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Home » Workshop » Auto Electrics » Polarity Changes

Are you positive about you classics earth?

Many British cars from the 1940s to late ‘60s were wired with a positive earth. There was a feeling that this would cause less corrosion, but with 20/20 hindsight it has just proved to be an annoying blind alley.  In the modern world a positive earth  means that you cannot use phone charging equipment, satellite navigation and other conveniences.

Project Shop is experienced in converting positive earth cars to negative. If your car needs this work done, we can do a basic conversion, or we can upgrade your electrical system at the same time, removing the old dynamo and installing a powerful alternator. This will keep your battery in better nick, make the car easier to start, give you freedom to upgrade your sound system, and allow you to use onboard sockets to charge phones and navigation units.

So how do you do it?

On basic vehicles a polarity change can take as little as a couple of hours, but it needs to be done carefully because mucking it up may well result in dialling 999 and asking for the fire service!

Safety First!!

Pretty simple but make sure the ignition is switched off. We always put the keys in our pocket in case someone else decided to switch it on.

The Battery

Disconnect both battery terminals and turn the battery through 180 degrees. You will most likely need to change the battery cables because they will be too short, but even if they are not it is good practice to do so and make sure that the the positive is red and the negative is black. DO NOT CONNECT THE POSTS UP YET, we have plenty more to do before this is safe to connect.

The Coil

Your coil will have two connections either side of the king lead. Depending on how old it is it will either be marked SW (SWitch) and CB (Contact Breaker) or a + (plus ) and – (negative). Essentially in either case you simply need to reverse these connections. If you have the older style of coil it is good practice to replace it. By reversing the polarity of the coil you will reduce the voltage to the spark plug by about 50% thus causing a weak spark and all the running issues associated. There is no such issue with the newer style of coil.


If your car is running points then there is nothing to change here, but if you have electronic ignition be sure to reverse the connections otherwise you will blow the electronic ignition module.


We need to change the ‘polarisation’ of the generator. Have a look at your dynamo, there should be a large (usually brown) wire and a smaller wire connected to a terminal marked with an F for Field. To do this you will need to follow these instructions:

  • Pull off the wire connected to the field terminal.
  • Connect the negative battery lead only.
  • Connect a ‘jumper wire’ about 1.5 metres long to the positive post of your battery.
  • Touch it against the F terminal, you should see a few sparks fly.  Repeat this stage a further 2 times
  • Remove the ‘jumper wire’
  • Reconnect your small brown wire to the F terminal

.Your dynamo is now polarised with a negative earth.


Remove the ammeter from the dash/instrument panel. Simply reverse the connections on the back. It is easy to see that it is done correctly because will simply read backwards if not!

Old Radios

If you are leaving the old head unit in the dash it is good practice to disconnect it completely from the vehicles wiring. They can get hot and combust if accidentally switched on for a period of time!!

Other electrical components

Most other electrical components will run perfectly well left alone but anything with a motor will need to be carefully checked before switching on. Most windscreen motors will work perfectly well as they are on a cam and will run both forwards and backwards, but heater motors, electric window motors and convertible hood motors will probably need to have there polarity changed. Simply change the wires around on these components.

You are now ready to connect the positive battery lead and give it ago! Good luck, or give us a ring and book your car in.

Home » Workshop » Auto Electrics » Polarity Changes
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