If you are lucky enough to have an auto-electrician or are interested in the electrical health of your fleet, read on – it’s not as complex as you may think.

The answer to the question posed in the introduction to this article is simply because the negative earth contact gets overlooked for attention in the rush to get back on the road. And ‘resistance’ to operate can flow back from a trailer. A recent case proves the point where a Freightliner ran sweetly when uncoupled from a trailer’s electrics and ‘powered-down’ to idle when coupled to the trailer.

The force motivating electrons to ’flow’ in a circuit is called voltage. Voltage is a specific measure of the difference in potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point. Without reference to two particular points, ‘voltage’ has no meaning.

Free electrons tend to move through conductors with some degree of friction, or opposition to motion. This opposition to motion is more properly called resistance. The amount of current in a circuit depends on the amount of voltage available to motivate the electrons – and also the amount of resistance in the circuit to oppose electron flow. Just like voltage, resistance is a quantity relative between two points. For this reason, the quantities of voltage and resistance are often stated as being ‘between’ or ‘across’ two points in a circuit.

When electrons move against the opposition of resistance, ‘friction’ is generated. Just like mechanical friction, the friction produced by electrons flowing against a resistance manifests itself in the form of heat.

This heat energy is enough to cause a highly resistant lampfilament to glow white-hot, converting energy and producing light, whereas the wires connecting the lamp to the battery – having much lower resistance – hardly even get warm while conducting the same voltage.

The standard units of measurement for electrical current, voltage and resistance are:

• Voltage measured in volts, symbolized by the letters 'E' or 'V'.
• Current measured in amps, symbolized by the letter 'I'.
• Resistance measured in ohms, symbolized by the letter 'R'.

The first – and perhaps most important – relationship between current, voltage and resistance is called Ohm’s Law, discovered by Georg Simon Ohm and published in his 1827 paper, The Galvanic Circuit Investigated Mathematically. Ohm’s principal discovery was that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given temperature. Ohm expressed his discovery in the form of a simple equation, describing how voltage, current, and resistance interrelate:

V = I R
In this algebraic expression, voltage (V) is equal to current (I) multiplied by resistance (R).

The water-and-pipe analogy is an interesting way of explaining Ohm’s Law. A water pump that exerts pressure (voltage) to push water around a ‘circuit’ (current) through a restriction (resistance), models how the three variables interrelate.

If the resistance to water flow stays the same and the pump pressure increases, the flow rate must also increase. Voltage pressure is the shock but with consistent current flow, it’s amps that are the human body killer. An electric perimeter fence is a good example – it has very high voltage that provides a nasty shock but the low amperage means that there’s not sufficient current to sustain the shock into killer mode.

What all this means is that USA sourced trucks with 12v systems do not need resistance in the system, for example: where 12v = 4 amps ? 3 ohms. The higher the resistance the lower the amps and a steady amps current is needed to drive electronic systems. 24v systems handle a drop in amps more easily and thus appear to be more robust against maintenance neglect.

Minimising extraneous resistance in electric circuits is essential to electronic reliability. And it’s a vicious cycle – resistance in electrical systems builds up heat that in turn creates contact point oxidation where resistance occurs. This oxidation turns into more resistance and more heat that increases oxidation even further and so on.

So why bother about the negative earth terminal contacts installed all over trucks? The problem is perception – why unbolt a cable from a chassis when there appears to be no corrosion near the bolt? But on the other side of the fastening – where it makes contact to the chassis – there is corrosion build-up and at least annually these fastenings need to be loosened and wire-brushed to ensure perfect contact and minimal circuit resistance.

All light vehicles have 12v systems and are becoming more electronic by the day. Letting passive resistance build up in a fleet can stop the wheels.

The last word goes to root-causeanalysis expert, Patrick Swan, who experienced puzzling and unreliable behavior on his 4 X 4 VW Kombi Synchro. The vehicle would mysteriously cut out and then, when started again, would run erratically. Swan traced this down to a corroded negative earth contact that he cleaned and treated with copperimpregnated grease, such as Engen Copper Compound grease. Having overcome this point of passive resistance, 16 years later the Kombi still runs on annual holidays without missing a beat.

In conclusion it’s worth repeating that without reference to two particular points ‘voltage’ has no meaning.

Material for this article has been referenced from:
Lessons In Electric Circuits, Volume I – DC
By Tony R. Kuphaldt