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.

Interior Lights

Interior lighting includes courtesy lights, map lights, and instrument panel lights.

(adsbygoogle = window.adsbygoogle || []).push({});

Courtesy Lights

Courtesy lights using ground side switches

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.

Courtesy lights using insulated side switches

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.… READ THE REST

Exterior Lights

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.… READ THE REST

Flash to Pass

 Flash-to-pass feature added to the headlight circuit

(adsbygoogle = window.adsbygoogle || []).push({});

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.… READ THE REST

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.

Most limit switches operate off of a cam on the motor

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.… READ THE REST

Headlight Switches

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.… READ THE REST


There are four basic types of headlights used on automobiles today:

  1. standard sealed beam
  2. halogen sealed beam
  3. composite
  4. high-intensity discharge (HID)

Sealed-Beam Headlights

Sealed-beam headlight construction

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.… READ THE REST