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MG MGA - Electrics

G'day All
I have a problem with our 59 coupe ( which has fairly original wiring )
I discovered that with battery disconnected, the positive battery lead has a direct to earth connection. This was as a result of my battery charger going into problem mode after about 1 minute of charge time whilst the battery was still connected in the car. Now that I have it out of the car, it is recharging nicely.


If I connect ohm meter to earth and pole on starter - should I get a direct earth ? - because I do.
Also, same situation with the brown wire from starter switch to regulator box

Thanks in anticipation

"Sparks away"


P R Macwhirter

both are coils.

G'day Mog Cattery

So if they are coils they should both have direct link to earth - cos if there's no link to earth, the coils won't energise and do their stuff.

So how can I have Positive wire from Battery ( car is of course Neg earth ) which terminates at starter switch and on the same terminal, have this brown wire going to earth ?? seems like a drain on the battery all the time.

If you or someone can explain ?
perhaps my battery charging error was something else but I still can't see how the positive battery terminal can be run to something earthed ?


"sparks still a problem "

P R Macwhirter

Sorry, you said that wire from starter switch to control box is short, or ok.

should not be earth, when running did the ignition light go off, must dash try Barneys

Phil, original wiring would be positive earth. Has yours been converted?
Dan Smithers

Just to clarify
There is a brown wire which runs from the input side of the starter switch to the control box
If I disconnect that and connect meter to that terminal (just removed from the starter switch) and to an earth. I get no resistance, ie a short to earth via the control box

Is that correct ? if not I'll probably start investigating the control box

Had a quick look at the link but rather not go there unless need to - looks a bit complicated
P R Macwhirter

Dan, yes converted many years ago and running well until I tried to charge the battery after being away for a month or so. Discovered what I believe to be a problem but not sure if it is !!
P R Macwhirter

Not sure I understand 100% what you are saying and certainly am no expert but would have thought you should expect to get continuity between lead to starter motor and earth, as the circuit will flow through the motor.
But not certain that is what you meant?

g Victors

Phil, While your battery is out of the car measure the resistance using your ohm meter across the positive battery cable and earth with everything switched off. It should be infinite unless you've got something wired in that permanently takes current, like a clock for example. If the resistance is low there is a fault. Try removing the brown lead from terminal A on the regulator or at the starter switch. If the resistance doesn't change then the fault is in the starter circuit.
From your description it sounds as if your starter switch may have gone short circuit or partially so.

Don't forget that the starter motor resistance is very, very low and unless you have a rather special meter it will register zero ohms. ................................Mike
m.j. moore

Do you have your battery in backwards? Original was pos ground to the frame.
Brian W.

G'day All, and thanks for comments
Let me recap as it appears to be hard to convey in written word.
1)My reference to positive terminal is correct as the car runs neg earth and has done so for 20 years.

2)The battery was indeed in the correct way around

3)there is a brown wire which runs from the input side of the starter switch ( pull type via knob on dash) to the black bakerlite control box

4) If I disconnect that brown wire front he starter switch and in isolation put analogue multi meter from that newly released brown terminal to earth, the needle swings right across indicating direct path to earth 0 ohms.

So, is that correct situation ?

And if it is, why does that not flatten the battery in short time as the current will flow from positive terminal, along battery lead to the same terminal to which this "direct to earth" brown lead is connected.

This is why I am wondering if there is a problem with wiring - battery wouldn't charge whilst in the car and connected.


P R Macwhirter

Phil, -- Internal elecrical resistance of the starter motor is very small, something like 0.03 ohms, so it will look like short to ground with the analog ohm meter.

See the MGA primary electric diagram here:
The thick brown wire on the starter switch is connected on the always hot battery terminal. If you disconnect this wire from the starter switch you can measure down stream resistance. For the MGA, when headlights and ignition switch are switched off (and you are not pressing the horn button) there should be nothing at all electrically connected in the whole car. There should also be no internal connection in the control box. Resistance from the brown wire to ground should be infinite (50-megaohms on the meter), meaning open circuit, no connection to anything.

When you find some resistance on the brown circuit, that means something downstream is connected and switched on (which is normal if true). When you find no resistance on the brown circuit, that means something downstream is shorted to ground (definitely bad). The way to diagnose this is to disconnect the brown wires at the control box then use the ohm meter (or a test light) to find which wire has the short. Then follow that wire downstream to the next junction, disconnect the wires there, and test again to see which downstream circuit has the short to ground.

I will venture an educated guess and suggest that you may find the lighting switched failed with an internal short to ground.
Barney Gaylord

Or are you saying that there is continuity to earth from the starter motor switch terminal that would normally be connected to the permanently "hot" lead, which is currently disconnected?
If so, it must simply be a case of faulty starter switch. Plenty about that on Barneys site and also in archives I am sure.

g Victors

Thanks to All,
It seems that Barneys response sets out what I need to do - find out what is shorted downstream of the box

Again, thanks to Barney and All,

I will investigate and report back in due course

P R Macwhirter

This thread was discussed between 21/04/2015 and 23/04/2015

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