FIGURE. A single-filament bulb. A lamp generates light through a process of changing energy forms called incandescence. The lamp produces light as a result of current flow through a filament. The filament is enclosed within a glass envelope and is a type of resistance wire that is generally made from tungsten. As current flows through the tungsten filament, it gets very hot. The changing of electrical energy to heat energy in the resistive wire filament is so intense that the filament starts to glow and emits light. The lamp must have a vacuum surrounding the filament to prevent it from burning so hot that the filament burns in two. The glass envelope that encloses the filament maintains the presence of vacuum. When the lamp is manufactured, all the air is removed and the glass envelope seals out the air. If air is allowed to enter the lamp, the oxygen would cause the filament to oxidize and burn up. FIGURE. A double-filament lamp. Many lamps are designed to execute more than one function. A double-filament lamp has two filaments so the bulb can perform more than one function. It can be used in the stop light circuit, taillight circuit, and turn signal circuit combined. It is important that any burned-out lamp be replaced with the correct lamp. The technician can determine what lamp to use by checking the lamp's standard trade number. TABLE. A TABLE OF SOME TYPICAL AUTOMOTIVE LIGHT BULB EXAMPLES.
Today's technician is required to understand the operation and purpose of the various lighting circuits on the vehicle. The addition of computers and their many sensors and actuators (some that interlink to the lighting circuits) make it impossible for technicians to just bypass part of the circuit and rewire the system to their own standards. If a lighting circuit is not operating properly, the safety of the driver, the passengers, people in other vehicles, and pedestrians are in jeopardy. When today's technician performs repairs on the lighting systems, the repairs must meet at least two requirements: They must assure vehicle safety and meet all applicable laws. The lighting circuits of today's vehicles can consist of more than 50 light bulbs and hundreds of feet of wiring. Incorporated within these circuits are circuit protectors, relays, switches, lamps, and connectors. In addition, more sophisticated lighting systems use computers and sensors. The lighting circuits consist of an array of interior and exterior lights, including headlights, taillights, parking lights, stoplights, marker lights, dash instrument lights, courtesy lights, and so on. The lighting circuits are largely regulated by federal laws, so the systems are similar between the various manufacturers. However, there are variations. Before attempting to do any repairs on an unfamiliar circuit, the technician should always refer to the manufacturer's service informations. This chapter provides information about the types of lamps used, describes the headlight circuit, discusses different types of concealed headlight systems, and explores the various exterior and interior light circuits individually.