Learner Zone
Motor Vehicle HubAutomotive Electrical
An automotive electrical system uses the alternator as the energy converter, batteries as the energy accumulators and the electrical equipment as consumers. The energy from the battery is supplied to the starter (consumer), which starts the engine.
Current flows along a single cable from the battery to the component being powered and back to the battery through the car’s metal body. The body is connected to the earth terminal of the battery by a thick cable. This type of circuit is called an earth-return system.
The strength of the current is measured in amps. The pressure that drives it around the circuit is the voltage. Modern vehicles use a 12-volt battery. If the battery voltage drops, less current flows, which means the components will not work.
current, voltage and resistance
The extent to which a wire resists the flow of current is called resistance. This is measured in ohms (Ω).
Thin wires conduct less than thick ones, because there is less room for the electrons to travel through.
The energy needed to push current through a resistance is transformed into heat. This can be useful in the very thin filament of a light bulb, which glows white hot. However, a component with a high current consumption must not be connected using such thin wires or they will overheat, blow a fuse, or burn out.
All the electrical units of measurement are interrelated. A pressure of 1 volt causes a current of 1 amp to flow through a resistance of 1 ohm. Volts divided by ohms equal amps.
Often the power consumption of a component will be stated in watts, which are calculated by multiplying amps and volts.
Familiarise yourself with the Ohm’s law triangle to better understand the way resistance, current and voltage relate to one another.
polarity
Electricity flows in one direction only, and some components work only if the flow through them is in the correct direction. Current flows from the positive pole (terminal) to the negative pole. Electrons flow from negative (-) to positive (+).
This acceptance of a one-way flow is called polarity. The negative (-) battery terminal is earthed and the positive (+) one feeds the electrical system. This is called a negative earth system, and when buying an electrical accessory one should check that it is of a type suitable for the car’s system. Fitting an accessory with the incorrect polarity will cause damage.
short circuits and fuses
If the wrong wire is used or if a wire breaks, this can cause an accidental short circuit which bypasses the resistance of the component. The current in the wire becomes dangerously high, which can melt the wire or cause a fire. To guard against this, ancillary circuits have fuses. The most common type of fuse is a short length of thin wire enclosed in a heatproof casing. Due its thin size, it can carry the normal current of the circuit without overheating.
There are many fuses, each protecting a small group of components, so that when one fuse blows it does not shut down the whole system. Many of the fuses are grouped together in a fuse box which is located among a cluster of components, as pictured.
series and parallel circuits
A circuit usually includes more than one component, such as bulbs. They can be connected in a series (one after the other) or parallel (side by side).
For example: a headlamp bulb is designed to have a degree of resistance so it consumes a certain current to glow normally. There are at least two headlamps in a circuit. If they were connected in a series, the current would have to go through one headlamp to get to the other. This means the current would encounter twice the resistance which halves the current, and the headlamps would glow weak. Connecting the bulbs in parallel means that electricity goes through each bulb once.
Some components must be connected in series. For example, the sender in the fuel tank varies its resistance according to the amount of fuel in the tank, and sends a small electrical current to the fuel gauge. These two components are connected in series so the resistance in the sender will affect the position of the gauge needle.
ancillary circuits
The starter motor has its own heavy cable direct from the battery. The ignition circuit furnishes the high-tension impulses to the spark plugs. The charging system includes the generator which recharges the battery. All the other circuits are called ancillary (subsidiary) circuits.
Most are wired through the ignition switch so that they work only when the ignition is switched on. This prevents someone accidentally leaving something switched on which causes the battery to go flat. Conversely, the side and tail lights are always wired independently of the ignition switch as these occasionally need to be left on when parked.
When fitting extra accessories that consume a heavy current, always wire it through the ignition switch.
Some ancillary components can be operated without the ignition turned on by turning the switch to the auxiliary position. A radio is usually wired through this switch, so that it can be played with the engine off.
Watch this short video to learn more about how the ignition system works.
wires and cables
Wire and cable sizes are classified by the maximum amperage that they can carry safely.
A complex network of wires runs through the car. To avoid confusion, each wire is colour coded. Most car handbooks and service manuals include a wiring diagram, and the colour codes are a useful guide to trace wiring.
Where wires run side-by-side they are bound together in a bundle, or in a plastic or fabric sheath to keep them tidy.
Wires stretch over the length of the car, with single wires or small groups of wires emerging where necessary. This is called the wiring loom.
charging and starting
Watch this short video to learn more about charging and starting systems.