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HOW THE SIDE AND REAR LIGHTS IN VEHICLES WORK

In addition to headlamps, most vehicles are legally required to have front side lights, rear lights, stop lights, rear reflectors, flashing directions indicators and a light to illuminate the rear number plate. New vehicles must now be fitted with a high-intensity rear fog lamp. All lights should be in full working order. Many vehicles incorporate 5-watt front side lights in the headlamp shells. Where the side light is separate, it is usually housed with a21-watt flashing direction indicator bulb in a light unit. The side light is behind the diffusing white lens, and the indicator bulb is behind an amber lens.

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At the rear, a typical lamp assembly will include a 21-watt direction indicator bulb behind an amber lens, a 21-watt reversing light bulb behind a diffused white lens, and probably a combined stop\rear lamp bulb with 5-watt and 21-watt filaments. The red lenses will incorporate reflectors. New cars may also have a separate high-intensity rear fog lamp, usually fitted with a 21-watt bulb behind a red lens.

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On most cars, the side, rear, and number plate lights arecontrolled by a single switch that is wired independently of the ignition, sothat the lights can be left ON when the car is parked. This switch will alsoprovide the supply to the instrument lighting, sometimes through a separateswitch and with a dimmer control which allows the intensity of instrument lighting to be varied.

The headlamps have three switches, an ON/OFF switch that is usually incorporated with the side lights switch, a dip/main beam switch that diverts current to the main filaments or dip filaments as required, and aspring-loaded switch that by-passes all other lighting switches and operates the main beams for signaling. On many cars, the headlamp flasher switch and dip switch do not pass current to the headlamps themselves, rather they operate the relays.

A relay is a rapid electromagnetic switch. When fitted to a headlamp flasher circuit, it would be placed near the headlamps in the circuit between the battery and headlamp main-beam filaments. Operating the headlamp flasher switch sends a small current to a coil in the relay. This current generates a magnetic field and causes a soft iron flap, to move and close two contacts.

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The contacts feed the comparatively heavy current from the battery to the main beam filaments. With the help of the relay, current travels to themain beams by the shortest route from the battery, thus reducing the amount of resistance. This gives brighter lights, and allows the use of smaller contacts on the interior flasher switch.

The stop-lights receive their current from a circuit that is live only when the ignition is switched on, and work only when the brake pedal is depressed. Pedal movement works a button-type switch or a hydraulic switch in a brake-pipe union. Because there are usually two or more bulbs in each light unit and they share a common earth connection with the car body, it is possible for a bad connection between a twin filament bulb and its holder, or the light unit and the car body, to produce some strange occurrences.

A poor earth can cause the indicators to come on when the rear lights are switched on, alternatively, the side lights may come on when the brake pedal is pressed. What happens is that the lack of a nearby earth connection causes the current to seek out other routes to earth in order to complete the circuit.

Where these routes pass through a near by light filament, the filament will glow dimly during the process. The problem can be cured by ensuring that all earth connections to the body are sound, and that all bulbs are clean and making a good connection with their holders.

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Originally posted 2018-12-07 11:28:47.

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