Lighting Circuits: Summary

The most commonly thought of light circuit is the headlights. But there are many lighting systems in the vehicle. Different types of lamps are used to provide illumination for the systems. The lamp may be either a single-filament bulb that performs a single function, or a double-filament bulb that performs several functions. The headlight lamps can be one of four designs: standard sealed beam, halogen sealed beam, composite, or high-intensity discharge (HID). The headlight filament is located on a reflector that intensifies the light, which is then directed through the lens. The lens is designed to change the circular light pattern into a broad, flat light beam. Placement of the filament in the reflector provides for low- and high-beam light patterns. Some manufacturers use concealed headlights to improve the aerodynamics of the vehicle. The concealed headlight doors can operate from vacuum or by electrically controlled motors. Some systems incorporate the use of IС chips into the concealed headlight door control. In addition to the headlight system, the lighting systems include: Stop lights. Turn signals. Hazard lights. Parking lights. Taillights. Back-up lights. Side marker lights. Courtesy lights. Instrument panel lights. The headlight switch can be used as the control of many of these lighting systems. Most headlight switches have a circuit breaker that is an integral part of the switch. The circuit breaker provides protection of the headlight system without totally disabling the head-light operation if a circuit overload is present. A rheostat is used in conjunction with the headlight switch to control the brightness of the instrument panel illumination lights.

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Interior Lights

A rheostat controls the brightness of the instrument panel lights
Interior lighting includes courtesy lights, map lights, and instrument panel lights. Courtesy Lights FIGURE. Courtesy lights using ground side switches. Courtesy lights illuminate the vehicle's interior when the doors are open. Courtesy lights operate from the headlight and door switches and receive their power source directly from a fused battery connection. The switches can be either ground switch circuit or insulated switch circuit design. In the insulated switch circuit, the switch is used as the power relay to the lights. In the grounded switch circuit, the switch controls the grounding portion of the circuit for the lights. The courtesy lights may also be activated by the headlight switch. When the headlight switch knob is turned to the extreme counterclockwise position, the contacts in the switch close and complete the circuit. FIGURE. Courtesy lights using insulated side switches. Reading and Map Lights Individual switches and controls to allow passengers in the vehicle to turn on individual lights are incorporated within most courtesy light systems. The system shown has individual two-position switches that allow the passenger to turn on a light. When the switch is pressed, it completes the circuit to ground for that light only. Instrument Cluster and Panel Lights Consider the following three types of lighting circuits within the instrument cluster: Warning lights alert the driver to potentially dangerous conditions such as brake failure or low oil pressure. Indicator lights include turn signal indicators. Illumination lights provide indirect lighting to illuminate the instrument gauges, speedometer, heater controls, clock, ashtray, radio, and other controls. FIGURE. A rheostat controls the brightness of the instrument panel lights. The power source for the instrument panel lights is provided through the headlight switch. The contacts are closed when the headlight switch is located in the PARK or HEADLIGHT position. The current must flow through a variable resistor (rheostat) that is either a part of the headlight switch or a separate dial on the dash. The resistance of the rheostat is varied by turning the knob. By varying the resistance, changes in the current flow to the lamps control the brightness of the lights. Video: Mercedes-Benz S63 AMG amazing interior lighting

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Exterior Lights

An example of a two-bulb taillight circuit
When the headlight switch is placed in the PARK or HEADLIGHT position, the front parking lights, taillights, side marker lights, and rear license plate light are all turned on. The front parking lights usually use dual-filament bulbs. The other filament is used for the turn signals and hazard lights. Most taillight assemblies include the brake, parking, rear turn signal, and rear hazard lights. The center high mounted stop light (CHMSL), back-up lights, and license plate lights can be included as part of the taillight circuit design. Depending on the manufacturer, the taillight assembly can be wired to use single-filament or dual-filament bulbs. When singlefilament bulbs are used, the taillight assembly is wired as a three-bulb circuit. A three-bulb circuit uses one bulb each for the tail, brake, and turn signal lights on each side of the vehicle. When dual-filament bulbs are used, the system is wired as a two-bulb circuit. Each bulb can perform more than one function. Taillight Assemblies The headlight switch controls parking lights and taillights. They can be turned on without having to turn on the headlights. Usually, the first detent on the headlight switch is provided for this function. Figure illustrates a parking light and taillight circuit. This circuit is controlled by the headlight switch. Thus the lights can be operated with the ignition switch in the OFF position. FIGURE. An example of a two-bulb taillight circuit. In a three-bulb taillight system, the brake lights are controlled directly by the brake light switch. In most applications, the brake light switch is attached to the brake pedal. When the brakes are applied, the pedal moves down and the switch plunger closes the contact points and lights the brake lights. On some vehicles, the brake light switch may be a pressure-sensitive switch located in the brake master cylinder. When the brakes are applied, the pressure developed in the master cylinder closes the switch to light the lamps. FIGURE. Operation of a brake light switch. The brake light switch receives direct battery voltage through a fuse, which allows the brake lights to operate when the ignition switch is in the OFF position. Once the switch is closed, voltage is applied to the brake lights. The brake lights on both sides of the vehicle are wired in parallel. The bulb is grounded to complete the circuit. FIGURE. Brake light operation with the turn signals in the neutral position. Many brake light systems use dual-filament bulbs that perform multifunctions. Usually, the...

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Flash to Pass

Flash-to-pass feature added to the headlight circuit
FIGURE. Flash-to-pass feature added to the headlight circuit. Many steering column-mounted dimmer switches have an additional feature called "flash to pass." This circuit illuminates the high-beam headlights even with the headlight switch in the OFF or PARK position. In this illustration, battery voltage is supplied to terminal B1 of the headlight switch and on to the dimmer switch. Battery voltage is available to the dimmer switch through this wire in both the OFF and PARK positions of the headlight switch. When the driver activates the flash-to-pass feature, the contacts in the dimmer switch complete the circuit to the high-beam filaments.

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Concealed Headlights

Concealed headlights enhance the vehicle's styling and aerodynamics
FIGURE. Concealed headlights enhance the vehicle's styling and aerodynamics. A vehicle equipped with a concealed headlight system hides the lamps behind doors when the headlights are turned off. When the headlight switch is turned to the HEADLIGHT position, the headlight doors open. Early systems used vacuum-controlled doors. Today most systems use electric motors. FIGURE. Most limit switches operate off of a cam on the motor. Electrically controlled systems can use either a torsion bar and a single motor to open both headlight doors, or a separate motor for each headlight door. Most systems will use limit switches to stop current flow when the doors are full up or full down. These switches generally operate from a cam on the reaction motor. Only one limit switch can be closed at a time. When the door is full up, the opening limit switch opens and the closing limit switch closes. When the door is full down, the closing limit switch is open and the opening limit switch closes. This prepares the reaction motor for the next time that the system is activated or deactivated. FIGURE. An electrically controlled concealed headlight system with a manual control knob. The electrically operated concealed headlight system provides a provision for manually opening the doors in the event of a system failure. FIGURE. Pop-up headlight system wiring schematic. Figure illustrates a system that incorporates an integrated chip (1С). Each motor has its own relay and limiting switches. When the limit switches are in the A-B position, the doors are full open. When the switches are in the A-C position, the doors are full closed. FIGURE. Current flow with the headlight switch OFF and the headlight doors closed. Figure illustrates another method used to operate the electric motors of a concealed headlight system. When the ignition switch is in the RUN position but the headlight switch is off, current flows through the ignition switch to the relay. The relay is energized because the coil is grounded through the headlight filaments. With the coil energized, the relay points close. However, the door closing limit switch is open. This results in a de-energized door closing field winding. When the headlight switch is turned to the HEADLIGHT position, current continues to flow to the relay coil through the ignition switch. However, current is also sent to the other side of the relay coil from the headlight switch. Voltage is equal on both sides of the relay coil, so there is no...

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Headlight Switches

(A) Instrument panel-mounted headlight switch. (B) Steering column-mounted headlight swith
The headlight switch may be located either on the dash by the instrument panel or on the steering column. It controls most of the vehicle's lighting systems. The most common style of headlight switch is the three-position type with OFF, PARK, and HEADLIGHT positions. The headlight switch will generally receive direct battery voltage to two terminals of the switch. This allows the light circuits to be operated without having the ignition switch in the RUN or ACC (accessory) position. When the headlight switch is in the OFF position, the open contacts prevent battery voltage from continuing to the lamps. When the switch is in the PARK position, battery voltage that is present at terminal 5 is able to be applied through the closed contacts to the side marker, taillight, license plate, and instrument cluster lights. This circuit is usually protected by a 15- to 20-ampere fuse that is separate from the headlight circuit. When the switch is located in the HEADLIGHT position, battery voltage that is present at terminal 1 is able to be applied through the circuit breaker and the closed contacts to light the headlights. Battery voltage from terminal 5 continues to light the lights that were on in the PARK position. The circuit breaker is used to prevent temporary overloads to the system from totally disabling the headlights. FIGURE. (A) Instrument panel-mounted headlight switch. (B) Steering column-mounted headlight swith. FIGURE. Operation with the headlight switch in the PARK position. FIGURE. Operation with the headlight switch in the HEADLIGHT position. The rheostat is a variable resistor that the driver uses to control the instrument cluster illumination lamp brightness. As the driver turns the light switch knob, the resistance in the rheostat is changed. The greater the resistance, the dimmer the instrument panel illumination lights glow. In vehicles that have the headlight switch located in the steering column, the rheostat may be a separate unit located on the dash near the instrument panel. Dimmer Switches The dimmer switch provides the means for the driver to select either high- or low-beam operation, and to switch between the two. The dimmer switch is connected in series within the headlight circuit and controls the current path for high and low beams. In the past, the most common location of the dimmer switch was on the floor board next to the left kick panel. The driver operates this switch by pressing on it with a foot. Positioning the switch on the...

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Headlights

Sealed-beam headlight construction
There are four basic types of headlights used on automobiles today: standard sealed beam halogen sealed beam composite high-intensity discharge (HID) Sealed-Beam Headlights FIGURE. Sealed-beam headlight construction. From 1939 to about 1975, the headlights used on vehicles remained virtually unchanged. During this time, the headlight was a round lamp. The introduction of the rectangular headlight in 1975 enabled the vehicle manufacturers to lower the hood line of their vehicles. Both the round and rectangular headlights were sealed-beam construction. The sealed-beam headlight is a self-contained glass unit made up of a filament, an inner reflector, and an outer glass lens. The standard sealed-beam headlight does not surround the filament with its own glass envelope (bulb). The glass lens is fused to the parabolic reflector, which is sprayed with vaporized aluminum that gives a reflecting surface that is comparable to silver. The inside of the lamp is filled with argon gas. All oxygen must be removed from the standard sealed-beam headlight to prevent the filament from becoming oxidized. The reflector intensifies the light that the filament produces, and the lens directs the light to form the required light beam pattern. FIGURE. The lens uses prisms to redirect the light. The lens is designed to produce a broad, flat beam. The light from the reflector is passed through concave prisms in the glass lens. Lens prisms redirect the light beam and create a broad, flat beam. The illustration shows the horizontal spreading and the vertical control of the light beam to prevent upward glaring. FIGURE. The prism directs the beam into (A) a flat horizontal pattern and (B) downward. By placing the filament in different locations on the reflector, the direction of the light beam is controlled. In a dual-filament lamp, the lower filament is used for the high beam and the upper filament is used for the low beam. FIGURE. Filament placement controls the projection of the light beam. Halogen Headlights FIGURE. A halogen sealed-beam headlight with iodine vapor bulb. The halogen lamp most commonly used in automotive applications consists of a small bulb filled with iodine vapor. The bulb is made of a high-temperature-resistant quartz that surrounds a tungsten filament. This inner bulb is installed in a sealed glass housing. With the halogen added to the bulb, the tungsten filament is capable of withstanding higher temperatures than that of conventional sealed-beam lamps. The halogen lamp can withstand higher temperatures and thus is able to burn brighter. In a conventional sealed-beam headlight, the heating...

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