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MG MGB Technical - Negative earth back to positive

The previous owner of my ´63 MGB has converted the electrical system from the standard positive earth to negative, but used the original dynamo.

I want to convert it back to positive earth. How do I do it?

Tore
Tore

I've never heard of anyone converting from -ve to +ve, but I guess you'd just have to follow this, but backwards, so to speak.

http://www.mgaguru.com/mgtech/electric/et202.htm
Derek Nicholson

Hmmm... The reason was that I felt it would be easier to replace the dynamo if it breaks down on a long trip if the system was positive earth as original, but may be I should rather learn the trick how to re-polarize the generator if I need a new one.

Tore
Tore

Many years it was standard policy to polarize any dynamo you replaced. I'm not sure it was necessary but it was in the instructions. Maybe something in the repair procedure caused it to lose it's polarity.
On the other hand maybe it was a myth.

Clifton
Clifton Gordon

Neither a dynamo or an alternator will start generating any voltage or current until they are 'pump primed'. In an alternator this is done externally from the current through the ignition warning light, but in a dynamo it has to be internally from residual magnetism in the iron that the field windings are mounted on. This genenerates a small voltage in the output, which is fed back to the field windings by the control box, which generates a larger magnetic field than the residual magnetism, which generates a bigger voltage in the output and so on until the required voltage and current is being output at which point the regulating functions of the, well, regulator, come into play.

A new dynamo out of the box will probably have no internal magnetism, or it could be in the wrong direction which could damage it, the regulator, wiring and all sorts. So all newly fitted dynamos should be polarised to ensure a) they will generate and b) it is in the correct direction. This is done by very briefly connecting battery voltage to the field terminal (just touching it and disconnecting it once briefly enough to generate a spark, known as 'flashing'), which generates a large magnetic field in the windings and causes the iron they are mounted on to take up the required magnetic polarity.

It's exactly the same process to polarise a dynamo on a negative ground car as a positive, the polarity of the car makes no difference at all.

In practice alternators gain a very small amount of residual magnetism while in use which means that they *will* generate an output voltage and current when the warning light is disconnected - eventually. The engine may have to be revved to 3000 rpm or so to start it, but after that they will charge normally down to the usual 600 rpm or so. If they drop below that they will stop charging and have to be revved to 3k again. However the polarity of an alternator is governed by its internal electronics, they are not easily reversible like a dynamo, and they should never be 'flashed' like a dynamo or you could destroy the electronics.
Paul Hunt 2

Tore,

All new dynamos now come as negative earth and if you want to use them on positive earth you have to repolarise them.

I would leave it as it is!!
Chris at Octarine Services

Thanks - I will leave it negative. I´m pretty good at engines and gearboxes, but sooooo incompetent when it comes to electrics!

Tore
Tore

This thread was discussed on 04/03/2007

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