Positive or negative earth

Most vehicles switched from positive earth to negative earth around the late 1950s. Series land rovers, however, remained positive earth until 1966, making the switch to negative earth in 1967 (originally indicated by a change to a black painted instrument panel and a warning plate on the radiator). Positive earth simply means the positive terminal of the battery is hooked (grounded) to the chassis or frame of the vehicle, whereas modern vehicles all hook the negative terminal to the chassis today.

Many old rovers have been switched over time to allow installation of “modern” equipment like radios and other accessories which were only designed for negative earth vehicles. In addition, if the dynamo or generator has been upgraded to an alternator the conversion to negative earth is usually necessary. Outside of those cases it is not necessary to change the vehicle over, though it is very important to be aware of which configuration a vehicle has for situations like boosting or battery charging.

Initially when contemplating this vehicle it appeared that someone had mistakenly installed the battery backwards. In the case of a positive earth vehicle with a dynamo/generator this would hardly be noticeable in terms of operation, other than the ammeter behaving backwardly, which this one was. The less obvious but important effect would be reversed polarity at the spark plugs which causes a 15% loss in spark heat along with faster deterioration of the ignition system.

The challenge in determining if this was actually the case is that the coil is the key to the whole thing, modern coils are marked with a (+) and (-) side whereas older ones are labelled (sw) and (cb) for switch and contact breaker. Positive and negative do not change, simply hook the coil up correctly for the ground of the vehicle, if positive ground then negative comes from the switch (ignition) and positive goes to the contact breaker (distributor); if negative ground then positive comes from the switch (ignition) and negative goes to the contact breaker (distributor), both will ensure a negative centre of the spark plug (coils are an amazing technology…see links to research resources at the bottom of this post).

But given the above, (sw) would then be (-) on a positive ground vehicle and (+) on a negative ground vehicle. Similarly (cb) would be (+) on a positive ground vehicle and (-) on a negative ground vehicle. There is a way to determine the direction of current at the spark plug with an analog voltmeter, or careful observation of the spark flare using a pencil (though not always so easy to see clearly).

The clues: would have been wired positive ground when new, had a positive ground wiring harness professionally installed in 2019, appeared to have original coil, sw/cb era coils are generally labeled for positive earth, but: battery currently installed for negative ground, coil wiring was reversed, ammeter was operating backwards (which is incorrect), spark appeared to flare towards block (which is correct), wire from coil to distributor appeared to be a replacement.

Though there is still a small degree of ambiguity, in the end it was decided the most likely scenario was that vehicle had been correctly switched to negative ground because some further cleaning revealed a “12 65” date stamped on the coil, meaning it was very likely original and made for a positive ground rover, and was therefore correctly reversed to have positive from the ignition switch going in to (cb) and negative out from (sw) to the distributor.

Coil markings: 12V, HA12, 451320, 12 65, LUCAS, sw, cb, Made in England.

The final piece that was left undone for some reason was that the ammeter wires had not been swapped to read correctly. A relatively simple and quick operation requiring only a flat screwdriver and a bit of care and the ammeter now reads correctly.

Links to resources for research: