chap42 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 42, the reader should
be able to:

•
•
•

Prepare for ASE Electrical/Electronic Systems (A6)
certification test content area “E” (Lighting System
Diagnosis and Repair).

Determine which replacement bulb to use on a given
vehicle.
Describe how interior and exterior lighting systems work.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 42, the reader should
be able to:

•
•

Read and interpret a bulb chart.
Discuss troubleshooting procedures for lighting and
signaling circuits.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
adaptive (or advanced) front light system (AFS)
brake lights
candlepower • center high-mounted stop light (CHMSL) • color
shift • composite headlight • courtesy lights
daytime running lights (DRLs) • feedback • fiber optics

hazard warning • high-intensity discharge (HID) • hybrid
flasher

Continued
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KEY TERMS:
Kelvin (K)
light-emitting diode (LED)
reference voltage (RF) • rheostat
trade number • Troxler effect
U.S. Department of Transportation (DOT)
xenon headlights

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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The lighting and signaling circuits represent two of the most
frequently serviced automotive electrical areas.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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LIGHTING
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.
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.

Continued
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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:
1.
2.
3.
4.
5.
6.

Headlights
Taillights
Side-marker lights
Front parking lights
Dash 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.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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BULB NUMBERS
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
Continued
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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.

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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.
Continued
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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
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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
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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.

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TYPICAL AUTOMOTIVE LIGHT BULBS

See the complete chart on Page 460 of your textbook.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Weird Problem—Easy Solution

A General Motors minivan had the following electrical problems:

•
•
•

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.

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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Heavy-Duty Automotive Bulbs
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.
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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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BRAKE LIGHTS
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:

1. Cruise control (deactivates when brake pedal is depressed)
2. Antilock brakes (ABS)
3. Brake shift interlock (prevents shifting from “park” position
unless the brake pedal is depressed)
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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No Cruise Control? Check the Third
Brake Light
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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 42–7 Typical brake light and taillight circuit showing the brake switch and all of the
related circuit components.

See the schematic
on Page 463 of
your textbook.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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HEADLIGHT SWITCHES
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.
Continued
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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 builtin circuit breaker, requiring
replacement of the switch
assembly.
Continued
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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.
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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.

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