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Polarity Published: 30th Oct 2018 - 0 Comments - Be the first, contribute now!

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No, we’re not talking motorsport but changing your classic’s earthing requirements. It’s a racing cert that you should do it – so here’s how, safely!

Are you positive about going negative with your classic? Well you should be because, while it may alter the originality of your oldie, the benefits far outweigh this rather small matter.

Wel over years ago saw the big change over of the polarity process, first started back in 1968. Motor vehicles were initially negative earth. The battery’s negative ‘pole’ was connected to earth (or ground) until the 1930s, when excessive corroding at the terminals saw a major switch over to the positive (+ post).

However, not every carmaker did this and, as a result, it lead to widespread confusion and it also ruined a lot of electrical equipment over the ensuing decades, until it was agreed in the automotive world to go back to negative earth, half a century ago.

If you have a positive-earthed classic, you may find that fitting electrical accessories is becoming increasingly difficult, so it’s a good idea to ‘switch over’. However, it isn’t just a case of swapping the battery leads and away you go but, as we reveal, it’s not exactly difficult either.


This item is not polarity-critical, so simply turning the battery around in its tray suffices (although make sure the leads are more than long enough), although make sure you remember what you’ve done (fit a sticker)! When using a charger, red is ‘positive’ as before.


Direct current generators are polarity-conscious and you can reverse their polarity by simply touching the battery live cable to the unit’s field terminal and ‘flashing’ it for a few seconds.


These are a lot more sensitive than an old dynamo ssytem and, while some old units, such as the Lucas 10AC and 11AC, can be reversed in polarity, many others can’t, so check with an auto electrician beforehand, as major damage to the diodes will no doubt occur.


These external control boxes are polarity universal and should be okay – but check if in doubt.

Ignition system

The biggest concern here is the ignition coil; technically a positive-earthed car delivers a fatter HT voltage, although it’s nothing significant. As a pay off, a negative-earthed car, thanks to less amperage in the coil’s primary circuit, means less burning at the contact breaker points.

On old coils there are two Low Tension terminals marked SW (switch) and CB (contact breaker). Going over negative earth means these leads must be reversed. Most coils are marked + and - anyway to be fair.

However, if you’ve fitted electronic ignition, then have a word with the manufacturer first as it may not work. On basic points replacement types it shouldn’t be a problem, but on models equipped with a separate amplifier it may be so check.


Nearly all supplementary instruments are happy whatever pole they are earthed to, but you may have problems with rev counters and ammeters. With electric tachos there may be provision at the back of the instrument to switch polarity; otherwise they are reversed by swapping the supply lead over, plus some have a different terminal for this purpose. If you don’t swap the supply leads over on an ammeter, it will show a reverse reading. Engine off, but lights on, the dial should show minus readings. If the meter reads ‘plus’, swap the leads over. Now start up the engine but with the lights off – if the gauge shows a discharge, then the dynamo hasn’t reversed its polarity properly. Clocks may be a problem and have to be professionally altered to work correctly.


Can differ here – some sets come with a simple screw to switch over the polarity, while others require a soldering job to swap the leads over. In any event, serious damage will occur if you mess it up! Old valve sets aren’t polarity conscious.

We have heard of owners who isolate the radio from the car (like in a shielded case) and run directly from the battery’s terminals with an in-line fuse for safety, but we cannot comment on this for its effectiveness.


With what are known as ‘permanent-magnet’ motors, like wipers and washer ones, reversing their polarity simply reverses their rotation, so you’ll soon know what’s wrong! Most heater fans only work one way, irrespective of polarity, but if you’ve fitted an electric engine cooling fan, check it works the correct way, too. Starter motors are not polarity conscious.

Not affected

Systems such as lights, fusing, fuel pumps and horns are not affected by the swap over – although if you have air horns, driven by a compressor they’re could be problems. On a postive note (sorry-ed) we carried out such a polarity swap on our Ford Zephyr Mk3 many years ago and have had no problems at all since the big switch over.


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