Every time you drive your car you rely on the electrical system. Find out more about what makes up your car's electrical system and how it all works together.
The battery stores electricity and supplies current to the car's electrical system when needed.
- The alternator/generator would normally fulfil this need when the engine is running but there are occasions when the power is needed and the engine is not running.
- An example of this would be when the starter motor is operated or the parking lights or radio used when parked with the engine off.
- Most cars have a 12 volt battery, which is really six separate two volt cells joined together.
- Each cell has a positive and negative terminal and the positive of one is connected to the negative of the next to give 12 volts.
- The individual cells contain a number of plates, divided by separators, and immersed in sulphuric acid.
- The positive plates are coated in lead peroxide and the negative cells in spongy lead.
- When current is drawn from the battery a chemical reaction takes place, which produces electricity.
- When the alternator/generator is charging the battery the chemical reaction happens in reverse.
- If current is continually drawn from the battery without recharging then it will go flat and will need to be charged from an outside source (battery charger).
- Check the level of the battery regularly and top up with distilled water as necessary.
- Keep the terminal connections tight and free from corrosion.
- Clean up any acid spills immediately.
- When replacing a battery always disconnect the negative terminal first and reconnect last.
- Don't mix the terminals up as this can damage the electrical system.
- If you are unsure about which connection goes where, call a qualified technician.
Older cars have a generator, sometimes called the dynamo, but all modern cars have an alternator - a more efficient way of charging the battery.
- The alternator is light and smaller and charges at lower engine speeds.
- A fanbelt drives the alternator; it doesn't generate electricity until the engine is running.
- A magnetic field called a rotor is at the centre of the alternator.
- There are conductors fitted to the side of the alternator called the stator.
- When the engine runs the rotor spins and produces electricity.
- This electricity is then sent to recharge the battery and feed current to the electrical system.
The starter motor is a small electric motor fitted with a gear called a pinion. The pinion engages with teeth on the flywheel (ring gear) and turns the crankshaft. When the ignition key is turned to the start position, the starter motor solenoid is engaged and current is drawn from the battery. This causes the starter motor to turn and engages the pinion with the ring gear. When the engine starts turning faster than the starter motor, the pinion disengages.
The ignition switch directs the current to the ignition system, the starter motor solenoid and the fuse box to feed the various accessories.
All cars have a selection of instruments and gauges.
- Every car has a speedometer, which can be operated by a cable from the transmission or wheels, or can be electrically operated by a transducer from the transmission.
- The transducer generates electrical current, the faster it spins the more current it generates and the speedometer converts this current into movement of the speedometer needle and odometer.
- A variable resistor in the fuel tank, which is attached to a float, operates the fuel gauge.
- Current is fed through the gauge to the resistor.
- The amount of resistance varies with the level of fuel in the tank and this determines where the needle on the gauge will sit.
- Remember that the fuel gauge is just an indication of the fuel level not an accurate analysis of much fuel is present.
- The temperature gauge is mounted in the instrument cluster and is operated by a sending unit mounted in a water jacket in the cylinder head.
- It works much the same as the fuel gauge, except that the resistance is varied by engine temperature rather than movement of a float.
- When the engine is cold the resistance in the sending unit is high and a small amount of current flows through the gauge.
- As the engine heats up the resistance becomes less and the needle in the gauge rises.
- The instrument cluster also contains warning lights for various electrical systems.
- Most lights work on the principle that when the circuit is inoperative the light is on.
The electrical current that operates the lights is drawn from the alternator when the engine is running, and the battery when the engine is running too slow for the alternator to produce enough current, or the engine is stopped.
- Most of the light globes have a single filament like a house globe, which heats up and glows when current is applied.
- However there are some exceptions; most headlamp globes have two filaments, which heat at different rates.
- The main beam glows brighter than the dipped beam.
- The brake light and rear parking light are often incorporated into the same globe. This globe also has two filaments.
- The indicators are operated through a flasher unit, which quickly switches on and off the current to the globes.
- A switch that is attached to the brake pedal operates the brake light.
- When the pedal is depressed the switch comes on and operates the lights, when the pedal is released the switch turns off.
The wipers are operated by a small electric motor. When the wipers are switched on, current is sent to the motor and this moves a set of linkages, which are attached to the wiper arms. Different wiper speeds are achieved by varying the voltage to the wiper motor.
Wiring distributes the current from the battery to all the electrical components. It also carries signals from sensors and switches. You may have noticed that all the wires in your car are not the same thickness. This is because some wires carry a large amount of current, like the connection between the battery and the starter motor and some carry very little current like the wires to the fuel injectors. A large current going through a small wire will cause it to overheat and melt. It is important to select the right size wire.
Fuses protect the wiring; they form a bridge in the electrical circuit. If the circuit overloads for any reason the fuse will blow and break the circuit. Most cars have more than one fuse panel. Your owner's handbook will indicate the position of the fuses and the size of fuse used for each circuit. The fuse contains a small wire inside, which is designed to melt before the wiring in the circuit melts. A visual check of the fuse will tell you if it is blown. You must always replace blown fuses with the correct amperage replacement. If a fuse blows a second time within a short period then have the vehicle checked by a qualified auto electrician.
There are many accessories on modern cars that are operated by the electrical system. All of the circuits are fused and if you experience a fault the first thing to check is the fuse.