Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Battery in wrong polarity, what is broken?


Last saturday it was the coming out for my MGB. The warehouse was dark and unfortunately I made a wrong polarity branchment. I noticed some current drain when I made branchment. It lasted for few seconds in wrong polarity and I corrected it. But since I notice that the only thing that seems not working is the alternator lamp indicator. Is it related to my mistake or is it some other electrical problem. The alternator appears to charge normally...


Jean Guy Catford

Hi Jean.

Alternators do not like this treatment.
I think you will need a new regulator pack for the alternator, and maybe a new diode pack.
It might be better to go for a recon alternator.
I wouldn't drive it without at least having a voltmeter in place, as you risk either a flat battery or a 'cooked' (overcharged) battery.

Assuming that the ignition was off, the only other things at risk would be the clock and the radio (depending on how (and if) a radio is fitted).

If you have a late model with some of the US mods in place there may be other problems with the in-line diodes.


Don has hit it. Alernator diodes and rectifier will fry very quickly with wrong polarity. So will the module on a petronix electronic ignition. (mine was due to the PO not marking that the polarity had been changed)
Bruce Cunha

How old is your MGB? You say the alternator is working OK. How do you know? I thought it was instant death for an alternator if you connect it wrong way round! Have you got an older car with a dynamo? That shouldn't be upset by being connected up wrong way round. You may have been lucky.

Mike Standring

What about voltage regulators, stablizers, condensors, etc?
rick ingram


It is a 1973 with an alternator. Something is wrong as the alt. pilot lamp on dash, which is not working anymore. It charges at 14V and the car will function without battery at more than 1500RPM. But I agree, something is wrong with it. I have another alternator with good electronic parts but with a faulty bearing, I will try to burrow the electronic parts. I do not know if is is a complicated task.


Jean Guy Catford

Any condensers/capacitors likely to be found in an MGB as individual components are not polarity sensitive, neither is the factory instrument voltage stabiliser. The alternator voltage regulator is, and it sounds like it is this that has failed, in part at least.
Paul Hunt 2


The trouble comes from the regulator pack.
Provided the diodes should largely support a -12V inverse tension which is a really low reverse voltage, and given that you would check your alternator charging capacities or about 13,5V at batteries connections, you may be sure that there is no risk resulting from your mistake . But please at first check out for the correct batteries voltage while engine is running. If you need it, I can Email you the diodes check procedure . Regards.

If you reverse the voltage to the alternator, whilst the voltage is still relatively low at 12v, the potential current is extremely high as the diodes represent a short-circuit across the battery and the only thing limiting the current is the inherent resistance of the alternator cables. It is this very high current through the diodes which destroys them, unless bad connections elswhere in the circuit have the effect of reducing the current to a 'safe' level, for a short duration anyway. Even then they have been severely stressed, and must be regarded as 'suspect' from then on. Even with one or two damaged diodes you can still get near normal charging voltages at lower currents, but the current delivering capacity of the alternater can be compromised, and will run hotter and noisier than normal.
Paul Hunt 2


Thanks all for your input, I will carefully check the real status of the charging system prior to go further.

Jean Guy Catford

This thread was discussed between 08/05/2006 and 14/05/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.