NEGATIVE EARTH

I clearly recall the first time I ever pulled out the wiring from behind a dashboard with a serious intent to try and do something with it, Haynes manual at the ready. Faced with all of that ageing automotive spaghetti and the worry of what might happen if I got it wrong, I was seriously tempted to cry chicken.

​With the help of a more experienced friend, a dab of logic and some patience, I realised that classic automotive wiring is fairly straightforward once you break the job down into manageable elements (individual circuits) and understand some basic principles.

Novices, worry not. I'm going to presume you know anything at all about wiring, and I'm starting with basic principles. Let's begin by looking at what is meant by negative and positive earth.

Realising that, in essence, all a component circuit is doing is switching either the negative or positive current from the battery and that the other side goes to earth, the whole thing will begin to seem a whole lot less intimidating.

 

Vehicles these days are wired "negative earth", but that wasn't always the case. There was no industry standard in the early days and it was entirely up to individual vehicle manufacturers to use whichever polarity they preferred. It was only with the advent of modern solid state components that a concerted shift was made  to negative earth.

In order to benefit from some modern components (such as electronic ignition, stereos, ammeters, or twelve volt phone chargers), many owners opt to reverse the polarity of their positive earth vehicles (which, before you ask, is very do-able, but needs a little advance planning before rushing into it).

Using the chassis as a common earthing point reduces the need for miles of extra cables (unless you have a car with a fibreglass unibody, of course!).

Negative earth circuitry at its most simplistic - the negative side of the battery is "earthed" (i.e. connected to the chassis). The positive side goes to a switch on your dashboard via a fuse, and then on to the component you want to switch on or off. The negative terminal on the component completes the circuit by also connecting to earth.

Basic automotive negative earth wiring diagram
Classic car dashboard wiring

Of course, it's seldom quite as simple as the diagram above, but that does sum it up in the proverbial nutshell.

 

If your vehicle is negative earth, you are switching the positive side of the cable coming from the battery, and negative side goes to earth.

 

If your vehicle is positive earth, you switch the negative side and take positive to earth.

The next step up from this basic circuit is to incorporate a relay in between your switch and the component. Relays are essentially another kind of switch, but ones that open and close when they get a signal from your dashboard or stick switches.

Relays are amazing bits of compact technology. They can turn the current from your battery from a constant feed into an intermittent one so that indicators can flash (which is why an indicator relay is often called a 'flasher can'). They can also help to moderate current so that wiring doesn't burn out.

Glossary of electrical terms

 

Ammeter - instrument to measure the current level (amps) in a circuit; useful for monitoring battery health

Ground : another term for earth (may be shortened to GND)

Live terminal : positive terminal

Polarity : the direction of current flow in an electrical circuit. There are two poles - negative and positive

Relay : a switch that opens/closes a circuit either electromechanically or electronically; often used to enable low amp switches and wires to work with higher amp components (such as headlights) without the risk of wires overheating and burning

Solid state : devices utilising solid semiconductors (such as transistors  and diodes) as opposed to valves

If you're tempted to play with your own heritage wiring, do take care. It truly isn't rocket science but, worst case scenario, if you get it wrong you could get hurt or have a very expensive bonfire.

J. F. M.

It's My Classic - the free online classic car and motorcycle magazine for people with a passion for heritage vehicles.