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.

Battery

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.

Coil

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.

Dynamo

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.

Ammeter

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.

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Comments
  1. Stewart says:

    I have a 1965 MGB Roadster, I want to put electronic ign on.
    Do I have to change my fuel pump or swap wires?
    Will RPM gauge still work?
    Heater motor?
    I just want to be sure converting to neg earth will be ok for me
    Thanks
    Stewart

    • simonbbc says:

      Thank you for your question.
      Here is a list of the changes you may need to make:
      Fuel Pump you can swap wires and this will be fine although we do offer a replacement low pressure fuel pump
      Rev counter can be resoldered inside see Internet article for possible help if not there are many articles on the internet about this.
      Radio may need changing as some radios are positive earth only and likewise negative earth only (if there is one installed)
      Heater will work regardless of the polarity

      I hope that this document above provides enough info to help with the conversion and helps in relation to the Coil, Battery, Ammeter and Dynamo / Alternator

  2. tony levitt says:

    Have you an electronic ignition system for an AC 2 litre saloon 1949 6 cylinder lucas distributor?

  3. Barrie Jones says:

    Original SU fuel pump should work fine without any wiring changes, but if it has an electrolytic condenser inside the lid this must be unsoldered and reversed.

    Wiring to wiper motor should be reversed, otherwise it will destroy its bearings because the thrust on the armature will be away from the thrust bearing.

    Heater (if fitted) should have fan wiring reversed or else the fan will suck instead of blowing. The fan motor has 2 wires plugged into the side. Just pull them off and swap sides.

    Fuel gauge has a voltage stabilizer associated with it. This is clearly marked POSITIVE EARTH. You will have to replace it with a negative earth unit from a later MGB.

  4. derek roberts says:

    New conrtol box fitted to ’64 998 cooper. Ignition light stays on when revving engine. Dynamo re- poarised to – earth. New brushes and commutator checked, fan belt tension good. Not sure what alse to check?

    • simonbbc says:

      This sounds like there is an issue with wiring to the coil or that the wiring is not connected at the coil.
      There is no reason why the ignition light would stay on unless wiring is incorrect. When you say new control box, what do you actually mean please?

  5. stephen mok says:

    i just brought a K3X replace my regular point, my car is 70 rolls royce silver shadow ,
    after received the parts ,no installation instruction attached,just want to know, is the red wire connected to the coil + and the black connected to the Coil -?pl advice.also want to know, should i change the coil?
    pl reply asap

    • simonbbc says:

      Thank you for your email.
      We are sorry that you were missing the K3X instructions, form this I assume that it was labelled on the K3XRoyce on the invoice. If this was the case then you have the correct item, I was worried that you had the K3X which is for the P5B or P6.
      So with that covered it certainly is the Red to the positive side of the coil and Black to the negative side.

  6. Gmc says:

    I accidently connected my 1953 GMC pick up as a positive ground and now my vehicle doesn’t want to start what should I check first

  7. Adam gittings says:

    Hi,
    I am looking for an electronic ignition kit for my 1961 Wolseley 6/99.It has the DM6 type distributor -it’s very original and I want to keep it positive earth.Do you have anything suitable? Will I need another coil?
    Thanks

  8. Tom Mcnair says:

    hi Tom here i have a 1963 bedford ca van i have just purchased a electronic ignition kif {kit 10}
    do i need to reverse wires on starter and dynamo after i reverse batery cheers Tom

  9. Tom Mcnair says:

    also forgot to ask do i reverse the coil as well thanks Tom

    • simonbbc says:

      If your vehicle is negative earth then you don’t have to do any of the above but you do need to follow the three steps, battery, coil and dynamo or alternator… all of these need to be swapped over in accordance to the steps above.

  10. John Beaumont says:

    Hi

    I bought a Jaguar XK140 recently that had been changed to negative earth but I am not sure what items were re-wired. There was a loud noise from the relay every time I switched on the overdrive and I cured this by reversing the leads on the illuminated switch (no idea why – but it worked).

    I have just discovered, by chance, that the heater rheostat switch is getting seriously hot when driving along. Could this be caused by the polarity change (or something else?) and are there any other components that I should be concerned about?

    Thanks
    John

    • simonbbc says:

      Yes this would be likely and it would be wise to check this with a negative earth switch. It is likely that some electrical components are not compatible to run in negative earth.

      • John Beaumont says:

        Thanks very much for the quick response. Where could I get a negative earth switch that would fit an XK140 and are there any particular other items I should check?

        John

  11. Banesh Bhatoolall says:

    I bought a 1966 Riley Elf that was last on road in 1982 but had no battery ! How do I know if it is positive or negative earth please ?

    • simonbbc says:

      Thank you for your post… If you check which side of the battery is to ground and also which side of the coil is wired to the Live side this will be the best way to tell.
      Please call up to the office on 01527 889 453 if you have any further questions.

  12. peter says:

    hi I would like to follow the very clear steps you gave to change my 1963 fb victor over from positive earth to negative,other than my amp meter do I have to alter the wiper moter wires as well ?thanks for your help p,cope

    • Simon says:

      It is common to have to change the two components’ wiring you mentioned when completing the conversion yes, however it can be different for some components so it is always good to double check on each individual part to be sure.

      Simon

  13. peter says:

    sorry I’m back again but I just need to get this right will changing from positive earth to negative earth mean I will have to change my feul gauge as well? thanks p,cope

    • simonbbc says:

      Usually the fuel pump is one component that does need to change after the polarity change so this would be good to double check with your vehicle. If you have a FuelFlow fuel pump then you can un these on both positive and negative earth vehicles.

      Simon

    • Simon says:

      It is likely yes but this depends on the actual gauge itself as some aftermarket gauges are capable of dual polarity.

      Simon

  14. tommy says:

    how wood i change my wifes 1957 hillman to neg earth please

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