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MG MGA - Electrical problem

I thought this would be easy enough to figure out for myself ... but I was wrong! So, any help would be greatly appreciated.

My twin 6v batteries are fully charged and I've cleaned all the terminals and leads, but I'm getting very little power through to where it matters. The symptoms are exactly the same as nearly flat batteries, ie the engine won't turn over and headlights, horn, wipers etc work but only just. I've disconnected the starter motor, but it makes no difference to the lights etc.

I've clearly got to work my way through the whole electrical system to find the problem, but I'm no expert and am hoping that somebody knows of 'an idiots guide' to help me? Failing that, maybe a suggestion or two about where the problem is most likely to be??


Robert Sinclair


These problems nearly always prove to be bad connections from the battery to the chassis. I note that you cleaned the battery terminals. Did you check the other end of the cable?

Steve Gyles

Robert, I'm with Steve on this one, but here's a way to prove it. Connect a voltmeter accross your two batteries, that is, right on battery terminal that goes to the earth lead and the battery terminal that is connected to the lead going to the front of the car. The voltmeter should read about 13 volts if your batter is fully charged. With the voltmeter thus connected (you will either need croc clips or a friend!) pull the starter knob. If the voltage drops to anything significantly less than about 6-8 volts, your battery has had it (or needs some remedial treatment to recover it). If the voltage hardly drops at all, you have a connection fault between the battery and the starter switch terminal or a bad earth.
Lindsay Sampford

Thanks for the feedback. I happened to have a brand new earth lead in my spares box, which I have fitted (connected to the battery holder), so I very much doubt that that's my problem. As far as the batteries themselves are concerned, they're only 6 months old (from MG Owners Club) but I am relying on my battery charger to tell me that they're fully charged. I've just ordered a voltmeter, online, so hopefully I'll have that in a couple of days time and will be able to do a proper check.

As an extra check, I've disconnected the battery lead to the starter solenoid, thereby isolating the starter motor, but the lights, horn, wiper etc are still pretty well non-existant!
Robert Sinclair

Okay, we assume the cables are clean. Did you check that the earthing point on the chassis was also clean bare metal. Same applies to the engine earthing strap; check that the strap and anchor points on the engine and chassis are also clean.

Steve Gyles

Robert, even a new battery could be ruined if it was allowed to fully discharge and stay that way for a short while. Under those conditions, lead sulphate forms on the plates insulating them from the electrolyte. Your battery charger would see a battery in such a condition as 'fully charged' because it would draw practically no current. In some cases, sulphate can reduce the effective surface area of the plates and thus the capacity of the battery. Such a battery would show a good open circuit voltage but be incapable of delivering enough current to light a headlamp properly or turn a starter. But this is all looking on the gloomy side, it is probably a bad earth connection!
Lindsay Sampford

I used a rotary wire brush in an electric drill to clean the paint from the surface where the battery earth lead lug connects to the chassis and the for the engine earth lead both on the engine block and engine mount/chassis frame. This gives a bright metal surface. Once connected i then painted the connection to protect it.

C Manley

I tend to agree with Steve and Lindsay in that you probably have a bad earth or bad battery connection.

Once you have your voltmeter (Maplins sell multimeters for less than 10), check the battery voltage as Lindsay advised.

Then if the battery voltage is ok you could probably isolate the problem by using battery jump-leads.

I would bridge each battery lead one at a time with the jump-lead and each time check the lights or try start the car.

If bridging one of the leads restores the connection then you have found your bad circuit.

I would check the main power feed first. (Before you start, check and double check the polarity on your car) You will need longish leads to connect the live connection on the battery to the starter switch.

Then check the battery-earth connection
You need to find a clean metal point on the car chassis to bridge the earth terminal to.

Then you could try bridging the engine earth strap as these do break.

Dont forget the short connection between the two 6 volt batteries too.

Best of luck

Colyn Firth

Robert, don't forget to check the cable to battery terminal connections (not just the terminals to battery connections) especially on the link cable. These often look good until you disassemble and check, finding black and green instead of bright copper wires!
N McGurk


I had a similar problem recently and diagnosed it fairly quickly. I am in Oxford and if you are not far away in Berkshire could come and help. I have a very good meter and some very useful jump leads that can reach from the battery to the starter to confirm if it is earthing problems or not. Mine was the engine earth strap. Everything was new but it just developed a problem suddenly. John

John Francis

Thanks for your offer John,

I am in Wokingham, but before dragging you down here let's see what happens when my new voltmeter gets delivered. I'm beginning to think that it's either the batteries (which will really be annoying because they aren't very old) or maybe the engine earth strap. I'll let you know.
Robert Sinclair


If it affecting your lights, horn, wiper etc then it can't be the engine earth strap.

The engine earth strap is to give a good connection to the engine which is insulated from the chassis by rubber mounts and will only affect the starter and the ignition.

Malcolm Asquith

Class time. Y'all take your seats please. Start with reference to wiring diagram here:

Disconnecting the starter motor from the starter switch is not necessary, as the switch is always open circuit until you pull to start. Engine earthing strap also has nothing to do with dim headlights, as the headlamps have a separate grounding connection to the chassis. Lights being dim is a loss of voltage issue, always a bad connection somewhere (unless the battery is dead). You are likely losing half the voltage somewhere to a high resistance connector.

You will need a volt meter for this diagnosis. Switch on headlights, expecting them to be dim (indicating low voltage across the lamps). While the lights are on, check voltage across each battery individually, placing test leads directly on the battery posts (not on the connectors). Each battery should read about 6 volts with a little current flowing. If not then charge the battery (or replace it if it cannot hold a charge).

Then connect the volt meter across two batteries, contacting the posts and including the cross over cable. Two batteries together should be about 12 volts with a little current flowing. If significantly less than 12 volts, then there is a bad connection in one of the cross over cable terminals.

If it passes that test, then move the test leads, one at a time from the battery post to the cable connector. If you then get less than 12 volts, that connector has a bad contact. If it passes that test, then move the test lead from the ground cable to the chassis to test the chassis grounding connection. If you still have 12 volts, then the chassis ground is okay. Significantly less than 12 volts indicates a bad ground connection.

Once all of the battery post connectors and the ground connection have passed the full voltage test, you can move to the forward end of the main battery cable where it connects to the starter switch. At the starter switch you have the same cable and same chassis as back at the batteries. So connect the volt meter between the main battery cable end (not on the bolt) and the chassis to verify that you still have 12 volts (with lights still on for the load).

From There you move long the conductor path one small step at a time looking for the drop in voltage. Keep one test lead grounded on the chassis, and probe with the other test lead on the starter switch connector stud. This should have the full 12 volts, same as the cable end. If not then the connection between the cable and the stud is bad.

If good there, then continue on. Next baby step is the brown wire terminal connected to the same stud, should have 12 volts. If no, then it's a bad connection. If good, then move to opposite end of the same wire, the brown wire at terminal "A" of the control box. Probe the wire end, then the "A" terminal screw, then he "A1" terminal screw, then the "A1" wire end, and they should all be 12 volts. Loss of voltage indicates bad connection.

It that pans out, then notice the "A1" wire is brown with blue stripe. Follow the to the ignition switch terminal where it connects with another browN/blUe going to the Lighting switch. Original MGA ignition switch has screw post terminals with both of these wires under one set screw. A replacement ignition switch may have paired Lucar push-on connectors (which are commonly a problem). Ground one lead of the volt meter on the chassis, and probe the input wire end, the terminal post, and then the output wire terminal (going to the lighting switch). IF you find less than 12 volts you find the bad connection.

From there follow the wire to the lighting switch. Ground the volt meter, and probe the input wire end, the input terminal post, each output terminal post, and each output wire end. Anything less then 12 volts reveals a bad connector. If you find 12 volts on the input post, and significantly lower voltage on output post, then there is a bad contact inside the lighting switch.

If you have 12 volts at the lighting switch output, the move farther along the Blue wire to the dipper switch. See diagram here:
Do the same tests here, ground the volt meter and probe dipper switch input wire end, input terminal screw, output terminal screw(s) and output wire end(s). Operate the switch to toggle between output terminals. Any loss of voltage indicates a bad connection. Voltage drop between input and output terminal screw indicates bad contact inside the switch.

If the dipper switch has 12 volts output, and the headlights are still dim, then follow the wires to right front corner of car where you will find double barrel (3-way) snap connectors for the blUe/White and blUe/Red wires. Ground the volt meter and probe each wire end in the snap connectors. These should still be all 12 volts. Any drop of voltage reveals a bad connection.

By this time at least the right side headlamp should be bright. If so, then you can check the snap connectors feeding the left side headlight for any loss of voltage. However, if you have 12 volts on the wire feeding the headlamp, and the lamp is still dim, then you likely have a bad ground connection for the lamp.

This test is a little different. Find the Black wire returning from the headlamp and terminating in another double barrel snap connector (along with more Black wires). Ground one lead of the volt meter, and probe the black wire returning from the headlamp. By this time you are likely to find some voltage on the Black wire that should be grounded and have zero volts. This reveals a bad connection in the grounding circuit.

ALL Black wires in the whole car should read ZERO volts in reference to chassis ground, because all Black wires are supposed to be grounded. If you ever find voltage on a black wire, you need to fix the bad connection somewhere between your volt meter probe and the chassis.

At some point it may dawn on you that most of these tests can be done with a test light rather than a volt meter. In most cases you either have a connection (bright light) or you don't have a connection (no light. In case of a high resistance joint that will cause a voltage drop while still conducting some current, the test light may be less bright or very dim (like the dim headlamp). Then you could use the volt meter to give some indication of how bad the connection might be depending on how much voltage drop you get across the connector.

You may also soon notice that current flowing through a high resistance connection will generate heat (as will any resistor carrying current). A hot connector is a bad connection, needing to be cleaned or replaced.
Barney Gaylord

When the meter arrives, a simple check of the battery earth is to connect one end to the battery earth terminal and the other to a good earth and then try the headlights. If the meter gives a significant reading, then it is the battery earth
Graham M V

Hi there
Here is a practical one, see if Barney agrees, disconect the black/positive wire from selenoid,that is the wire going to battery, not starter!! use spare battery from your daily driver or jumper cables to your daily car and hook positive to selenoid and negative to engine, all is ok? no? if ok move negative to chasis, ok? move negative to your existing battery (you do not have to disconect the negative) ok?
That will eliminate or locate ground problem only. You can check positive from battery to selenoid only the same way. Get the car out of gear!


Chances are good that you will have more than one bad connection. I finally gave up on my car and replaced the entire wiring harness - made a huge difference! Good luck sorting it out. Don't forget to check the voltage to your fuel pump and the connections at the fuse block.
Don Carlberg
Don Carlberg

The first jumper connection might make the starter work, but maybe not the lights. The second connection might make the lights work, but maybe not the starter. Depends on where the fault lies. Then what's next, since you went to the work of disconnecting the hot cable form the starter switch. Reconnect it or do some other test first? What you are proposing is using another battery and jumper cables and maybe high current to test certain sections of the electrical system.

I hate having to disassemble cables or fiddling with high current jumper cables just for diagnostic work. A test light and a systematic probing is so much easier. When you get the hang of it, there are so many more tricks to narrowing down the search quicker.

For instance, turn the headlights on and pull the starter. It the lights go out it's a loose battery cable. If the lights stay bright and no cranking, there is a disconnect is in the starter circuit. If the the lights go very dim but not out, and it cranks very slow or not at all, then the starter is drawing too much current, possibly a shot at the starter input stud to and plate. If the lights are dim but the starter works, then it's not a heavy cable connection, but something farther along in the smaller wiring. You can narrow it down that much in five seconds without getting out of the driver's seat.
Barney Gaylord

This all sounds a bit over the top to me. We're dealing with a simple '50s car here, not an Apollo command module!
Robert, do the test with the voltmeter and tell us what you find.
Lindsay Sampford

Thanks guys. I really appreciate all the advice. I think I'd better test the batteries properly before doing anything else(when my new voltmeter arrives!) and then take it from there.
I'll let you know the outcome, so .. watch this space!
Robert Sinclair

this kind of thing happened to my brother in law's boat.

Clean tight connections at the battery(s) and at the ground straps.

We first swapped batteries. Even with a new battery, his battery cable ends were so mangled that we couldn't really get them tight enough for good contact - very slow cranking starter. The boat's terminals were too far gone to save...years of being tightened with improper tools had made them impossible to cinch up. I cut, stripped and cleaned the ends, replaced the battery terminals (with bolt on units) and we then were able to get it to start the way it should. Huge difference.

Even a small amount of corrosion will hinder current flow, so clean the inside of the terminals and the outside of the posts well with a brush and sandpaper. Cover the battery terminals with some anti-corrosion compound (Lubrimatic electrical contact grease is a good choice - or similar. Lots of people use Vaseline.)

Make sure all your connections are sound, and you'll be OK. Start with the battery and ground connections, then look at the starter switch, starter motor, and voltage regulator terminals. Don't forget that you have a chassis ground for the wiring harness at the front (near right front fender in front of radiator), and at the rear (right rear, just abaft of gas tank). These are important for proper operation of the lights, so while you're at it, pull those and clean them as well. Connect them back up using star washers to break the paint to ensure good contact if needed.

AJ Mail

There is another factor to consider. Was the wiring harness replaced?

The bullet ends on the majority of replacement harnesses are not the same size in diameter as the original Lucas bullet ends and will be a questionable fit and electrical contact in the connectors.

I discovered this by accident when checking the fit of a sub harness on my car to discover that the entire harness had this issue.

I cured the problem by buying the proper Lucas bullet ends, cutting off the undersized crimped on ones and soldering and swaging them onto the new harness. Now the fit in the connectors is tight.

Another problem to be aware of is that the connectors themselves sometimes crack causing poor contact but you can't see this inside the vinyl cover. Only testing the connections as Barney has pointed out will indicate a problem.

S L Bryant

OK guys, here's the latest!

I have just spent the last hour playing with my new voltmeter, and have discovered that I've got a problem with one of my 6v batteries. But before splashing out on a new one, I'd like confirmation that I'm right (especially as this battery is less than a year old and has hardly been used!!)

With both batteries fully charged, I get readings of 6.5v and 6.8v from them individually, and 13.3v for the two in series. BUT, when I turn the headlights on and try again, one of them immediately drops down to 4.2v, giving me a reading of 10.5v overall.

I get the same reading (10.5v) at the starter switch, and at the control box (terminals A and A1), but it drops down to 9.5v at the fuse unit (terminal A3) with ignition on. Finally, the fuel pump voltage is slightly lower again, at 9v.

Now, 10.5 volts still sounds quite good to me, even though it's less than it should be, but is that really why my engine's not cranking, or is there something else? I hooked it up to my other car today, using jump leads, and although it started without too much difficulty, it was sounding much less lively than it used to.

And now for my final question. Should I get myself another new 6v battery, or ditch the 6s and go for a modern 12?
Robert Sinclair

Spend half as much as for 1 6 volt and get a 12 volt battery. Using 6 volt batteries costs over 4 times as much as a 12 volt battery. I use BCI size 35 batteries in my cars. Then you can make a box about the size of a battery. Use it to store small tools and emergency parts in and place it on the other battery rack,
R J Brown

One thing you might try is charging the batteries individually with a 6V charger. If one battery has a significantly different charge level from the other, a 12V charger may falsely indicate fully charged.

Don't know about there, but here, most auto parts stores have the capability to test batteries and will do it for free.

If you do need to replace, and are not concerned about concours originality then a single 12V is cheaper and more reliable.
Jeff Schultz

Also, droping down to 10.5 volts with just the headlights on sounds like it could be a shorted cell. Lead acid cells are about 2V each so a 12V battery has 6 cells and a 6V battery has 3. Not much you can do if a cell has shorted internally except replace the battery.
Jeff Schultz

Robert - looks like the battery is dead - 10 volts isn't enough to crank the engine - you should be able to return it under guarantee, although they don't like sitting doing nothing all year!. I have fitted a 12 volt battery - a Fiat Punto diesel van one - that fits into the cradle OK - good luck - cheers Cam
Cam Cunningham

Robert ,may all your troubles be little ones !
I fitted an Oddasey dry cell 12v, which whilst small, will deliver a continuius 1800 amps for 20 + secs. Average starter draws about 150 amps , it holds charge for months on end with out trickle charging. It s expensive but will last for ever, it fits in with no mods, except maybe to the leads.... Sean
P.S. as Barney suggests use a test light as a volt meter can give a 12v reading with mininum current draw, and can be miss leading
S Sherry

Robert, as Cam says "they don't like sitting doing nothing all year!" Lead acid batteries, whether they are 6volt or 12volt, do need attention if they are not in regular service. Ten hours charge once a month will do it, or you could invest in one of the 'smart' chargers on the market now. They monitor the battery condition and automatically give it a charge when it is needed. They are also said to be able to recover sulphated batteries, which is probably what's wrong with yours if it has been idle for a long while. So it might be possible to get your 6volt batteries working again with some long term treatment with a smart charger such as the CTEK XS3600. Such chargers are certainly a good investment if you have a vehicle that stands idle for months at a time.
Lindsay Sampford

Robert, 6 volt batteries do not have the same life as sealed 12 volt batteries and of course the cost of pair are a lot more than a single equivalent 12 volt. I struggled with a pair of 6 volts for years and they were always a little problematic and seemed not to hold a charge well. I changed to a single 12 volt battery 18 months ago without needing any alterations to carriers and still rate it as one of the best changes I have made to my A. Middle of winter, sub freezing, I still know it will spin the engine fine.
J H Cole

JH, the only trouble I had with my 6volt batteries turned out to be a faulty dynamo. They now crank the engine with great gusto, even in the dead of winter.
Lindsay Sampford

LS, my experience only, I may have been unlucky but more than one classic car battery supply has commented on the reduced life of 6 volt batteries.
J H Cole

OK, you've all convinced me to swap my 2 6v batteries for a new 12v. Cam has suggested a Fiat Punto van battery, but do any of you UK guys have other suggestions? I don't want to have to modify the carrier, unless that's unavoidable, and neither do I want to struggle too much to get it in and out!!
Robert Sinclair

See near end of this page:
Barney Gaylord

Thanks Barney.
Robert Sinclair

JH, I've had over 4 years on my 6volts so far, and they were fitted to the car when I got it, so I don't know how old they are. I have given them some tlc since I've owned the car. This includes some weeks on the smart charger and several charge/discharge cycles. They will run the headlights for over 3 hours with them still shining brightly.
Lindsay Sampford

I seem to recall that when you change to a 12V battery you need to give some thought to which way round you require the terminal posts to be.

The first battery I purchased would fit physically into the cage but the terminal posts were round the wrong way, so I had to change the battery for a different one.

Some but not all battery suppliers, will provide the same battery in both configurations, or as in Barney's link with the posts in the centre so they are reversible.

J Bray

Just to wrap up this thread ....
After discovering that one of my 6v batteries was only pushing out 4 volts (presumably a dead cell?) I decided to switch from twin 6 volts to a single 12. Barney's link gave me a couple of options to choose from, so I flipped a coin and ordered the Varta Blue B35 from Advanced Battery Supplies. I placed my order at 10.30am yesterday, it was delivered at 09.30 this morning, fully charged and ready to go, and was fitted by 11.00. What a great solution! It fits perfectly, easy to move the earth lead to the opposite carrier, and ...... the engine's turning over and firing like a youngster again (maybe I should hook myself up to one of these things!) Great result, so thanks again for all the tips and advice.
Robert Sinclair

John, I would think the only consideration is where the 'live' battery post is going to be with regard to the power lead that runs to the front of the car if the battery is going to be placed in the right hand carrier. A new earth lead would have to be made up to suit anyway. If the battery was to go in the left hand carrier, a new power lead, front to back would be needed, so making a longer earth lead if needed, to reach a terminal that was further away than the 6 volts terminal would have been, is a minor thing.
Lindsay Sampford

RS, re Lindsay's point, it seems to me that you have to arrange it so that the live battery post is behind the driver in the offside position because there's not much spare play in the lead. As LS says you need a longer earth lead for the original fixing but you could drill and bolt to a nearer location. I needed a longer lead to get back to the battery cut off switch.
J H Cole

pic of battery

J H Cole

Do not let the cable drag on the propshaft. Do install a hold-down clamp.

Now I just wish we could buy those batteries in in North America. Varta is owned by Johnson Controls, but so far they are not sold here.
Barney Gaylord

JH, it might be a good idea to put an in-line fuse close to the battery terminal on that small power lead. I'm presuming that is the memory lead for your radio. If it is, it has a long way to go, and being small, there is always a chance of it chafing and earthing out somewhere, with disatrous results! If it is just memory, a 3 amp fuse would be more than enough.
Lindsay Sampford

re. Battery, I used one made frome EXIDE, which is a US Company as far as i know.


Siggi, -- Details please. Picture? Part number? Dimensions?
Barney Gaylord

LS, its a charging lead. A year or more ago there was much discussion about installing a plug in charging point for a battery conditioner (after Steve Giles). Since I was having trouble with my 6 volts I put this in and used it for a while every time the car was in the garage. It did work but it's not used any more so there's no current draw and I'm assuming that it doesn't need fusing.
J H Cole

JH, it doesn't know it's not being used! It can still earth out if it's dangling about. You ought to fuse it or remove it, I wouldn't like you to go up in flames!
Lindsay Sampford

OK you've convinced me I'll put something like a 5amp? fuse in.
J H Cole

all I've got:
FIAT #00071730452 Battery # 1059B
bottom size is 175 by 175mm, fits nice in original battery frame. height is about 220mm.
It was out of a Fiat Punto, a very small car certainly not available in US.
Sorry for the metric dimensions.

Okay, got it. I finally found the small size battery on the Exide web site. As far as I can tell, it is still not available in North America. even though Exide Technologies and Johnson Controls are both North American companies.
Barney Gaylord

This thread was discussed between 08/11/2011 and 19/11/2011

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