Exterior lighting is controlled by the headlight switch, which is connected directly to the battery on most vehicles. If the light switch is left on, it can drain the battery. Most headlight switches contain a built-in circuit breaker. If excessive current flows through the headlight circuit, the circuit breaker will momentarily open the circuit, then close it again. The result is headlights that flicker on and off rapidly.
Continued NOTE: This flickering on and off is misunderstood by many drivers and technicians. Because the flickering is rapid, many people believe that the problem is caused by a loose headlight or by a defective voltage regulator.
This feature allows the headlights to function, as a safety measure, in spite of current overload. The headlight switch controls the following lights on most vehicles:
Front parking lights
Interior (dome) light(s)
NOTE: Because these lights can easily drain the battery if accidentally left on, many newer vehicles control these lights through the computer. The computer keeps track of the time the lights are on and can turn them off if the time is excessive. The computer can control either power side or ground side of the circuit usually through an electronic control module.
The number used on automotive bulbs is called the bulb trade number, as recorded with the American National Standards Institute (ANSI), and the number is the same regardless of the manufacturer. Amber-color bulbs that use natural amber glass are indicated with an “ NA ” for natural amber at the end of the number (for example, #1157NA). A less expensive amber bulb that uses painted glass is labeled “ A ” for amber (for example, #1157A). See Figure 42–1
Figure 42–1 Bulbs that have the same trade number have the same operating voltage and wattage. The NA means that the bulb uses a natural amber glass ampoule with clear turn signal lenses. Continued
The trade number also identifies size, shape, number of filaments, and amount of light produced, measured in candlepower . Candlepower of a #1156 bulb, common for backup lights, is 32. A #194 bulb, for dash or side-marker lights, is rated at 2 candlepower. The amount of light produced by a bulb is determined by the resistance of the filament wire, which also affects the amount of current (in amps) required by the bulb. See Figures 42–2 and 42–3. It is important that the correct trade number of bulb always be used for replacement to prevent circuit or component damage. The correct replacement bulb for a vehicle is usually listed in the owner’s manual or service manual. See Figure 42–4 and the Bulb Chart.
Figure 42–2 This single-filament bulb is being tested with a digital multimeter set to read resistance in ohms. The reading of 1.3 ohms is the resistance of the bulb when cold. As soon as current flows through the filament, the resistance increases about 10 times. It is the initial surge of current flowing through the filament when the bulb is cool that causes many bulbs to fail in cold weather as a result of the reduced resistance. As the temperature increases, the resistance increases. Continued
Figure 42–3 Close-up a dual-filament (double-filament) bulb (#1157) that failed. Notice that one filament (top) broke from its mounting and melted onto the lower filament. This bulb caused the dash lights to come on whenever the brakes were applied. Continued
Figure 42–4 Bulbs #1157 or #2057 are typically used for taillight and front parking lights. These bulbs contain both a low-intensity filament for taillights or parking lights and a high-intensity filament for brake lights and turn signals.
TYPICAL AUTOMOTIVE LIGHT BULBS See the complete chart on Page 460 of your textbook.
All of these problems were caused by just one defective 2057 dual filament bulb, as shown in Figure 42–5. Apparently, the two filaments were electrically connected through the corrosion observed between the terminals of the bulb. This caused the electrical current to feed back from the brake light filament into the taillight circuit, causing all the problems. See Figure 42–6 for another example of a weird bulb problem.
A General Motors minivan had the following electrical problems: Weird Problem — Easy Solution
The turn signals flashed rapidly on the left side.
With the ignition key off, the lights-on warning chime sounded if the brake pedal was depressed.
When the brake pedal was depressed, the dome light came on.
Figure 42–5 Corrosion caused the two terminals of this dual-filament bulb to be electrically connected. Figure 42–6 Often the best diagnosis is a thorough visual Inspection. This bulb was found to be filled with water, which caused weird problems.
Many automotive bulbs have the same operating parameters (same wattage, voltage, amperage, and candlepower) yet have different trade numbers. Some numbers are for standard duty, others have heavier filament wire or additional filament support, which qualifies them for a different trade number. Fleet-duty designation represents a durability increase; heavy-duty designation identifies the most severe service bulb. Heavy-Duty Automotive Bulbs Therefore, if the specification for your vehicle gives a trade number under the “regular” heading, you can safely switch to a bulb listed under the “fleet-duty” or “heavy-duty” heading. For best operation of turn signals and consistent brightness of bulbs, the switch of trade numbers should include all similar bulbs of the type being replaced. See the bulb equivalence chart on Page 462 of your textbook.
Brake lights use the high-intensity filament of a double-filament bulb. (The lower intensity filament is for the taillights.) The brake light switch is a normally open (N.O.) switch but is closed when the driver depresses the brake pedal. Since 1986, all vehicles sold in the United States have a third brake light commonly referred to as the center high-mounted stop light ( CHMSL ). The brake switch is also used as an input switch (signal) for the following:
Cruise control (deactivates when brake pedal is depressed)
Antilock brakes (ABS)
Brake shift interlock (prevents shifting from “park” position unless the brake pedal is depressed)
A common cause of an inoperative cruise control, especially on General Motors vehicles, is a burned out bulb in the third stop light. The cruise control uses the filaments of the third brake bulb (CHMSL) as a ground and shuts off the cruise if the bulbs are burned out (open). See Fig. 42-7. No Cruise Control? Check the Third Brake Light
Figure 42–7 Typical brake light and taillight circuit showing the brake switch and all of the related circuit components. See the schematicon Page 463 of your textbook.
The headlight switch operates the exterior and interior lights of most vehicles. On non-computer-controlled lighting systems, the headlight switch is connected directly to the battery through a fusible link and has continuous power or is “hot” all the time. A circuit breaker is built into most headlight switches to protect the headlight circuit. The interior dash lights can be dimmed manually by rotating the headlight switch knob, which controls a variable resistor ( rheostat ) built into the headlight switch. The headlight switch also contains a built-in circuit breaker that will rapidly turn the headlights on and off in the event of a short circuit. This prevents a total loss of headlights.
Figure 42–8 Typical headlight circuit diagram. Note that the headlight switch is represented by a dotted outline indicating that other circuits (such as dash lights) also operate from the switch.
If headlights are rapidly flashing on and off, check entire circuit for possible shorts.
The circuit breaker controls only the headlights. The other lights controlled by the headlight switch are fused separately. Flashing headlights may be caused by failure in the built-in circuit breaker, requiring replacement of the switch assembly. Continued
Removing a Headlight Switch Most dash-mounted headlight switches can be removed by first removing the dash panel. To get the dash panel off, the headlight switch knob usually has to be removed. Some knobs can be removed by depressing a small clip in a notch in the knob itself. Others are removed by depressing a spring-loaded release, which allows for removal of the entire headlight switch knob and shaft, as shown in Figure 42–9 Headlight switches mounted on the steering column are removed as part of the turn signal and wiper switch assembly. Many can be easily removed, whereas others require the removal of the steering wheel and so forth. See service info for year and model on which you are working.
Figure 42–9 To remove the headlight switch from a vehicle that uses a knob and shaft, a release button has to be pushed to release the shaft. After the knob and shaft assembly has been removed, then the retaining nut can be removed from the headlight switch so it can be removed from the dash.
Low-beam headlights contain two filaments: one for low beam and the other for high beam. High-beam headlights contain only one filament and have two terminals. Because low-beam headlights also contain a high-beam filament, the entire headlight assembly must be replaced if either filament is defective.
Figure 42–10 Typical headlight socket connections. Some vehicles may be different. The high- and low-beam connections must be determined by visual inspection.
Sealed-beam headlights can be tested with an ohmmeter.
A good bulb should indicate low ohms between the ground terminal and both power-side (hot) terminals. If either the high-beam or the low-beam filament is burned out, the ohmmeter will indicate infinity (OL).
According to U.S. federal law, all headlights, regardless of shape, must be able to be aimed using headlight aiming equipment. See Figures 42–12 and 42–13. Also see the photo sequence on headlight aiming at the end of the chapter.
Continued Figure 42–11 All vehicles sold in the United States must have provision for the use of mechanical aiming devices. Even the halogen bulb units with plastic or glass lenses have locating points and adjustment screws.
Figure 42–12 Typical headlight-aiming diagram as found in a service manual. See the diagramon Page 465 of your textbook.
Figure 42–13 Many composite headlights have a built- in bubble level to make aiming easy and accurate.
Composite headlights are constructed using a replaceable bulb and fixed lens cover that is part of the vehicle. The replaceable bulbs are usually bright halogen bulbs, which get between 500° and 1300°F (260° and 700°C) during operation.
Figure 42–14 Typical composite headlamp assembly. The lens, housing, and bulb sockets are usually included as a complete assembly. Never touch the glass of any halogen bulb with bare fingers because natural oils of the skin can cause the bulb to break when it heats during normal operation.
Halogen sealed-beam headlights are brighter and more expensive than normal headlights. Because of their extra brightness, it is common practice to have only two headlights on at any one time, because the candlepower output would exceed the maximum U.S. federal standards if all four halogen headlights were on. Before trying to repair a problem that only two of four lamps are on, check the owner’s or shop manuals for proper operation.
CAUTION: Do not attempt to wire all headlights together. Extra current flow could overheat wiring from the headlight switch through the dimmer switch and to the headlights. The overloaded circuit could cause a fire.
Halogen bulbs fail for various reasons. Cause for halogen bulb failure and their indications are: Diagnose Bulb Failure
Gray color—low voltage to bulb (check for corroded socket or connector)
White (cloudy) color—indication of air leak
Broken filament—usually caused by excessive vibration
Blistered glass—indication that someone has touched the glass
Never touch the glass ampoule of any halogen bulb. The oils from your fingers can cause unequal heating of the glass during operation, leading to a shorter-than-normal service life. Figure 42–15 Notice the broken filament in this halogen headlight bulb.
Parts and Operation High-intensity discharge ( HID ) headlights produce a distinctive blue-white light that is crisper, clearer, and brighter than light produced by a halogen headlight. High-intensity discharge lamps do not use a filament like conventional electrical bulbs, but rather contain two electrodes about 0.2 inch (5 mm) apart. A high-voltage pulse is sent to the bulb, which arcs across the tips of electrodes producing light.
Continued Unlike a halogen bulb, the HID bulb has no filament. It creates light from an electrical discharge between two electrodes in a gas-filled arc tube. It produces twice the light with less electrical input than conventional halogen bulbs.
Figure 42–16 The ignitor contains the ballast and transformer needed to provide high-voltage pulses to the arc tube bulb.
HID lighting system consists of the discharge arc source, igniter, ballast, and headlight assembly.
Two electrodes are contained in a tiny quartz capsule filled with xenon gas, mercury, and metal halide salts. HID headlights are also called Xenon headlights . Lights and support electronics are expensive, but they should last the life of the vehicle unless physically damaged. Continued
HID headlights produce a white light, usually low in red giving the lamp a blue-white color. The color of light is expressed in temperature using the Kelvin scale. Kelvin ( K ) temperature is the Celsius temperature plus 273 degrees. Color temperatures include:
Figure 42–17 HID (Xenon) headlights emit a whiter light than halogen headlights and usually look blue compared to the parking light on the side.
The HID ballast is powered by 12 volts from the headlight switch on the body control module. The HID headlights operate in three stages or states:
The temperature of the light indicates the color of the light. The brightness of the light is measured in lumens. A standard 100-watt incandescent light bulb emits about 1,700 lumens. A typical halogen headlight bulb produces between 1,000 and 2,000 lumens and a typical HID bulb produces about 2,800 lumens. What is the Difference Between the Temperature of Light and the Brightness of Light? Continued
Start-up or Stroke State When the headlight switch is turned to the on position, the ballast may draw up to 20 amperes at 12 volts. The ballast sends multiple high-voltage pulses to the arc tube to start the arc inside the bulb. The voltage provided by the ballast during the start-up state ranges from 600 volts to +600 volts, which is increased by a transformer to about 25,000 volts. The increased voltage is used to create an arc between the electrodes in the bulb.
Run Up State After the arc is established, the ballast provides a higher than steady state voltage to the arc tube to keep the bulb illuminated. On a cold bulb, this state could last as long as 40 seconds. On a hot bulb, the run-up state may last only 15 seconds. The current requirements during the run-up state are about 360 volts from the ballast and a power level of about 75 watts.
Continued Steady State The steady state phase begins when the power requirement of the bulb drops to 35 watts. The ballast provides a minimum of 55 volts to the bulb during steady state operation.
Bi-Xenon Headlights Some vehicles are equipped with bi-xenon headlights, which use a shutter to block some of the light during low-beam operation and then mechanically move to expose more of the light from the bulb for high-beam operation. Because xenon lights are relatively slow to start working, vehicles equipped with bi-xenon headlights use two halogen lights for the “flash-to-pass” feature. Failure Symptoms indicating bulb failure:
A flickering light
The lights go out (this is caused when the ballast assembly detects repeated bulb restrikes)
Color change—color of the lamp may change to a dim pink glow
Diagnosis and Service High-intensity discharge headlights will change slightly in color with age. This color shift is usually not noticeable unless one headlight arc tube assembly has been replaced due to a collision repair, and then the difference in color may be noticeable.
If the arc tube assembly is near the end of its life, it may not light immediately if it is turned off and then back on immediately. This test is called a “hot restrike” and if it fails, a replacement arc tube assembly may be needed or there is another fault, such as a poor electrical connection, that should be checked.
WARNING: Always adhere to all warnings because the high-voltage output of the ballast assembly can cause personal injury or death.
Parts and Operation A system that mechanically moves the headlights to follow the direction of the front wheels is called adaptive ( or advanced ) front light system , abbreviated AFS . The AFS provides a wide range of visibility during cornering. The headlights are usually capable of rotating 15 degrees to the left and 5 degrees to the right (some systems rotate 14 degrees and 9 degrees).
Continued Figure 42–18 Adaptive front lighting systems rotate the low-beam headlight in the direction of travel.
Figure 42–19 A typical adaptive front lighting system uses two motors—one for the up-and down movement and the other for rotating the low-beam headlight to the left and right.
The vehicle has to be moving above a predetermined speed, usually above 20 mph (30 km/h), and the lights stop moving when the speed drops below about 3 mph (5 km/h).
AFS is used in addition to self-leveling motors so the headlights remain properly aimed regardless of how the vehicle is loaded. Continued
In a vehicle that is equipped with an adaptive front lighting system, the lights are moved by the headlight controller outward, and then inward, as well as up and down as a test of the system. This action is very noticeable to the driver and is normal operation of the system.
NOTE: These angles are reversed on vehicles sold in countries that drive on the left side of the road, such as Great Britain, Japan, Australia, New Zealand, and others. Continued
Daytime running lights ( DRLs ) involve operating front parking lights or the headlights (reduced current and voltage) whenever the vehicle is running. Studies show that DRLs reduce accidents. Daytime running lights primarily use a control module that turns on either the low- or high-beam lamps. Some vehicles turn on lamps when the engine is running but delay operation until a signal from the vehicle speed sensor indicates the vehicle is moving. To avoid having the lights on during servicing, some systems will turn off the headlights whenever the parking brake is applied. Others will only light headlights when the vehicle is in drive gear. See Figure 42-21
Figure 42–21 Typical daytime running light (DRL) circuit. Follow the arrows from the DRL module through both headlights. Notice that the left and right headlights are connected in series, resulting in increased resistance, less current flow, and dimmer than normal lighting. When the normal headlights are turned on, both headlights receive full battery voltage, with the left headlight grounding through the DRL module.
The headlight switch controls the power or hot side of the headlight circuit. The dimmer switch can be either foot operated on the floor or hand operated on the steering column. The popular steering column switches are actually attached to the outside of the steering column on most vehicles and are spring loaded. To replace most of these types of dimmer switches, the steering column needs to be lowered slightly to gain access to the switch itself, which is also adjustable for proper lever operation.
Figure 42–22 Most vehicles use positive switching of the high- and low-beam headlights. Notice that both filaments share the same ground connection. Some vehicles use negative switching and place the dimmer switch between the filaments and the ground.
Current is sent to the dimmer switch, which allows current to flow to either high-beam or the low-beam filament of the headlight bulb.
An indicator light illuminates on the dash whenever the high beams are selected.
When the brakes are applied, the brake switch is closed and the stop lamps light. In many vehicles, the stop and turn signals are both provided by one filament. When the turn signal switch is closed, the filament receives current through the flasher unit. When brakes are applied, the current flows to the turn signal switch. The high-mounted stop is fed directly from the brake switch. If neither turn signal is on, current through the turn signal switch flows to both rear brake lights. If the turn signal switch is operated, current flows through the flasher unit on the side activated and directly to the brake lamp on the opposite side. If the brake pedal is not depressed, then current flows through the flasher and only to one side.
Figure 42–23 The brake lights are powered directly from the brake switch on a vehicle that uses separate bulbs for brake lights and turn signals.
The brake switch receives current from a fuse that is hot all the time.
The turn signal circuit, is controlled by a fuse that is ignition switch controlled. When the turn signal switch is moved in either direction, corresponding turn signal lamps receive current through the flasher unit.
Figure 42–24 The typical turn signal switch includes various springs and cams to control the switch and to cause the switch to cancel after a turn has been completed.
The turn signal switch is mounted within the steering column and operated by a lever. Moving the lever up or down completes the circuit through the flasher unit and the appropriate turn signal lamp
A turn signal switch includes cams and springs that cancel the signal after the turn has been completed. As the steering wheel is turned in the signaled direction and returns to its normal position, the cams and springs separate the turn signal switch contacts. Continued
In systems using separate filaments for the stop and turn lamps, the brake and turn signal switches are not connected. If the vehicle uses the same filament for both purposes, then brake switch current is routed through contacts within the turn signal switch. See Figure 42–25. By linking certain contacts, the bulbs can receive either brake switch current or flasher current, depending upon which direction is being signaled. Figure 42–26 shows current flow through the switch when the brake switch is closed and a right turn is signaled. Steady current through the brake switch is sent to the left brake lamp. Interrupted current from the turn signal is sent to the right turn lamps.
Figure 42–25 When stop lamps and turn signals share a common bulb filament, stop light current flows through the turn signal switch. Continued
Figure 42–26 When a right turn in signaled, the turn signal switch contacts send flasher current to the right-hand filament and brake switch current to the left-hand filament.
Light-emitting diode ( LED ) brake lights are frequently used for high-mounted stop lamps (CHMSL) for several reasons including : Why are LEDs Used for Brake Lights? Faster illumination. An LED will light up to 200 milliseconds faster than an incandescent bulb, which requires time to heat the filament before it is hot enough to create light. This faster illumination can mean the difference in stopping distances at 60 mph (100 km/h) by about 18 feet (6 meters) due to the reduced reaction time for the driver of the vehicle behind. Longer service life. LEDs are solid-state devices that do not use a filament to create light. As a result, they are less susceptible to vibration and will often last the life of the vehicle. Aftermarket replacement LED bulbs that are used to replace conventional bulbs may require the use of a different type of flasher unit due to the reduced current draw of the LED bulbs. See Figure 42–27.
Figure 42–27 A replacement LED taillight bulb is constructed of many small individual light-emitting diodes. NOTE: Some vehicles are equipped with outside mirrors that include an LED arrow that flashes with the turn signal. These LEDs are not service-able separately, but are serviced with the mirror as an assembly.
A turn signal flasher is a metal or plastic can containing a switch that opens and closes the turn signal circuit. The unit is usually installed in a metal clip attached to the dash panel to allow the “clicking” noise of the flasher to be heard by the driver. The turn signal flasher is designed to transmit the current to light the front and rear bulbs on only one side at a time. The U . S . Department of Transportation ( DOT ) regulation requires that the driver be alerted when a turn signal bulb is not working. This is achieved by using a series-type flasher unit. The flasher unit requires current flow through two bulbs (one in the front and one in the rear) in order to flash. If one bulb burns out, the current flow through only one bulb is not sufficient to make the unit flash; it will be a steady light.
Figure 42–28 Two styles of two-prong flashers.
These turn signal units are often called DOT flashers. When the turn signal flasher unit is old, the lights will flash more slowly (both sides affected equally). The contact points inside the flasher unit may become corroded and pitted, requiring higher voltage to operate. To restore normal operation, replace the signal flasher unit.
Bimetallic Flashers Bimetallic flashers units were the first type used and have generally been replaced with other types that have a longer service life. A bimetallic flasher unit uses strips of two different metals bonded together. Because the two metals expand at different rates, when electrical current flows through the strip, it bends and when current stops, the strip cools and unbends. At the end of the strip is a contact which makes and breaks the electrical turn signal circuit. If one bulb burns out, either in the front or rear of the vehicle, the flasher will not flash on the side with the bad bulb. Because it requires current to cause the metal strip to bend, not enough current is flowing through one bulb to make the strip bend. As a result, the driver is notified of a burned out bulb on one side and therefore meets the Federal requirement.
Hybrid Flasher A hybrid flasher, also called a flasher relay , is a type of flasher unit that contains an electronic timing circuit and a mechanical relay which opens and closes the turn signal circuit. This type of flasher is more expensive than the bimetallic-type flasher but it is longer lasting because the timing and control of the turn signals is performed by two different parts inside one housing. The electronic circuit uses a timer circuit as well as a current sensing circuit. If the current sensing circuit detects a lower than normal current flow through the turn signals, it automatically flashes at a faster rate (twice as fast as normal). This faster flash rate indicates to the driver that a fault has been detected in the turn signal circuit and meets the DOT requirement. If all bulbs are burned out, the flasher will not flash at all.
Solid-State Flashers Solid state flashers use an electronic switching circuit and an electronic timing circuit to operate the turn signals. Because these units do not use a mechanical switch or contacts to open and close the turn signal circuit, these flashers last a long time, but cost more than bimetallic or hybrid units. Similar to hybrid flashers, solid state flashers use a current sensing circuit and will flash rapidly if a bulb is burned out.
Continued NOTE: If the turn signals do not operate at all, check the bulbs before replacing the turn signal flasher unit.
Hazard Warning Flasher The hazard warning flasher is a device installed in a vehicle lighting system with the primary function of causing the left and right turn signal lamps to flash when the hazard warning switch is activated. Secondary functions may include visible pilot indicators for the hazard system and an audible signal to indicate when the flasher is operating. See Figure 42-29. Combination Turn Signal and Hazard Warning Flasher The combination flasher is a device that combines the functions of a turn signal flasher and a hazard warning flasher into one package.
Figure 42–29 A hazard flasher uses a parallel resistor across the contacts to provide a constant flashing rate regardless of the number of bulbs used in the circuit.
A typical hazard warning flasher is also called a parallel or variable-load flasher because there is a resistor in parallel with the contacts to provide a control load, and therefore, a constant flash rate, regardless of the number of bulbs being flashed.
Electronic Flasher Replacement Units Older vehicles (and a few newer ones) use thermal (bimetal) flashers that use heat to switch on and off. Newer vehicles use electronic flashers that use microchips to control the on/off function. Electronic flashers are compatible with older systems, but thermal flashers are not compatible with new systems. Eventually a thermal flasher will burn out (it may be 100,000 miles or more), and will be replaced. Electronic flashers do not burn out, and they provide a crisper and faster “flash” of the turn signals. Another reason to use electronic flashers might be if upgrading to LED taillamps, or lights. These newer LED bulbs only work with electronic flashers.
The easiest way to know which type of flasher can be used is to look at the type of bulb used in the taillamps and turn signals. If it is a “wedge” style (plastic base, flat and rectangular), the vehicle has an electronic flasher. If it is a “twist and turn” bayonet-style bulb, then either type of flasher can be used.
Puddle lights are lights on the bottom of the outside rear-view mirror that shine downward when the door is opened. They are called puddle lights because they light the area where the driver and passenger are going to step, thereby illuminating any hazards such as water puddles. What Are Puddle Lights?
Most turn signal flasher units are mounted in a metal clip that is attached to the dash. The dash panel acts as a sounding board, increasing the sound of the flasher unit. Most four-way hazard flasher units are plugged into the fuse panel. Some two-way turn signal flasher units are also plugged into the fuse panel. How do you know for sure where the flasher unit is located? With both the turn signal and the ignition on, listen and/or feel for the clicking of the flasher unit. Some service manuals also give general locations for the placement of flasher units.
A question that service technicians are asked frequently is why the side marker light alternately goes out when the turn signal is on and is on when the turn signal is off. Some vehicle owners think that there is a fault with their vehicle while actually it is normal operation. The side-marker light goes out whenever the lights are on and the turn signal is flashing because there are 12 volts on both sides of the bulb (see points X and Y in Figure 42–30). Normally, the side-marker light gets its ground through the turn signal bulb. Why Does the Side-Marker Light Alternately Flash?
Figure 42–30 The side-marker light goes out whenever there is voltage at both point Xand Y. These opposing voltages stop current flow through the side-market light. The left turn light and left park light are actually the same bulb (usually a #2057) and are shown separately to help explain how the side-marker light works on many vehicles.
Courtesy light is a generic term primarily used for interior lights, including overhead (dome) and under-the-dash (courtesy) lights. These interior lights can be operated by rotating the headlight switch knob fully counterclockwise (left) or by operating switches located in the doorjambs of the vehicle doors. There are two types of circuits commonly used for interior lights. Most manufacturers, except Ford, use the door switches to ground the courtesy light circuit. See Figure 42–31.
Figure 42–31 A typical courtesy light doorjamb switch. Newer vehicles use the door switch as an input to the vehicle computer and the computer turns on or off the interior lights. By placing the lights under the control of the computer, vehicle engineers have opportunity to delay lights after the door is closed and shut them off after a period of time to avoid draining the battery.
Many Fords use door switches to open and close the power side of the circuit.
Newer vehicles operate the interior lights through the computer or through an electronic module. Exact wiring and operation of these units differ Consult service literature for exact model on which you are working.
The switches in the doorjamb simply signal the computer that a door has been opened. The computer controls lighting to help prevent accidental battery drain. For example, if the vehicle door has been left open, the computer can open the circuit and prevent a dead battery. The schematic rarely shows exactly how the circuit works. However, the service information walks you through the diagnosis. With a service manual, or computerized information service, the technician is never lost. Always follow the procedures exactly! Even if the service procedure sounds long and involved, the procedure will lead you to the correct diagnosis. Service Manual Diagnosis
Some vehicles are equipped with illuminated entry, the interior lights are turned on for a given amount of time whenever the outside door handle is operated while the doors are locked. Most vehicles equipped with illuminated entry also light the exterior door keyhole. Some vehicles equipped with body computers use the door handle electrical switch of the illuminated entry circuit to “wake up” the power supply for the body computer.
Fiber optics is transmission of light through special plastic (polymethyl methacrylate) that keeps the light rays parallel even if the plastic is tied in a knot. These strands of plastic are commonly used in automotive applications as indicators for the driver that certain lights are functioning. Some vehicles are equipped with fender-mounted units that light whenever the lights or turn signals are operating. Plastic fiber-optic strands, which often look like electrical wire, transmit the light at the bulb to the indicator on top of the fender so that the driver can determine if a certain light is operating.
Fiber-optic strands also can be run like wires to indicate the operation of all lights on the dash or console. Fiber-optic strands are also commonly used to light ashtrays, outside door locks, and other areas where a small amount of light is required. The source of the light can be any normally operating light bulb, which means that one bulb can be used to illuminate many areas. A special bulb clip is usually used to retain the fiber-optic plastic tube near the bulb.
When current that lacks a good ground goes backward along the power side of the circuit in search of a return path (ground) to the battery, this reverse flow is called feedback or reverse-bias current flow. Feedback can cause other lights or gauges that should not be working, to work. Feedback Example A customer complained that when the headlights were on, the left turn signal indicator light on the dash remained on. The cause was a poor ground connection for the left front parking light socket. A corroded socket did not provide a good enough ground to conduct all current required to light the filament of the bulb
The front parking light bulb is a dual filament: one filament for the parking light (dim) and one filament for the turn signal operation (bright). The two filaments of the bulb share the same ground connection and are electrically connected. When all the current could not flow through the bulb’s ground in the socket, it caused a feedback or reversed its flow through the other filament, looking for ground. The turn signal filament is electrically connected to the dash indicator light; therefore, the reversed current on its path toward ground could light the turn signal indicator light. Cleaning or replacing the socket usually solves the problem if the ground wire for the socket is making a secure chassis ground connection.