And the 15AC alternator. I think was the first type without the plastic end cover (don't know what the plastic end covered one was)

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Sure was! Zero reason to have to keep it that way. Once you have the battery swapped the right way around, it takes 10 seconds to convert the car to negative earth. 5 seconds if the car has the push button solenoid

I have a mind to convert the 997 Cooper back to +ve ground just for fun. I don't have a radio fitted to the car ..

All you need to do is take the big spade off the generator, swap the battery around, start the engine and drag the big spade across its connection to the genny lightly, like you strike a match. This creates sparking which reverses the polarity of the permanent magnetic field in the generator. Connect it back up, and away it goes in opposite polarity.

There were +ve ground alternators made for a while, but convention took over and -ve ground became the norm for the motor industry. Zero difference between the two, its just that theres a large difference if you make alternators or electronics for the cars. Gauges didn't care about polarity, but tachos and radios sure do!

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Umm s what happens when you dont change the polarity? And you hook the battery up the other way? (I know it blows u alternators)

Or how doyou tell which it is supposed to be?

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Nothing, unless you want to fit a radio. Then sadness occurs when it blows up...

Or did you mean the genny polarity? It will change when you create the sparking ( To Strike the Field its called) through the passage of current. All you are doing is creating a tiny tiny amount of magnetism in the rotating part of the generator. This seeds a process which creates a little bit of charge, which feeds back to create a larger charge and so on until the genny comes up to speed and voltage.

Its a little bit of a leap of faith, but its really quite easy.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

So if it was positive earth and you hook the battery up negative earth. It starts and no problems occur? then why change the polarity of the generator?

Then if it is hooked up negative earth by accident as above, then radio and whatever else will all work as supposed to correct?

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Generators don't work like a little electric motor does when you reverse the voltage. Its through the type of construction of the motor. When the current reverses direction, the magnetic field also changes direction to compensate, and it turns the same way as the other polarity. Its counter intuitive I know if you have ever played with little DC motors.

In an AC motor of the same construction like a drill motor (yes they are wired the same way, Series-Wound motors), the voltage and current changes direction every 1/50th of a second, but the motor continues to turn the same way. A car starter motor will run on AC as well as DC because of this motor construction, Series motors are also called universal motors for this ability to run on whatever current is provided to them.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

You can polarize the generator to neg earth without touching the generator wiring:
After swapping the battery around,
1.Take cover off voltage regulator.
2. Close the cutout contacts by pushing down for a couple of secs, then use a screwdriver to push the moving contact back up so they open.

Don't forget to swap the two + & - primary wires on the coil around. If you don't, folklore says the coil's H/T spark voltage will be reduced.

Note, IANAE... but I know this works, I've done it lots of times way back when.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Once the polarity is shifted you are supposed to change the wires around on the ignition coil. It will start and run just fine with the coil reversed, but the coil doesn't work to full efficiency.

Oh and if you have a modern electronic ignition, tacho, radio or anything with a transistor in it, don't even consider swapping your polarity. Sadness follows.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.