Car Parts and Accessories

Current to a car’s electrical system is supplied by the battery when the engine is not running and by an alternator (on older cars a generator) when the engine is running. An alternator is fitted as it more easily supplies the current at lower speeds for the ever increasing number of electrical accessories in modern cars.

All the electrical supply is at the voltage of the battery (usually 12 volts) or the generator (approx. 14 volts), except the supply to the spark-plugs, which is boosted by the ignition system to as much as 60 000 volts as required.

One of the main functions of the electrical system is to provide ignition, in the form of a spark, to the compressed petrol/ air mixture in the cylinders. Another is to start the engine with the starter motor.

A car’s electrical system is divided into circuits, all with different basic functions and different controls. They are: the ignition circuit; the starter circuit; the charging circuit; the lighting circuit; and accessories circuits, which are sometimes controlled by the ignition switch, and in most cases are protected by a fuse. A blown fuse, more often than not, is a symptom of trouble elsewhere than in the fuse itself, such as the overloading of a circuit.

The electrical components in a car are connected through switches to one side of the battery. The other side of the battery is connected to the car body or chassis. In this way the circuit to any component is completed through the body of the car, which becomes one ‘wire‘ of the circuit, the earth return.

This method of earthing not only saves the cost of about 30 m of copper wire, but also reduces the possibility of disconnections and simplifies fault finding and the fitting of extras.

Different thicknesses of wire are used to enable the required current to pass without causing overheating of the wire. The connection between the starter motor and the battery, for instance, is extremely thick in comparison with the other wiring because it has to carry a much higher current—up to 300 to 400 amp.

Circuit diagrams in car handbooks and workshop manuals are not always easy to follow, although the symbols used are standard in most of them.

Diagrams bear little or no relationship to the actual positions of wiring or components in the car and, for reasons of space, are considerably abbreviated. But such diagrams show all accessories fitted as standard equipment.

Cable colours are usually indicated by letters, and a coding table is then given to permit easy identification. But remember electrical work is always finicky; take care.

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