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MG MGB Technical - Dodgy alternator or batteries?


At one point on the motorway when I had most electrical stuff on (wipers at fast setting, heater, headlights and radio) I noticed the ignition light started to glow very faintly. This stopped after a bit, and I haven't quite noticed it come on in the same way since.

I have a voltmeter that plugs into the cigarette lighter, and have taken some readings:

Engine off - 12.5V
Idling (nothing else on) - 13.6V
running above 1000rpm (nothing else on) - 13.9-14.0V
running with headlights and wipers and heater - 13.4-13.5V

The fan belt seems ok tension-wise, but I have noticed that it is getting a bit more difficult to start more recently in that it feels like the batteries (2x 6V) are slightly lacking in juice when cranking the engine. How can I tell if its the batteries or the alternator that's the problem? Or was it just that I happened to have quite a lot of electrical demand going on (I don't often need to use the wipers at max speed!), and it was just a one off thing?


cheers


(1973 MGB)
Nat

Nat,
the red light shouldn't come on at motorway speeds by just using what the factory has fitted to the car but to me your readings seem ok considering a voltmeter that plugs into the cigarette lighter, to me as a total non-expert it could be poor wires/leads/cables and/or connections, even to or from alternator.

Paul Hunt and his site would help with your actual question. - http://www.mgb-stuff.org.uk/basicelec2.htm#charging

But from decades of experience of using various classics year round, rather than technical knowledge or know how, see below.

First thing I always suggest with sorting any car running problem particularly electrical is to check the battery(ies) and fully recharged on a SLOW charger.

In your case I'd suggest you remove both batteries from the car and recharge them slowly with a low amp old charger or modern trickle charger or battery conditioner, overnight at least longer if possible, either individually at 6v or joined as a pair at 12v.

Before charging check the batteries posts for crud and clean if required, also check electrolyte levels and top up if required and check level again after charging.

The B having two batteries means extra cables and connections to join them together so more potential for problems hence many owners switch to a single 12v battery, saving the additional cables and connections and additional costs of the 6v batteries.

Whilst the batteries are being charged you can get on with sorting the rest.

If your battery leads still have those horrible caps with the screw going through them check they're not flared out so become loose fitting and also that they're clean inside and out, you can't see the crud under the caps. If the leads have post clamps check they're clean inside and out.

Check the cabless particularly at the start of their insulation, and look at the connections at the other ends.

All cables, leads, wires and earths and their connections have to be in good condition and clean, secure and protected.

Particularly check the main cables themselves and their connections, the one to the starter is long, check it's connection to starter too.

Check the starter motor is bolted up tight and no signs of crud or corrorision.

Check the engine/gearbox earth and its connections are in good condition and clean, secure and protected.

Now check the wires and connections to and from the alternator are clean, secure and protected, clean and repair as required.

Once you know the batteries are fully charged you have a better datum point to start from and will be able to know if they loose their charge too quickly from real life use.

Cleaning and if required repair other wires and connections on the other electrical items (and switches, earths, etc.) will also help as we've now entered the cold, damp and wet period of the year which will show up weaknesses.

The car battery is one of the most oversold items on a car, they don't last forever but often faults with them are really faults from those using them, or those responsible for the car's upkeep.

HTH.
Nigel Atkins

ETA: I meant to put the 12.5v for engine off might be a bit low but it's via a fair bit of wiring and cigarette socket and it depends how accurate your voltmeter is - but it might also show your batteries need a good fully slow recharge whatever the cause of their discharge.

I only ever give batteries one chance to be sorted and if they need more than that I change them but I don't abuse the batteries. I expect many years of reliable very low, if any, maintenance use out of a car battery.

I can't stand a classic that won't start, often difficult to sort a car that won't start and I've found often, especially as the weather gets damper and colder, a car won't start because the owner/user has caused the battery (or its connections) to be faulty. Modern batteries going back decades have been very well made and very reliable - if the correct and suitable battery has been fitted and not a "good" spare battery or one that was a good price and physically fits but isn't man enough for the job or abuse.

My old 4-amp analogue battery charger and old battery conditioner have very rarely been used on the cars we've had over the decades but have helped out a few neighbours a few times.

A good habit to get into with any vehicle but particularly a classic is not to turn any electrical item on (unless safety) until you have started the engine, and to turn off all electrical items before turning the engine off.

This also allows you to start the engine and have it running for about 20 seconds before pulling off so that you can listen for any unusual or different noises, same for before you turn the engine off, leave the engine to settle down for about and listen for any unusual or different sounds.

Now when I was a young girl it was all fields around here, many things have changed since then . . .
Nigel Atkins

Those voltage should be OK, the highest being 14v with no load is a bit lower than ideal, but I've found these plug-in and even dash gauge digital meters can read low.

The warning light glows when there is a difference in voltage between the white at the ignition switch and the brown/yellow at the alternator. One cause is bad diodes in the alternator, but given your voltages that doesn't seem likely.

Other than that in theory they should always be the same, but voltage-drops in switches, wiring and connections are inevitable in cars of our age so with everything turned on there are bound to be small differences between those two points. The Lucas Fault Diagnosis says:

"The acceptable volt-drop figure for most circuits is 10% of system voltage (1-2v on a 12v system) but there are exceptions to this rule as in the case of the starter circuit where the maximum voltage drop is 0.5v."

Ideally measure the voltage using a pukka multi-meter on the brown and brown/yellow at the alternator, and the brown, white and green at the fusebox, with the engine running at 1000+rpm with everything turned on and the differences between the voltages will show you where you are getting volt-drops.

Whilst the battery may be getting a bit 'soft' with the colder weather especially if you are not doing much mileage, it's nothing to do with this symptom because with the engine running and the alternator charging as it seems to be the battery is not supplying any voltage or current to the circuits.

To test the battery measure voltages when cranking, with one of the coil LT wires removed so it doesn't try to start. Put a voltmeter right on the posts of the battery - each in turn in the case of twin 6v adding the voltages together, then on the battery cable stud and the starter body. The battery voltage is a good indication of battery condition - 10v is good, and the difference between the two readings shows how much is being lost in all the connections. Anything less than 1v is satisfactory, I have seen 3v being lost and that is definitely worth doing something about.

paulh4

Nat,
there's another battery test and loads of battery info I could put up, or Paul can give or you can get from his excellent site, but unless you have spare suitable battery(ies) in good condition and fully charged getting your B's batteries fully (slow) charged is a must for assessment of the batteries and your, possible, charging and starting issues.

This, in addition to also checking and cleaning and repairing as required the cables, wires, switches and connections, could make all the difference and possibly resolve the issue(s) or at the very least help with problem solving and probably help the efficiency of the systems (don't forget to check all earths and their connections).

If you're going to be using the car through winter then autumn (or early winter now) is a good time to do servicing and maintenance work to prepare the car for the additional strain that the cold, damp and wet weather will place on it and the longer hours of darkness.

I've used all my classics through all the winters I had them and none were/are garaged, all sit outside 365 (6). They were designed for year round use, and in some ways they're better than modern cars in some conditions but they do require a bit more effort from the owner and driver. If you need or want any tips I have loads of experience of year round use of classics.
Nigel Atkins

See below link, this is my go to battery tester (not this exact one, mine’s a Draper brand), it claims to put a 100A load on the battery, I measured mine and from memory it was nearer 70A. Measuring the battery under this sort of load gives you an indication of the internal resistance which is what will cause problems when you try to start a car. Using a normal voltmeter doesn’t give any indication of internal resistance, unless the battery is under load such as the starter. My father had one for commercial vehicles that put a much bigger load on the battery, similar to the picture below. You could also get small versions that would test individual cells, which was ok for batteries with exposed lead bars on the top but no use for modern batteries.

https://www.ebay.co.uk/itm/BRAND-NEW-6V-12V-100Amp-Car-Battery-Load-Drop-Charging-System-Analyzer-Tester/163798682469?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D225114%26meid%3D1bc70ff2474c49a88be9bad3dd7d4a3c%26pid%3D100005%26rk%3D2%26rkt%3D12%26mehot%3Dag%26sd%3D174498067350%26itm%3D163798682469%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithDarwoV3BBEV2b%26brand%3DUnbranded&_trksid=p2047675.c100005.m1851







R.A Davis

I remember seeing battery testers as in the images in the 60s and they were old then. A battery specialist may well still have one or similar as in the link. If that's only taking 70 amps i.e. less than when cranking - and it would be unusual for the average car owner to be able to find that out - I'd be concerned at it indicating 10v was 'weak'. Sounds like something a battery retailer would have :o) Won't break the bank though.

All you need is a voltmeter, testing at the battery posts and the starter connections when cranking, and that will tell you about the battery as well as all the connections and even highlight problems with the starter.

Many perceived problems with batteries are possibly nothing more than having lost capacity due to inadequate charging. As I say 14v is on the low side, and I remember reading about Mercedes selecting alternator voltage regulators for the high end of the normal range to counteract battery warranty claims. Dynamos could charge at up to 15.5v whereas alternators even within spec can be as low as 14.3v. The Lucas Fault Diagnosis Manual states that a discharged battery will only regain half it's capacity when charged from the on-board system and needs external charging to regain full capacity. My occasional-use V8 battery is 17 years old now and maybe three times over that period I've boost charged it at about 16v when it seems to be getting a bit slow to crank. Conditioning chargers often have a program that charges with pulses of 20v which does the same thing, although 'maintenance' chargers usually do not. Not that I'm a fan of leaving those on all the time, they can conceal impending battery failure (they probably all fail in the end) which will strand you when you are away somewhere without the charger connected. I'd rather know about that in advance from the cranking speed.
paulh4

Thanks everyone!

I will borrow a battery trickle charger from a friend once lockdown ends and see how that goes.

I'll get the multimeter out over the weekend and check the connections as well.

cheers
Nat

Nat,
don't bother too much about checking with a multimeter, check all with your eyes, and clean and repair as required.

Same with those battery testers, the average car owner doesn't need them if they have average hearing. If the battery(ies) were taken to a place that uses one of those testers it'd probably have a very wide part of the range on it that will say to replace the battery!

It's true a battery(ies) can get fully charged and still be no good but you can soon tell. A good check (provided your starting system is in good order) is to see how well the battery(ies) turns over the engine (disconnect a wire so it won't start the engine), allow a few seconds rest and repeat the starts and rests 5-10 times and listen how quick and how much the the engine turning over drops off. Fully recharge the battery(ies) if you do this, shouldn't take too long if the battery(ies) is in good condition.

Unless you have a good quality multimeter (the cheap digital ones can be unreliable especially over a year old I've found) and really know how to use it (which I don't) I think you can easily be mislead by the multimeters use and readings.

A lot of servicing and maintenance often boils down to just clean (and lubricating). Cleaning the inside of a light's lens and the bulb, say every 10 years or after dirty storage or body work or repainting, can make the lights brighter! Don't use polish on the plastic shiny reflectors though just damp and dry wipe or dust.

Following the same principle, if required, cleaning the electric connections (and earth) to the light can aid brightness (then secure and protect). Doing the same to other electrical items, connections, switches will cut down on the resistance and help with the item working fully.

For winter, or anytime, keep safe, see and be seen, keep clean the windows inside and out, mirrors, all the lights (exterior of I mean now) and the reflective number plates.

Classics have small dim lights compared to modern cars, especially on the rear, which is why I like reflective number plates (that were used in the 60s too), good area of passive lighting.

Good luck.


Nigel Atkins

All 'hearing' will tell is that there seems to be a problem.

The voltage tests will tell you whether that is battery, connections, or maybe starter.

I can understand how someone who can't use a meter might be nervous doing them, but otherwise you might as well dismantle the car and rebuild it from scratch. Or pick up the car by the radiator cap, rotate everything underneath it one quarter-turn clockwise, discard and replace, then buy a new radiator cap :o)
paulh4

Paul, you wag, I meant you'll hear if the battery(ies) has issues with turning over (starting) because if it don't you stop there, recharge the batteries and drive the car.

We don't really know if there is a problem and can't start problem solving effectively without the batteries being fully charged and just charging them might resolve the problem. In the meantime, if it's not already been done a bit of preventative maintenance will help generally and perhaps prevent future problem which a meter may or may not pick up at the moment.

Would you really buy one of those battery testers for the one time every ten, twelve, years when the battery might be getting pasted it.

The multi-meters are fine for problem solving if they're reliable and the user understands exactly how to use them and the figures they give, how many threads have been on here alone where this has not been so.

If your guidance is followed with a good multimeter then things would be fine but you can't be there looking over the shoulder and see when mistakes are made.

I might give you the task to teach me - are you ready for that, I'm a very questioning student. :)
Nigel Atkins

I would never buy one of those battery testers. It tells you nothing more than a multi-meter and cranking will, and a multimeter can tell you a lot more than that battery tester ever will. But then I bought my first multi-meter in about 1965, still have it, one of three now. Maybe because of that I would never assume someone doesn't know how to use a multi-meter, especially if they have already said they have one.
paulh4

Paul, people have cars doesn't mean they fully understand them or are good drivers - this group certainly includes me and I'd say I'm slightly above average in these areas only because the average is low.

I'm really annoyed that the multimeters only come with annoying pinpoint probes and there don't seem to be the kits with all the other adapters and clips so that both hands are holding the probes with the unit out of sight so you can't see the readings rather than having both hands free and being in a position to actually see the unit and reading.

As I've put before the last two cheap digital multimeters gave unreliable readings soon after the guarantees had run out, but I rarely use one anyway. The current one I was bought is a more expensive digital so hopefully more reliable and longer lasting but still a PITA to use.

I have experience of the testing equipment being over relied on, by myself and others, some electrical professionals, instead of also using eyes, ears and often brain.

Just before typing this I saw a neighbour that unofficially dabbles with cars totally illuminate the, to me, blind side of a raised bonnet, himself and background. Two cars side by side with jump leads across, you can probably guess what I think he done wrong and I've warned him about doing so before. He's fine though, the car he's trying to get started isn't, he's just filled it with petrol and it's a diesel.

Nigel Atkins

“I would never buy one of those battery testers. It tells you nothing more than a multi-meter and cranking will”

This assumes the battery is installed in a car and you can use a multi-meter. I have several AVO’s including my late father’s model 12 complete with 900A external shunt, several digital meters including a Fluke and a FLIR but if I want a quick indication of the condition of the battery I use my £15 battery tester.
R.A Davis

Bob,
at £15 it'd probably be a lot more accurate than a modern £15 digital multimeter, and the multimeter wouldn't be more versatile in use if the readings can't be relied on.

Having put that I've just seen a digital multimeter for £3.47! And it looks like the £8, £10 and £12 ones of a few years back, possibly part of the general great race to the bottom of quality and longevity, and waste of materials and resources.
Nigel Atkins

Nigel, for general fault finding I think some of the cheap analogue meters are preferable to cheap digital ones. If you’re looking for accuracy you need to spend a bit more but for a ball park figure either will do, the issue I have with some of the digital meters is the auto range function, unless you’re paying attention you could be reading 0.2V, 2.0V or 20V.

What I do find amusing is some of the cheap analogue meters have mirrors behind the scale, they may not be very accurate but at least you won’t have a parallax error 😊
R.A Davis

Bob,
I didn't even think about the existence of cheap analogue, the last time I looked for myself many years ago new analogue meters were just being made again but at, say 50 start, at the time so not worth it for the once or twice a year use I'd have.

My wife bought me a cheap digital multimeter after the continuity part of my very old, very basic Draper tester gave up the ghost. The Draper still functioned as a battery tester and after the next cheap digital multimeter went to recycling I got the tester part of the Draper working again but I had to install and remove the battery after every use as there was a buzzing.

(Paul, nothing showed on the meter reading by using my ears and possibly nose told me not to leave the battery in.)

By accuracy I meant with the last cheap digital multimeter I tested it before using it on my midget's car battery, it showed about 10v, which using my brain I knew would be wrong so I dispensed with its use on that occasion and used more basic but reliable methods.

Next time I thought I'd try using it again, as always I tested it before use, again on the Midget's car battery this time it showed about 11v but I kept it on the battery and its reading climbed about 0.1v every second - my battery was recharging itself!

So I kept the self-charging Midget car battery and put the meter in the recycling (probably to landfill).

Last Xmas (I loathe Xmas, I've not bought Xmas presents for anyone, including myself, for possibly decades, yet others give me Xmas presents, despite being asked and told not to, just to make me even more grumpy) I was given a Martindale MM39, two-year warranty, PITA to use digital autoranging multimeter.

But - I am expecting to soon be given full tutorials on how to use it, even if it remains a PITA to use.

Nigel Atkins

"This assumes the battery is installed in a car "

Yes it does, but they're the only ones I'm interested in :o) Also relevant to the OP.

I don't know why you find a mirror scale on an analogue meter amusing, that is (or use to be) a sign of a quality as on my 50 year-old AVO. A mirror scale on a digital, now that would be amusing!

I've had two digital auto multi-meters i.e. with tach and dwell and both packed up after a year or two, but at least they were a source of additional probes and clips. Then found a NOS gunson's Testtune analogue at the Classic Car Show and that has stood me in good stead ever since. In contrast probably 20 years ago someone sent me a Radio Shack pocket digital that just does volts, ohms and continuity and I probably use that more than any of the others as it doesn't need any setting-up.

Those of you who are still awake will have spotted that makes four meters, not three.

Some time ago a regular contributor to the MGOC magazine was advising people how to use jump leads. He said before clipping them to the first battery clip the other ends together to stop them flapping about and possibly shorting to something ...
paulh4

Just in case there is any doubt in people's minds, DO NOT clip the ends of jump leads together!
paulh4

“I don't know why you find a mirror scale on an analogue meter amusing” I would thought this was obvious, I assume (always a dangerous thing) that you know how to use the mirror, but for those that don’t, the mirror is to ensure you’re looking straight down on the meter, you do this by ensuring the actual needle is directly over the reflection in the mirror. This is aid to reading the meter accurately and on the likes of an AVO, which should be capable of an accurate reading, allows you read the reading accurately.

On a cheap Chinese meter, see link below, where the reading is probably more of an indication than an accurate reading, I think it’s probably a waste of time, hence the amusement. This would be similar to putting a mirror behind a modern car speedo, it may help you read the speed more accurately but won’t make the speedo any more accurate which can read up to 10% fast, with most manufacturers seeming to set them to read between 5 and 10% fast.

https://www.ebay.co.uk/itm/Analogue-Multimeter-Multi-Read-Electrical-Circuit-Tester-Meter-AC-DC-BLUE-SPOT/112527468264?epid=1037229369&hash=item1a3328e6e8:g:SmAAAOSwnehc-pVS
R.A Davis

"... the mirror is to ensure you’re looking straight down on the meter, you do this by ensuring the actual needle is directly over the reflection in the mirror"

Exactly, so in what way is a meter having a mirror-scale amusing? That was the point of my comment, not "Why does it have a mirror?".

Using a meter known to be inaccurate is something else ...
paulh4

Paul, I think you’re missing the point but I’ll leave it there to save boring everyone further
R.A Davis

My ChemE prof always stressed that we recognize the difference between accuracy and precision. "In measurement of a set, accuracy is closeness of the measurements to a specific value, while precision is the closeness of the measurements to each other." (Wikipedia).

All digital devices give tremendous precision (3 or 4 decimal places) with very questionable accuracy. Most analog devices give reasonable accuracy with low precision. Comparr sliderule with pocket calculator.

Oh! How we moderns mangle the language.

Stepping off my soapbox now and heading toward the roast turkey.

Happy T'giving.

Jud
J K Chapin

My ChemE prof always stressed that we recognize the difference between accuracy and precision. "In measurement of a set, accuracy is closeness of the measurements to a specific value, while precision is the closeness of the measurements to each other." (Wikipedia).

All digital devices give tremendous precision (3 or 4 decimal places) with very questionable accuracy. Most analog devices give reasonable accuracy with low precision. Compare sliderule with pocket calculator.

Oh! How we moderns mangle the language.

Stepping off my soapbox now and heading toward the roast turkey.

Happy T'giving.

Jud
J K Chapin

Well I didn't know about the mirror but can understand it and apply the logic to the gauges in the Midget, particularly the fuel gauge that sits right in front my capacity intolerant navigator. We have to make a petrol station visit if it shows half or less, to be fair half is less than 3 gallons at the most.

At the moment both my only car, the Midget, and my wife's modern both show +2mph (at 30, 40 and 50 mph that I've noticed). I had the Midget speedo set this way for continuity and to make me think about safety cameras' accuracy and precision.

Amazingly I think I usually use those words correctly, but then again I might have them confused or flattering myself.

The incident with my neighbour's car seems more remarkable that it wasn't serious the more I think about it. Both my wife and I saw it as we both happened to be looking in that direction. It was a grey overcast day but still very much daylight yet there was a very clear yellow flash and glow that lit up my neighbour who was facing our way but with the raised car bonnet blocking the view of the engine bay and about 45 yards between us. My wife thinks she saw sparks(?) going up about 2 foot above the bodywork. I can't think it was fuel but fumes igniting from the battery. I've warned him before about using jump leads correctly and battery explosions but obviously he thinks he'll always get away with it.
Nigel Atkins

This thread was discussed between 21/11/2020 and 26/11/2020

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