How to: Convert a vehicle from Positive Earth to Negative Earth

Positive to Negative Earth Conversion

First up, why do we have Positive Earth and Negative Earth in cars…?

During first half of the 20th century some manufacturers used positive ground, while others used negative ground, and some even switched form positive earth to negative earth more than once.

When car radios became a popular accessory, ignition static could be a problem, and it was good if all vehicles might have the same electrical polarity to reduce ignition noise on the radios. We still had a mixture of positive and negative earth vehicles until the mid 1960’s. With the advent of practical portable alternators it was a convenient time to standardize, and all vehicles thereafter were built with electrical return path to the negative battery post. For a couple of years in the mid 1960’s there were a few positive earth alternators, but most people have never seen one.

For that reason, if your car is still a positive earth vehicle, it pays to switch it over.

A 1958 MGA… Originally positive earth.

How to convert your car from positive earth to negative earth:

Make sure that the ignition to the car is in the off position before you start.


Disconnect both of your battery terminal connections, and turn the battery around 180 degrees. If it is not possible to turn the battery, the cables must be lengthened. Now connect the cable that leads to the starter to the positive post on the battery. Position the ground cable so that it can be connected to the negative post of the battery, you may need to purchase new connectors, but do not connect it now.


Look at the wiring on the ignition coil. One side is probably marked SW (SWitch and the other is probably marked CB (Contact Breaker). Your ignition coil may be marked with a + (plus) and a – (minus), instead of SW and CB, as it may have had the coil replaced at some time. Reverse these wire connections. Essentially …….

a) for contact points, if the ignition low tension coil wiring is reversed, the coil will “pull” spark, rather than “push” the spark, and you will be losing about 50% of your voltage at the spark plugs and have a weak spark. Electrons prefer to leave a high-temperature, high energy surface (like the center electrode of the spark plug) rather than a low-temperature (low energy) surface like the arm of the spark plug. More voltage/spark is available if the electrons move in the correct direction, in the direction they prefer, from the high-temperature center electrode of the spark plug to the relatively low-temperature arm of the spark plug.

b) For electronic ignitions that will only work with negative earth you must reverse these wire connections or you will blow the electronic ignition module.


Look at the Generator. You should see a large brown wire and a smaller wire connecting to a terminal marked F (for Field). Pull off the wire to the Field terminal. Connect a short jumper wire to the large brown connection (make sure you have metal-to-metal contact). Touch the jumper “lightly / slightly” to the Field terminal on the generator. You should see sparks, touch again (more sparks), and again (again more sparks). Now disconnect the jumper wire from the large brown connection, and re-connect the Field terminal wire. Your generator is now polarized negative earth.


Remove the ammeter from the instrument panel leaving wiring connected and reverse the brown wire connections on the back side of the ammeter. It is easy to tell if you have done this correctly the gauge will simply read “backwards”.

Replace the ammeter in the instrument panel.

Your car is now wired for negative ground (negative earth).

You can now reconnect the battery with a new negative earth cable and install a new radio, tape player, CD player, radar detector, or even plug devices into your cigar lighter without worrying about frying components.