off the shelf refurbished one. 

I used them to refurbish original Lucas M45 Starter Motors for my 1955 Land-Rover, and found them to be competitive whilst providing a good service.

Mark

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I had troubles with a new digital temperature gauge which was using the grounds behind the dash. When the load increased the elderly TR6 grounds were rising considerably above 0V due to the old wiring having a higher resistance than it should and slightly corroded connectors. This made the temperature gauge over read badly and also made the voltmeter under read variably according to the load. EG turn up the radio or use the cigarette lighter, GND rises to 0.8V, voltmeter drops from 12.8 to 12V and electronic temperature gauge goes from 85C to 102C as the ground the gauge sees is now 0.8V higher than the ground the sensor sees at the cylinder block..

I solved this by running in a new fat, good quality ground from the battery negative to the grounds behind the dash.

This maybe of no use at all but I thought I'd mention it as a possibility which is worth checking out before splashing the cash on an alternator since the voltmeter also uses those old grounds.

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Just to +1 fatjon - I have replacement gauges that were performing poorly, as they were new ones they did not benefit from a voltage regulator so the poor earth made the problem effect all the gauges.

Will be a simple thing to fix and as an American friend of mine used to say - it's a bit like chicken soup, it can do no harm but might do some good...:)

Does the ignition warning light come on?

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Good comments.

The ignition light doesn't come on and the battery charges fine. In fact if I didn't have the Voltmeter I would have no idea that there is any kind of problem. Of course, if I didn't have a fuel guage I wouldn't know I was running out of petrol until it was too late.

I quite like the idea of refurbishing the alternator and the Repair kit isn't expensive at all, so I think I'll give it a go anyway. If the problem persists then I will check the earths. Up until now though I've had no other indications of poor earth's, but that doesn't mean they are perfect of course.

I'll get a repair kit and let you know the outcome. Might be in a few weeks time now though.

Cheers, Rob.

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On 5/2/2022 at 3:39 PM, RobH said:

Yes -  Waldi and Ed are right of course,  you should first check the meter in case it is lying to you. Given the intermittent nature of the fault that probably means connecting another meter up and comparing the readings while driving. 

Your alternator  should be an  ACR type and repair kits are fairly easily available but most seem to include bits you may not really need like a new regulator and diode stack, though if you are going to do a refurb maybe its worth replacing the lot anyway.  Some include new bearings too.

e.g.

https://www.rotatingelectrics.co.uk/collections/alternator-parts-repair-kits/products/lucas-alternator-acr-large-repair-kit-15161718-mob-lra100

 

Changing the brushes and slip ring isn't difficult but does need some soldering.  See these links for photos:

https://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?t=4513

https://www.classiccars4sale.net/classic-car-how-to-guides/feature-articles/alternators

 

+1

I had similar symptoms, cured by changing slip rings and bushes. Mid-ulster rotating electrics supplied good quality parts quickly and for a very competitive price.

Mike.

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  • 2 weeks later...

Used the kit from Mid-Ulster, so changed bearings, slip ring, brushes, diodes and a silver box thing.

Nothing obviously wrong but annoyingly broke one of the wires to the slip ring. Actually it broke very easily and I wonder if this could have been on its way out. Maybe it’s not an original part or it’s been refurbed before but the centre brush was not worn at all. Maybe it wasn’t pressing properly on the slip ring and it was this causing my symptoms.

Anyway, everything done now, but I’d like to test it before connecting it electrically to the car. Can someone please tell me how to test it? I have 3 wires in the connector that goes on the back of the alternator. A thick brown one in the middle, a thinner brown and white one at the top (could be brown and yellow) and a thinner brown wire at the bottom.

I’m thinking to fit it in the car and start the engine and check the output from the 3 wires, what should I get please? And should I check anything before fitting it in the car?

Thanks in advance

Rob

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Unless you have a >30A ammeter there isn't much you can do in the way of tests Rob.

Fit the alternator to the car but before you connect anything up, connect a 12v bulb between one of the brown output leads and battery positive. If the bulb lights there is a short circuit in the alternator.  If the bulb doesn't light it is safe to connect everything up. 

Turn on the ignition and see the warning light comes on. Start the engine and see the light goes out.  With the engine running at 3000rpm the voltage at the brown wires should be between 13.6 and 14.4 volts but since you have a voltmeter on the car anyway, it should just show a normal reading. 

 

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Hi Rob,

Thanks for your reply. I’ll do as you say and fingers crossed all will be well. It’s been interesting exercise in any case. I’ll report back when installed/tested.

Cheers

Rob

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Sorry Rob H

Just read your reply again and I’m not sure I understand the test with the bulb. So this is done with the alternator not plugged in but the battery connected? (I mean battery not isolated). Then put the bulb between one of the brown wire connectors on the alternator and the battery live, or do I need to plug the alternator in first? Sorry to sound so stupid but I’m a mechanical guy not electrical…….as you can probably tell.;-)

Cheers

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Yes that's right - don't connect the alternator up until you've done the test.  The bulb goes between one of the large spade terminals and the battery live.  

 I'm sure you must have connected everything up correctly inside the alternator but there is just the possibility the diode pack is faulty.  If it is the battery can discharge through the alternator and do some damage. Test the bulb first by connecting it across the battery then connect it to the alternator.  If the bulb does not light it shows everything is OK and it is safe to connect the alternator properly. 

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2 hours ago, RobH said:

Unless you have a >30A ammeter there isn't much you can do in the way of tests Rob.

 

 

Rob,

The alternator on my car is presently ( fingers crossed for saying it) working. I have a non working one and have just bought a rebuild kit to rebuild it as a spare. I do not want to rebuild it, take the one off my car and then fit the rebuilt one as the one on my car is a new old stock Lucas.

Could you please explain how to test the rebuilt unit off the car with a >30 ammeter.

Thanks

Graham

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Pages 36 and 37 here:

https://www.gomog.com/allmorgan/LucasFault.pdf

You would need to arrange some means of driving the alternator if you want to do it off the car Graham.  Also they quote engine revs for the tests - alternators are geared higher so 3000rpm engine is possibly 6000 rpm alternator depending on the pulley ratios. 

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Hi Rob H

I installed the alternator and did the test with the light bulb as you said. When I connected it between the battery live and one of the large spade terminals the bulb flashed on and off slowly. Not very bright and at about the speed of an indicator. I guess this means something is wrong. I connected the diode pack to the same terminals as the ones I took out but I’m not entirely sure that the diode pack was identical, could this be the problem? Any ideas please?

Thanks Rob Y.

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That is a bit weird Rob.  What bulb did you use - not by any chance an l.e.d one was it?  Was there anything else connected to the alternator?  

The diodes should block any major current flow into the alternator but there is a tiny static current drain to the regulator. That is only a couple of milliamps - far too little to light a normal bulb.  If it was any larger it would flatten the battery if the car was left standing for a length of time.  Was the regulator a new one too?

 

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Oh dear I was hoping you weren’t going to say that.

Bulb was a standard filament car bulb which shone brightly when connected to the battery.

Nothing else was connected to the alternator at the time. If the regulator is the flat square metal box with a yellow and black wire that attach to the brush box then yes, this was new.

 I think I will get a new alternator and then play around with this one to try and get it working and use it as a spare. I would like to try and fix it of course, I don’t like to be beaten by machinery! Any further thoughts much appreciated.

Cheers

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This is the insides of the alternator as connected for that test.  The regulator is the circuitry inside the purple rectangle, and B+ is the two large spade terminals.  (+ is the connection for the ignition lamp).  You can see that from B+ there are two resistors to earth, R1 and R2, totalling 5kOhms, which will draw 2.5milliamps continuously from the battery, even when the ignition is off, but that is not enough to power an incandescent lamp.  With nothing else connected to the regulator it shouldn't do anything.  The pulsing you see is the real oddity.

I guess it is possible that if one of the rectifier diodes is leaky, current could find its way via the triple (the three diodes on the right) and the field winding to the regulator, and that may do odd things. 

Of course, the drawing is an original Lucas regulator and an aftermarket one may well look a bit different, but it should act the same. 

alttest.jpg.6c875fb0674412c652234cd6b095566e.jpg

 

I think what I would do is measure all the diodes to see if any are leaky, or maybe just put the old rectifier stack back and repeat the test. 

 

 

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Ok, I don’t know anything about leaky diodes or how to test them but I do understand how to put the old rectifier stack back in. I can be sure I’ve got the connections the same as before too, so I’ll do that.

I will let you know how it goes, probably later this week. I think I’ll still get a new one, I’m a bit nervous with this one, but would be happy with it as a spare if I can get it working. I’m going to get a 40w soldering iron too, my 25w one struggled with the slip ring soldering.

 I tried the test again with a different bulb 5w 12v filament and spinning the alternator by hand to see if that made any difference. Exactly the same result, bulb pulsed regularly but obviously with a low current as it glowed  quite faintly.

Thanks for the quick replies on this Rob H.

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