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.

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.

An example of a two-bulb taillight circuit

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.

Operation of a brake light switch

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.

Brake light operation with the turn signals in the neutral position

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 filament of the dual-filament bulb, which is also used by the turn signal and hazard lights, is used for the brake lights (the high-intensity filament). In this type of circuit, the brake lights are wired through the turn signal and hazard switches. If neither turn signal is on, the current is sent to both of the brake lights. If the left turn signal is on, current for the right brake light is sent to the lamp through the turn signal switch terminal 5. The left brake light does not receive any voltage from the brake switch because the turn signal switch opens that circuit. The left-rear lamp will flash as the turn signal flasher provides pulsed voltage into switch terminal 3 and out terminal 8.

Brake light operation with the turn signal in the left-turn positio

FIGURE. Brake light operation with the turn signal in the left-turn position.

Note: Because the turn signal switches used in a two-bulb system also control a portion of the operation of the brake lights, they have a complex system of contact points. The technician must remember that many brake light problems are caused by worn contact points in the turn signal switch.

All brake lights must be red and, starting in 1986, the vehicle must have a center high mounted stop lamp (CHMSL). This lamp must be located on the center line of the vehicle and no lower than 3 inches below the bottom of the rear window (6 inches on convertibles). In a three-bulb system, wiring for the CHMSL is in parallel to the brake lights.

Wiring of a CHMSL in a three-bulb circuit

FIGURE. Wiring of a CHMSL in a three-bulb circuit.

In a two-bulb circuit, the CHMSL can be wired in one of two common methods. The first method is to connect to the brake light circuit between the brake light switch and the turn signal switch. This method is simple to perform. However, it increases the number of conductors needed in the harness.

Wiring the CHMSL into the two-bulb circuit between the brake light switch and the turn signal

FIGURE. Wiring the CHMSL into the two-bulb circuit between the brake light switch and the turn signal.

Two-bulb taillight circuit incorporating a CHMSL into the brake light system

FIGURE. Two-bulb taillight circuit incorporating a CHMSL into the brake light system.

The most common method used by manufacturers is to install diodes in the conductors that are connected between the left- and right-side bulbs. If the brakes are applied with the turn signal switch in the neutral position, the diodes will allow voltage to flow to the CHMSL. If the turn signal switch is placed in the left-turn position, the left light must receive a pulsating voltage from the flasher. However, the steady voltage being applied to the right brake light would cause the left light to burn steady if the diode was not used. Diode 1 will block the voltage from the right lamp, preventing it from reaching the left light. Diode 2 will allow this voltage from the right brake light circuit to be applied to the CHMSL.

Turn Signals

Typical turn signal switch location

FIGURE. Typical turn signal switch location.

Turn signals are used to indicate the driver’s intention to change direction or lanes. Hie driver will actuate a turn signal switch that is usually located in the steering column. In the neutral position, the contacts are opened, preventing current flow. When the driver moves the turn signal lever to indicate a left turn, the turn signal switch closes the contacts to direct voltage to the front and rear lights on the left side of the vehicle. When the turn signal switch is moved to indicate a right turn, the contacts are moved to direct voltage to the front and rear turn signal lights on the right side of the vehicle.

Turn signal circuit with the switch in the left-turn position

FIGURE. Turn signal circuit with the switch in the left-turn position.

A flasher is used to open and close the turn signal circuit at a set rate. With the contacts closed, power flows from the flasher through the turn signal switch to the lamps. The flasher consists of a set of normally closed contacts, a bimetallic strip, and a coil heating element. These three components are wired in series. As current flows through the heater element, it increases in temperature, which heats the bimetallic strip. The strip then bends and opens the contact points. Once the points are open, current flow stops. The bimetallic strip cools and the contacts close again. With current flowing again, the process is repeated. Because the flasher is in series with the turn signal switch, this action causes the turn signal lights to turn on and off.

The flasher uses a bimetallic strip and a heating coil to flash the turn signal lights

FIGURE. The flasher uses a bimetallic strip and a heating coil to flash the turn signal lights.

The hazard warning system is part of the turn signal system. It has been included on all vehicles sold in North America since 1967. All four turn signal lamps flash when the hazard warning switch is turned on. Depending on the manufacturer, a separate flasher can be used for the hazard lights than the one used for the turn signal lights. Hie operation of the hazard flasher is identical to that of the turn signal. The only difference is that the hazard flasher is capable of carrying the additional current drawn by all four turn signals. And, it receives its power source directly from the battery. Figure shows the current flow through the hazard warning system.

Current flow when the hazard warning system is activated

FIGURE. Current flow when the hazard warning system is activated.

Neon Third Brake Lights

Some vehicles use neon lamps for rear high-mount brake lights. These lights are more energy efficient and turn on more quickly than the regular lights. Behind the third brake light lens is a single neon bulb. Since neon bulbs have no filament, the neon bulb should last much longer than a regular bulb.

The neon bulbs turn on within 3 milliseconds after being activated. Halogen lamps require 300 milliseconds. The importance of this quickness is that it gives the driver behind the vehicle an earlier warning to stop. This early warning can give the approaching driver 19 more feet (6 meters) for stopping when driving at 60 miles per hour (96 kmh).

LED Exterior Lighting

LED taillight assembly

FIGURE. LED taillight assembly.

Many car manufacturers use LED lighting technology for several exterior lighting functions. The most common use of LEDs is in the CHMSL. Other uses include taillight assemblies, side marker lights, and turn-indicating outside mirrors. LEDs used in rearlighting applications (especially the CHMSL) provide one means of increasing traffic safety. Driver reaction times in response to the brake light function is shorter for CHMSLs equipped with conventional incandescent bulbs than for those equipped with LEDs. This is due to the shorter LED illumination time of less than one millisecond. Another advantage of LED lighting is the extended life of the LED compared to a bulb.

An example of the use of LED technology in rear-lighting systems occurs in the Cadillac STS. Each tail lamp assembly has thirty points of illumination by using two vertical boards, each board consisting of fifteen LEDs. Hie CHMSL is approximately 1/2 inch (12 mm) thick and consists of seventy-eight points of illumination.

Cornering Lights

Cornering lights are used to provide additional illumination during turns

FIGURE. Cornering lights are used to provide additional illumination during turns.

Cornering lights are lamps that illuminate when the turn signals are activated. They burn steady when the turn signal switch is in a turn position to provide additional illumination of the road in the direction of the turn. Vehicles equipped with cornering lights have an additional set of contacts in the turn signal switch. These contacts operate the cornering light circuit only. Hie contacts can receive voltage from either the ignition switch or the headlight switch. If the ignition switch provides the power, the cornering lights will be activated any time the turn signals are used. If the contacts receive the voltage from the headlight switch, the cornering lights do not operate unless the headlight switch is in the PARK or HEADLIGHT position.

Cornering light circuit powered through the ignition switch

FIGURE. Cornering light circuit powered through the ignition switch.

Fog Lights

For increased safety when driving in snow, sleet, heavy rains, and heavy fog conditions, some vehicles are equipped with fog lights. Fog lights can also be installed as an after-market accessory. Headlights will reflect off heavy, dense fog and cause a white haze that can reduce visibility. Fog lights emit a specialized beam to penetrate through the snow, rain, or fog, providing the driver with a better and safer field of vision.

Fog lights are installed on each side of the vehicle, generally low on the front fascia. Due to their mounting location, fog lights illuminate below the normal line of sight. This minimizes the amount of reflected light, to help the driver see better.

Typical fog light circuit

FIGURE. Typical fog light circuit.

Common fog light circuits use a relay. Also, most fog light circuits are wired so the fog lights will come on only if the headlight switch is in the PARK or Low beam positions.

Back-up Lights

Backup light circuit

FIGURE. Backup light circuit.

All vehicles sold in North America after 1971 are required to have back-up lights. Back-up lights illuminate the road behind the vehicle and warn other drivers and pedestrians of the driver’s intention to back up. Figure illustrates a back-up light circuit. Power is supplied through the ignition switch when it is in the RUN position. When the driver shifts the transmission into reverse, the back-up light switch contacts are closed and the circuit is completed.

Many vehicles equipped with automatic transmissions incorporate the back-up light switch into the neutral safety switch. Most manual transmissions are equipped with a separate switch. Either style of switch can be located on the steering column, on the floor console, or on the transmission. Depending on the type of switch used, there may be a means of adjusting the switch to assure that the lights are not on when the vehicle is in a forward gear selection.

A combination backup and neutral safety switch installed on an automatic transmission

FIGURE. A combination backup and neutral safety switch installed on an automatic transmission.

Side Marker Lights

Side marker lights are installed on all vehicles sold in North America since 1969. These lamps permit the vehicle to be seen when entering a roadway from the side. This also provides a means for other drivers to determine vehicle length. The front side marker light lens must be amber and the rear lens must be red. Vehicles that use wrap-around headlight and taillight assemblies also use this lens for the side marker lights. Vehicles that surpass certain length and height limits are also required to have clearance lights that face both to the front and rear of the vehicle.

Wrap-around headlights serve as side marker lights

FIGURE. Wrap-around headlights serve as side marker lights.

The common method of wiring the side marker lights is in parallel with the parking lights. Wired in this manner, the side marker lights would illuminate only when the headlight switch was in the PARK or HEADLIGHT position.

A side marker light wired across two circuits

FIGURE. A side marker light wired across two circuits.

Many vehicle manufacturers use a method of wiring in the side marker lights so that they flash when the turn signals are activated. The side marker light is wired across the parking light and turn signal light. If the parking lights are on, voltage is applied to the side marker light from the parking light circuit. Ground for the side marker light is provided through the turn signal filament. Because of the large voltage drop across the side marker lamp, the turn signal bulb will barely illuminate. In this condition, the side marker light stays on constantly.

Current flow to the side marker light with the parking light on

FIGURE. Current flow to the side marker light with the parking light on.

Side marker operation with the turn signal switch activated

FIGURE. Side marker operation with the turn signal switch activated.

Side marker light with the turn signal light and parking light on

FIGURE. Side marker light with the turn signal light and parking light on.

If the parking lights are off and the turn signal is activated, the side marker light receives its voltage source from the turn signal circuit. Ground for the side marker light is provided through the parking light filament. The voltage drop over the side marker light is so high that the parking light will not illuminate. The side marker light will flash with the turn signal light. If the turn signal is activated while the parking lights are illuminated, the side marker light will flash alternately with the turn signal light. When both the turn signal light and the parking light are on, there is equal voltage applied to both sides of the side marker light. There is no voltage potential across the bulb, so the light does not illuminate. Hie turn signal light turns off as a result of the flasher opening. Then the turn signal light filament provides a ground path and the side marker light comes on. The side marker light will stay on until the flasher contacts close to turn on the turn signal light again.