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MG Midget and Sprite Technical - Electrickery

I'm having issues starting my car (1979 MG midget 1500, UK spec) when cold. I don't know whether to suspect the battery, alternator, wiring or starter motor.
It exhibits the symptoms of an almost dead battery, and after several attempts to start, the battery seems to die (even the interior light dims) although the headlights still appear bright and radio/car stereo still works. The car starts fairly easily via push/roll start and after a 10-15 minute drive, will re-start immediately, without issue (first turn of the key). I have noticed that whilst idling at traffic lights (especially if indicators and fan on), the vehicle will seem to lose charge via lights dimmer, radio turns itself off, indicator flash frequency slowing. This is remedied by increasing engine revs.
If the car is left for a few hours then I have the same issues starting again. I have a battery isolation switch and this usually helps but not always.
I don't have a multimeter (but have ordered a cheap one from China) but have done some testing with a 12V light tester. Apart from a very, very dim light when all off and the battery connected, the only thing drawing a current is the radio backup, and that is also quite dim. With the isolator open (battery disconnected), there is no discernable light.
I have taken the battery to 2 local auto parts shops (both offering free testing). The first tested out of the car and said all good. The second tested in car, after only a 5 minute drive to charge, and tested with lights on/off. That test resulted in battery good but needing re-charge.
I am confused. If the battery is good, then why does it lose charge? Sometimes drain is over several days, sometimes hours. If the alternator is bad, then how does it charge the battery while driving?
Any ideas?
Steve Travis

Even a cheap multimeter will be better than a test light and will give far more measurable and comparable numbers. And for the 5 or whatever they cost they're invaluable.

I assume its all 12V electrics on this (I'm not that familiar with midgets or older cars) but it sounds like the alternator isn't doing a very good job and is on its way out, i.e. enough to keep it topped up while running but not enough to recharge the battery.

The odd bit is the headlights and radio still being ok when the interior lights dim. The interior lights draw FAR less power so I would expect to be the last to go.

Does the car start consistently from jump leads on another car? If so the starter is ok.

Does the test light get much brighter when the engine is running compared to engine off? It should be, otherwise the alternator isn't really charging the battery.

You can test the alternator by disconnecting it from the car and spinning it up by hand connected to the test light via the rectifier, you should see something, and even if not connected it should stop fairly quickly when you stop spinning it by hand.

W Turner

I haven't tested with the engine running, I'll give that a go tomorrow.
I have used jumper leads twice, both times started almost immediately, so starter motor good.
The interior lights are LED, so even less power than normal incandescent light.
Also, should have mentioned that I have upgraded the headlights and installed relays, likewise the horns. Relays are connected directly to the battery positive terminal and to the negative cable on the car, that is, the cable that goes from battery negative terminal to vehicle, at the vehicle side.
Also, its winter here in Australia. Temperature lately has been 0-14 celcius.
Steve Travis

just my views

test bulb is good enough for most things, multimeters can just add confusion sometimes unless you really know what you're doing

a good battery should hold it's charge for a good while

I'd take the battery out of the car check the electrolyte level in all cells top up as required, clean terminals and fully charge the battery on trickle charge at least over night

after charging test with bulb tester, then leave the battery a good few hours or next day and test again to see if it has lost its charge

this will at least tell you how well the battery holds it charge and hopefully give you a fully charge battery to work from

the cold will highlight battery failings, if you have a electric cooling fan it might be the battery needs to be a bit more powerful

anything that has a motor generally wants more power and takes more out of the battery like motors on cooling fan, wiper, heater, casseste and of course the biggest drainer of all the starter

of course all wires, leads and connections need to be clean, secure and protected
Nigel Atkins

You have multiple poor connections, varying with time, temp, vibration, and investigation. Start at the battery and work out, both hot and earth sides.
Buy a meter and learn to measure voltage drops across circuits and sections thereof.

And that ain't winter!

FR Millmore

Steve, difficult to line up all the symptoms - but I would be sure to check your engine earth strap - both ends - as if this is a bad contact starting and charging will suffer - potentially without affecting lights etc... I had similar issues with a Mini a long while ago and found just tightening the earth strap fixed it..
John Barber

<<And that ain't winter!>>

I'll second that.

It's the middle of summer here and we just about made 16 degrees today.
Dave O'Neill2

0C is cold
Nigel Atkins

0-14F is winter, but not really bad winter

FR Millmore

certainly not a bad winter by our temps but it's a case of what you're used to or prepared for

number one cause of breakdown here I think is still battery faults, or more accurately batteies allowed to go faulty as faulty batteries are rare
Nigel Atkins

The cheap multimeter has arrived and I have some basic test results but not really a lot wiser.
The battery seems to be holding charge, still measuring 12.36V after being disconnected for a week. The alternator may be suspect - I measure 14.08V across the battery terminals but when I turn on all the electrics (heater fan, stereo, lights, indicator) the voltage across the terminals drops to 11.9V at idle and 12.3V at 2500 rpm.
Setting the multimeter to 10A mode, disconnecting negative cable from battery terminal and measure from earth strap connection point on car body to negative battery terminal, I get 0.05, which drops to 0.04 when I disconnect the stereo power (the reading actually starts at 0.24 but drops within 2 seconds to 0.04). It remains at 0.04 even when all fuses are removed.
Is this sufficient to cause the battery drain? Where do I look for its cause? Do I need a "bigger" alternator?
Steve Travis

The 12.35 over time tells you the battery is OK.
The 14.08 tells you the alternator is charging, since it is about what it should be, and is more than the battery was before you started the car.
The drop when loaded tells you that the whizzys from the alternator are getting lost in the multitude of bad connections between alt & batt.

With that much loss, you should be able to feel the worst ones as warm or hot connection points - be careful you can burn your hand. Most likely and first to check is at starter solenoid where batt cables and alt feed and all power take-off for car come together.
Other than connections, everything is normal.

It is remotely possible for certain alt faults to give correct high voltage but insufficient current, but quite rare.

Put your meter on V, connect from one side of a connector group to the other while fully loaded - running at 2500, hot, everything ON, Read voltage - should be zero, but up to .25 might be OK. I've seen as much as 6-8 V. That is LOSS. The total drop from Alt+ to Batt+ should be 0.25 or less, same for Batt - to any earth point on the car, but especially the alt case.
That is, total system V loss for hot plus earth return is 0.5V max under full load. If more, find the bad connections and fix them.

Want more detail, email me ask for "Electrobabble".

FR Millmore

Excellent description FR!

I wish that there was some way to make your post a sticky, for future easy reference to all.


Norm Kerr

I haven't read all the comments above so apoligies if this repeats anything.

These are my views which are by no mean expert when regarding electricity but.

1. sounds like youre running on the alternator whilst going... nothing wrong with that... if you put a multimeter across the battery when running it should read 14v.

2. Your battery seems to be charging... but discharging so you have a short.... prob the engine earth or a light or something

C L Carter

just to expand on what FRM's put and flesh out what I meant by >>of course all wires, leads and connections need to be clean, secure and protected<<

most mechanical and electric servicing and repairs on these cars boil down to just cleaning and lubircating or protecting for electrics

battery needs to be in good condition and the battery lugs need to be fully clean, the cables clamps clean and fitted sucurely and protected with spray stuff or Copperease to protect against fur or gung build up

(the battery itself should be held secure by the clamping bar so it doesn't move at all)

work from the battery forward and make sure all wires, cables and connections are clean - secure (tight) and that wires and cables aren't frayed or broken - and protected (wires sleeved, connections insulated or covered with protective stuff)

earths are very important especially for starrting, the earth cable to battery and the earth straps from engine to body work and I think a 79 may have another earth strap

for things like spade connectors if they are tight (and well protected) just taking them off and putting them back on a few times can sometimes be enough to clean them if the terminal they connected to isnt furred or gunged up

I should have put earths in my first post, usually do as after low battery it'd be the next thing I'd think of from your original post

as FRM has put, if you check and clean, secure and protect all your connections now ans sort out any faulty cables you'll know they're in good condition and will help to prevent future problems and make other problems easier to find and repair
Nigel Atkins

I will start selling "books with that in" when I get around tuit. (that is a great line Daniel - thanks!)

Meantime, I am giving away what I am supposed to be making a living at; I am a loose girl and give the goods to anyone who asks - "Electrobabble". and her sisters! And I take bribes too, champagne for breakfast or diamonds or old boots full of cash or a fiver down me pants.

FR Millmore

I'm in accord with FRM on the losses when the car is running but I'm also concerned about the 0.04 (40mA) drain when everything is off. This will over time drain the battery to the point where starting will be difficult. If my maths is correct this will take around 10 to 14 days.
You will need to find the source of the draw and with all the fuses out (don't forget in-line ones) you should start looking further. Try disconnecting electrical items which have a direct supply eg the alternator etc. and look for damaged insulation.

M McAndrew

Thanks guys. It'll be Monday before I get a chance to work on it. I'll report back.
I had already cleaned up the posts and terminals, but only at battery and earth strap from battery to car. I had also removed all fuses and still reading 0.04 drain.
Steve Travis

I think of that one as a battery lead/cable, the earth strap(s) I'm thinking of is the one engine or g/box to body and another one as per Archive posts for 1500

drain, I don't know - curtessy light, boot, cigarette lighter, something with a constant feed or a short, too late to think now
Nigel Atkins

Unplug Alt and check. The rarish alt fault is some bad diode dead-ode, which can give both the drain and low current delivery.

FR Millmore

I just did a quick test before going to work (start in 20 minutes). The car failed to start and gave dead battery symptoms.
I tested the voltage before attempting, with battery disconnected I read 12.3V. After the failed attempt, the battery still read 12.3V across the posts. However, with multimeter from battery negative post to car (point where negative cable attached to car), it only read 11.1V?
Next, I disconnected the alternator plug and tested amp drain (coincidence, I read FRMs post above after testing) and the 0.04 drain disappeared! So that's probably it, faulty alternator?
Steve Travis

Not clear here.
"After the failed attempt, the battery still read 12.3V across the posts." - Good
" However, with multimeter from battery negative post to car (point where negative cable attached to car), it only read 11.1V?"
>>If this means you were measuring from one end of the cable to the other, and got 11.1, then the cable or post connection is DEAD! Should be zero. Even at full load everything ON, it should be less than 0.1V.

Dead cables KILL alts. Note the signs and warnings in various places "Do not run engine with battery cables disconnected" That is what you are doing with a bad cable or very bad connections. This is exactly the sort of thing which tends to be intermittent.

People who - "Want more detail, email me ask for "Electrobabble" - know this.

FR Millmore

FRM - sorry for causing confusion. I have a battery isolation switch (knob) which disconnects the negative terminal. With the terminal disconnected, I measured 11.1V, so 1.2V lost going around the circuit. This was with the alternator still connected. With the battery connected, it reads zero.
I hope that clears it up.
Steve Travis

Start by confession I haven't read all of this thread. But have you done a cold cranking test on the battery? Your battery may be taking a charge, but not holding it and this would show up if you measure the volts across the battery terminals whilst cranking the engine on the starter (without starting, so disconnect the LT to the coil). Beginning with a fully charged battery showing around 12.5v the reading should not drop below 10.6v when cranked for a full 15 seconds (time it, its longer than you think!). If it goes much lower (say below 10.3v) its time for a new battery, whatever other problems you may have.

If for reasons previously suggested ( poor connections, bad earths etc) the battery is appearing to go flat when trying to start a contributing factor could be that the starter is faulty and drawing too much current. Worn brushes or damaged field coil will do this. Effectively providing a dead short
Guy W

When I first started having issues, I suspected the battery and took it back to the place of purchase for testing. They said that it is rated at 330 CCA but holding 355 CCA, so as good as new (it is one year old). One two occasions, I have used jumper cables to start and both times started easily, so I assumed that this discounted any issues with the starter motor?
Steve Travis

OK, so battery sounds as if it is good.

Starter motors can appear to work ok, but be faulty and draw a higher current than designed. If the supply to the starter is good, as with a good battery plus good connections, then it may work. But if the supply side is suspect then poor connections can contribute to a failing starter and it failing to provide the torque needed,whilst at the same time draining the battery. To me the most suspect item would be the earthing straps between chassis and engine block. As others have been pointing out.
Guy W

Okay, I'll get on to the earth straps on Monday.
However, the 0.04 amp draw is related to the alternator, probably bad diode as FRM suggested. Is that fixable, or best to get a new one?
Steve Travis

the connections to this isolation switch need to be correct size, in good condition and clean, secure and protected but that aside -

now I think I've got this right and others will say if not

if you have a jump lead you could try connecting from the battery earth lug to near the starter motor on the bellhousing/engine as this would supply a direct earth to see if that's the problem

careful obviously when attatching and disconnecting the jump lead to battery

if it fires up easily (allowing for battery charge/condition) you'll know its an earthing issue

I think sometimes unless you really know what you're doing (unlike me) the use of a multimeter can over complicate matters and going back to more basic and perhaps the use of a test bulb can be a better starting place until you progress
Nigel Atkins

ETA (as previous didn't take)

the jump lead I mean would be from a set of thick car-to-car jump leads and connected in addition to the battery earth cable and other earth leads and straps on the car
Nigel Atkins

"FRM - sorry for causing confusion. I have a battery isolation switch (knob) which disconnects the negative terminal. With the terminal disconnected, I measured 11.1V, so 1.2V lost going around the circuit. This was with the alternator still connected. With the battery connected, it reads zero.
I hope that clears it up. "

>>Nope, got considerably worse. I have now no idea what you are measuring. Can you draw a picture somehow?

This is especially incomprehensible: " With the terminal disconnected, I measured 11.1V, so 1.2V lost going around the circuit."
>>With the terminal disconnected there should be nothing going around the circuit, and no voltage to measure.

FR Millmore

FRM - I disconnected the battery via the isolation switch, then put multimeter black lead on battery negative terminal and multimeter red lead on car body, so the circuit is completed via the multimeter.
Steve Travis

"the circuit is completed via the multimeter"
No. N0 current flows through a voltmeter - by design, so the circuit is NOT completed. You are measuring the difference between voltage in the battery and "earth". Your measurement indicates that "earth" is getting a few volts from somewhere, and disconnecting the alt seems to have eliminated that. BUT, it does not tell you anything about the state of the cable and connections except that they are in place and connected in some manner, however bad.

The alt failure is usually in the rectifier stack, but experience indicates that the regulator is generally also dead, and it is not clear where the failure starts. It may often be from "running the engine with cables disconnected" as above - AKA "multitude of bad connections" - my first post. Rectifier + reg can be replaced, but a good Auto Electric Shop can usually check and repair the entire alt for less than you can buy the parts. Involves soldering parts in, plus tests they have the tools and knowledge for and you do not.

I think I am now sending everybody their copy of Electrobabble. If I missed anyone from my email mess, yell again!

FR Millmore

Update: I've cleaned the cable ends and connection points at battery, solenoid, earth strap and alternator.
New test results:
Car off:
Battery positive (B+) to B- = 12.45V (battery was charged prior to commencing clean-up)
Car on, all electrics off:
B+ to B- = 14.2V idle, 15-16.5V at 2500 rpm
Car on, all electrics on:
B+ to B- = 12.6V idle, 14.6V at 2500 rpm
Car off (after above tests):
B+ to B- = 12.65V
B+ to Alternator case = 12.65V
B+ to cylinder head = 12.65V
B+ to starter motor = 12.65V
40mA drain still present unless alternator cable disconnected.
I noted that battery fluid had bubbled out of the top of the battery vents after the "vehicle on" tests, I assume from over-charging? Also, whilst warming the car to operating temperature prior to tests, I noticed that the interior light would cut out/flicker at above 2000 rpm but return to full intensity at idle, I think maybe due to voltage spikes?
I believe the above indicates a defective alternator. The alternator has a stamp to say that it was reconditioned/refurbished in 1991, so probably surprising that its lasted as long as it has.
If I'm correct, then what are the recommendations for a new alternator? Stick with the original Lucas 17 ACR? Look into Bosch/GM Delco alternative? Others? A local guy is selling a Lucas alternative (apparently manufactured for Moss Motors USA) on ebay ( - thoughts?
Thanks for all the advise so far.
Steve Travis

All good except
"Car on, all electrics off:
B+ to B- = 14.2V idle, 15-16.5V at 2500 rpm"

Too high, should be 14.5 max. indicates either regulator/alt fault, or high resistance between alt and batt.
You will see the the Jag mentioned had similar high alt output at first, until I fixed all the earth and hot connections all the way back to the Battery, when it returned to 14.5 more or less.

Measure B+ at alt to B+ at batt, with car running and everything ON, should be less than 0.5V, pref .025V

Then B+ batt to anything hot with engine OFF but all electrics ON, again should be less than 0.5V. This eliminates the alt connections but tests the batt + cable and solenoid connections.

Do you have faint glow of the IGN light getting brighter as engine speed increases? (look at night or shield from daylight) This is an indication of a bad diode in the alt, which does cause high output V, and will eventually blow the bulb and cook the Alt.

FR Millmore

IGN light only illuminates/flickers when car is about to stall, otherwise not visible at all (dark outside now, so very easy to tell).
Measuring B+ to "hot" with engine off revealed 0.09-0.13V loss.
Measuring B+ to A+ with engine on showed 0.25-0.5V loss, the 0.5V at 2000rpm, 0.25V at idle.
This time, the alternator was struggling to power the electrics and charge battery (below 12V, even at 2000rpm). Battery now reading 9.74V B+ to B-.
Alternator confirmed?
Any opinions on the alternator that I linked to?
Steve Travis

43 amps doesn't sound a lot

check the socket is the correct type for your plug (mine was but was upside down)

check if you get a new fan and pulley or if you need to transfer your existing and that the pulley is same size

this is just an example for you -

for more momey you can also get these as chromed items

you can also get 16/17 ACR, 65 amp too

all with or without pulley and fan

or you could swap to a much smaller and lighter Japanese model but you'd have to check fixings

or you could get your existing one repaired
Nigel Atkins

ETA: you could ask your ebayer if it is 43 amps or is it more as he put >>Identical to the Lucas but with higher ampage output<< in his answer
Nigel Atkins

I believe that the original only output 35 amp (?), so 43A is the higher amount. The problem with most alternatives is the tyranny of distance. Not only delivery/postage cost (around 30GBP), but time.
I like the sound of a smaller, lighter and higher output Japanese unit but how do I check fixings without buying first? I don't have an angle grinder to modify the housing/case, either.
I've sent queries to the local one and the one Nigel linked to.
Steve Travis

in that case Steve you could ask BBc why this is 5 less and marked as for a 1500 -

these are heavy units so postage could be expensive
Nigel Atkins

Not convinced re Alt.

"Measuring B+ to "hot" with engine off revealed 0.09-0.13V loss."
When measuring losses, load must be specified; the greater the load, the greater the loss, and the clearer the diagnosis. If you are getting that drop with only the infinitesimal load of the meter, it is huge; if with 15A of lighting load, decent.
Are you measuring from the actual Batt posts, or from some part of the cable/clamp/wire attached to it? These are commonly the point of greatest fault in heavy power connections.

"This time, the alternator was struggling to power the electrics and charge battery (below 12V, even at 2000rpm). Battery now reading 9.74V B+ to B-."

It should take a couple of hours sitting with lights ON to pull a fully charged good battery down that low, if you are measuring correctly. And the CCA you mentioned is NOT a measure of Battery capacity, but of heavy load (starter) delivery - it sounded good, but could be an intermittent fault. Amp-hours is the measure of "normal load" capacity; it is Amps that can be delivered for some time before Batt V drops below spec - so 10A for 10 hr = 100 AH, as does 20A for 5 hr, or 1A for 100 hr.

This sounds more like a very bad connection again, but possibly internal to the battery - not uncommon and very vexing to find, as they tend to be intermittent! And such will give crazy alt & system voltages. I had a customer car that had a new battery fitted, drove 50 miles and died - alt dead, dead melted dead. Car came on a truck, and I hooked a charger up to the battery and smoked my charger - new battery that morning, had started the car three times and drove the 50, had a dead short internally!

Again - bad connections kill alts - and internal battery connections count as near the worst of bad connections. You have to remember that the alt/reg is sensing Batt V to decide what to do, so if Batt V is low or high or erratic - or appears so at the reg, strange stuff will happen
I would not fit a new alt without a thorough professional check of the battery including a heavy discharge test.

FR Millmore

Thanks Fletcher. So, do I take the battery back to the auto supplies shop for testing or an auto electrician/mechanic?
Steve Travis

Further, FRM earlier you advised that B+ to A+ should be less than 0.5V, my readings were less than 0.5V?
Also, the battery reading B+ to B- dropped from 12.6V to 9.6V after approx 5 minutes, car running and everything turned on. Is it possible for a faulty alternator to cause this much draw (given that the lights, fan, radio wouldn't) or more likely bad battery or connections?
Steve Travis

Non interested but competent party advised for evaluation.

As I keep saying, any loss measurements must include load. There is no load including a bad alt that could cause that kind of drop in B voltage in that time, without getting so hot it would smoke and fry thy leetle paws. So, I think it is a bad connection, inside or outside the battery.

OTOH, it might be possible to do that by hooking the battery up backwards so the alt is uncharging the batt, but I think it would kill the alt immediately, as it did my charger when I did said brain dead exercise last night - but I had put a fuse in its leads!
i have seen generator cars (3.8 MKII) that mysteriously ran for several months with the battery backwards - we never could figure out how - it killed the new battery in 4 or 5 months.

FR Millmore

Wow FRM is human after all. How to explode an air of invisibility but to connect a battery the wrong way round to a charger....
Thank heaven for fuses, easily replaced and, because the numpties keep changing them when something is obviously not right, cheap as chips....

BTW the 3.8MkII sounds like an interesting tutorial!

M McAndrew

Was dark, couldn't see the colours or signs, in hurry - brain fades.
Problem with being a positive earth guy working on negative earth vehicles! And having just taken the battery out of one vehicle and put it in another where it is in the same position but reversed! Got the battery changed as it got dark, but needed charge. I had even sorted the clips in the shop before I walked out in the dark, repeating "Right hand positive. left hand negative". Big spark when the charger made contact. Scared me - I found that little 6 Amp charger in a barn 35 years ago and it is still my only one.

That Jag was funny - a gorgeous car owned and transported from England by a fussy English owner, was his DD along with an MGA Deluxe, which he drove most of the time - Jag was for formal and trips. Had around 40,000 and 6 years on it, and he decided to replace the perfectly OK battery "just because". No small feat in a MKII! But the car was always lazy starting and the lights would dim at idle after that, so he finally brought it in and admitted that he had fooled with it. We checked the system, all good, but the battery voltage was always low. Since you cannot see the battery terminals or marks back behind the heater box and nearly touching the bonnet, we didn't look - and who would expect the battery in backwards after 6 months? We finally decided we had to take the battery out and try another, and I noticed the terminals as I dragged it out on its side as you must do if you do not want to remove the bonnet. My boss did not believe me when I told him, and actually came and measured the terminals to see which post they had been on, since he thought I had got confused wrestling with the thing.
Put in a new battery and lit it up = all good.

FR Millmore

This thread was discussed between 10/07/2012 and 08/08/2012

MG Midget and Sprite Technical index

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