How car electrical systems work
The electrical system of a vehicle is a closed circuit having an independent power source the battery. It uses a small fraction of the power of a household circuit.
A typical electrical system
Other than the main charging, starting and ignition circuits, there are other circuits that power lights, electric motors, the sensors and gauges of electrical instruments, heating elements, magnetically operated locks, the radio and others.
All Circuits are opened and closed either through switches or by relays – remote switches operated by electromagnets.
Current flows across a single cable from the battery to the component being powered, and then back to the battery through the car’s metal body. The body is connected to the earth terminal of the battery through a thick cable.
In a negative (-) earth-return system, the current flows from the positive (+) terminal to the component that is being operated. The component is earthed to the car’s body, which is earthed to the negative (-) terminal of the battery.
This type of circuit is known as earth-return system any part of it connected to the car body is said to be earthed.
The strength of the current is measured in the unit of amperes (amps); the pressure that drives it round the circuit is known as voltage (volts). Modern cars have a battery of 12 volts. Its capacity is measured in amp per hours. A 56 amp per hour battery should be able to deliver a current of 1 amp for 56 hours, or 2 amps for 28 hours.
If there is battery voltage drop, less current flows, and which is not enough to make the components work.
Current, voltage and resistance
The extent to which a wire resists the flow of current is known as resistance, and is measured in ohms.
Thin wires conduct less easily as compared to thick ones, because there is less room for the electrons to travel through.
The energy required to push current through a resistance is transformed into heat. This can be helpful, for example 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 with the help of wires which are too thin, or the wires will overheat, blow a fuse, or burn out.
All the electrical units of measurement are interrelated: a pressure of 1 volt results in a current of 1 amp to flow through a resistance of 1 ohm. Volts divided by ohms equal to amps. For instance, a light bulb with a resistance of 3 ohms, in a 12 volt system, consumes 4 amps.
This means it must be connected using wires thick enough to carry 4 amps easily.
Often the power consumption of a component will be stated in watts, which are found by multiplying amps with volts. The lamp in the example intakes 48 watts.
Positive and negative polarity
Electricity flows from a battery only in one direction, and some components work only when the flow through them is in the correct direction.
This acceptance of a one-way flow is known as polarity. Usually, the negative () battery terminal is earthed and the positive (+) one feeds the electrical system in the car.
This is known as a negative earth system, and while buying an electrical accessory a radio, check that it is of a type suitable for your car’s system. Fitting a radio with the incorrect polarity will harm the set, but most car radios have an external switch for setting the polarity to suit that of the vehicle. Switch to the correct setting before starting the fitting process.
Short circuits and fuses
If the wrong-sized wire is used, or if a wire is broken or disconnected, this can cause an accidental short circuit which around the resistance of the component. The current in the wire may become dangerously high and melt the wire or may cause a fire.
Fuse box Relay for electric fan Flasher unit
The fuse box is usually located in a cluster of components, as illustrated here. The box is shown after taking off the cover.
To guard this, ancillary circuits have fuses.
The most common type of fuse is a short length of thin wire enclosed in a heatproof casing which is usually glass.
The size of the fuse wire is the thinnest that can carry the normal current of the circuit without overheating, and it is rated in amperes.
The sudden surge of high current in a short circuit melts the fuse wire, or blows breaking the circuit.
When this takes place, see if there is a short circuit or a disconnection and if there is one, then install a new fuse of the correct amperage rating.
There are a lot of fuses, each protecting a small group of components, so that one blown fuse does not lead to the shutdown of the whole system. Many of the fuses are grouped together in a fuse box, but there might also be line fuses in the wiring.
Series and parallel circuits
A circuit usually has more than one component, like bulbs in the lighting circuits. It matters whether they are connected in series one after another or in parallel side by side.
For example, a headlamp bulb, is designed to have a degree of resistance so that it consumes a certain current to glow normally.
But there are always at least two headlamps in the circuit. If they were connected in series, electric current would have to go through one headlamp to get to the second one.
The current would encounter the resistance twice, and the double resistance would have halve the current, so that the bulbs would glow only with low energy.
Connecting the bulbs in parallel means that electricity goes through each bulb once.
Some components need to be connected in series. For instance, the sender in the fuel tank varies its resistance in accordance to the amount of fuel in the tank, and ‘sends’ a small electrical current to the fuel gauge.
The two components are connected in series in such a way that the varying resistance in the sender will affect the position of the needle on the gauge.
The starter motor has its own heavy cable, directly coming from the battery. The ignition circuit furnishes the high-tension impulses towards the sparkplugs; and the charging system has the generator, which recharges the battery. All the other circuits are known as ancillary (subsidiary) circuits.
Most are wired through the ignition switch, so that they work only when we switch on the ignition.
This prevents you from accidentally leaving something switched on which might cause the battery to go flat.
The side and tail lights, however, which you may need to leave turned on when the car is parked, are always wired independently of the ignition switch.
While fitting extra accessories like a rear window heater which consumes a heavy current, always wire it through the ignition switch.
Some ancillary components can be operated without the ignition being turned on by turning the switch to the ‘auxiliary’ position. A radio is usually wired through this particular switch, so that it can even be played with the engine off.
Wires and printed circuits
The instrument connections to this printed circuit are removed by squeezing the integral catches on every end.
Wire and cable sizes are classified by the maximum amperes that they can carry safely.
A complex network of wires runs throughout the car. To avoid confusion, each wire is coded in colour (but only within the car: there is no national or international system of colour-coding).
Most car handbooks and service manuals have a wiring diagram which can be difficult to follow.
However, the colour-coding is a useful guide for tracing wiring.
Where wires run side-by-side they all bound in a bundle, in a plastic or fabric sheath, to keep them tidy and less difficult to fit.
This bundle of wires stretches over the length of the car, with the help of single wires or small groups of wires emerging where necessary, and is known as the wiring loom.
Modern cars usually need room for many wires in confined spaces so some manufacturers now use printed circuits instead of bundles of wires, concisely at the rear of the instrument panel.
Printed circuits are plastic sheets on which copper tracks have been ‘printed’. Components are plugged directly into the tracks.
A few of the modern cars have flexible printed circuits. The copper tracks are printed in ribbons of flexible plastic, which change the whole wiring system.
Different types of Electrical Switches in a Car are-