chap46 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 46, the reader should
be able to:

•
•
•

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

List the safety precautions for working around an airbag.
Describe the procedures to diagnose and troubleshoot
airbag faults.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
airbag • arming sensor
clockspring
deceleration sensor • dual-stage airbags
event data recorder (EDR) • event file
integral sensor
occupant detection systems

Continued
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KEY TERMS:
passenger presence system (PPS) • pretensioners
squib • supplemental air restraints (SAR) • supplemental
inflatable restraints (SIR) • supplemental restraint system
(SRS)
unbelted

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

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SAFETY BELTS AND RETRACTORS
Safety Belts Used to keep the driver and passengers secured to the
vehicle in the event of a collision. Most safety belts include threepoint support and are constructed of nylon webbing.

The support points include two points on either side of the seat for
the belt over the lap and one crossing over the upper torso, which
is attached to the “B” pillar or seat back.
Every crash has three types of collisions:
Collision 1—The vehicle strikes another vehicle or object.
Collision 2—The driver and/or passengers hit objects inside
the vehicle if unbelted.
Collision 3—The internal organs of the body hit other organs
or bones, which causes internal injuries.

Continued

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

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If a safety belt is being worn, the belt stretches, absorbing a lot of
the impact, thereby preventing collision with other objects in the
vehicle and reducing internal injuries.
Figure 46–1 (a) Safety belts are the primary restraint system. (b) During a collision the
stretching of the safety belt slows down the impact to help reduce bodily injury.

Continued
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Belt Retractors Safety belts are also equipped with one of the
following types of retractors:
Nonlocking retractors, which are usually only used on
recoiling lap belts
Emergency locking retractors, which lock the position of
the safety belt in the event of a collision or rollover
Emergency and web speed-sensitive retractors, which allow
freedom of movement for the driver and passenger but lock
if the vehicle is accelerating too fast or if the vehicle is
decelerating too fast.

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

By James D. Halderman

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See Figure 46–2 for an example of an inertia-type seat belt locking
mechanism.
Safety Belt Lights and Chimes All late-model vehicles are
equipped with a safety belt warning light on the dash and a chime
that sounds if the belt is not fastened. See Figure 46–3.
Some vehicles will intermittently flash the reminder light and sound
a chime until the driver and sometimes the front passenger fasten
their safety belts.

Continued
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Figure 46–2 Most safety belts have an
inertia-type mechanism that locks the
belt in the event of rapid movement.

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

Figure 46–3 A typical safety belt warning
light.

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PRETENSIONERS
A pretensioner is an explosive (pyrotechnic) device that tightens
the seat belt as the airbag is being deployed. The purpose of the
pretensioning device is to force the occupant back into position
against the seat back and to remove any slack in the seat belt.
See Figure 46–4.
Pretensioners are explosive devices that could be ignited if
voltage is applied to the terminals.
Do not use a jumper wire or powered test light around the wiring
near the seat belt latch wiring. Always follow the vehicle
manufacturer’s recommended test procedures.
Continued
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CAUTION: The seat belt pretensioner assemblies must be replaced in
the event of an airbag deployment. Always follow the vehicle
manufacturer’s recommended service procedure. Pretensioners are
explosive devices that could be ignited if voltage is applied to the
terminals. Do not use a jumper wire or powered test light around the
wiring near the seat belt latch wiring. Always follow the vehicle
manufacturer’s recommended test procedures.

Figure 46–4 A small explosive
charge forces the end of the seat
belt down the tube, which removes
any slack in the seat belt.

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AIRBAGS
Airbag passive restraints are designed to cushion the driver (or
passenger, if the passenger side is so equipped) during a frontal
collision. Airbags may be known by many different names
including the following:
1. Supplemental restraint system (SRS)
2. Supplemental inflatable restraints (SIR)
3. Supplemental air restraints (SAR)
Most airbags are designed to supplement the safety belts in the
event of a collision, and front airbags are meant to be deployed only
in the event of a frontal impact within 30 degrees of center.
Continued
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Operation
Parts included in a typical airbag
system include:

1. Sensors
2. Airbag (inflator) module
3. Clockspring wire coil in
the steering column
4. Control module
5. Wiring and connectors
Figure 46–5 A typical airbag system
showing many of the components.

Force required to deploy a typical
airbag is equal to the force of a
vehicle hitting a wall at over 10
miles per hour (16 km/hr).
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 46–6
A simplified airbag deployment circuit. Note that
both the arming sensor and at least one of the
discriminating sensors must be activated at the
same time. The arming sensor provides the
power and either one of the discriminating

sensors can provide the ground for the circuit.

To cause inflation, the arming
sensor is required to close, to
provide the power-side voltage
to the inflator module.
Before the airbag can inflate,
the squib circuit also must
have a ground.
The ground is provided
through the actuation of either
the forward or the passenger
discriminating sensor.
Two sensors must be
triggered at the same time
before deployment.
Continued

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

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Types of Airbag Inflators Two different types of inflators used:
1. The Solid fuel type uses sodium azide pellets and when ignited,
generates a large quantity of nitrogen gas that quickly inflates
the airbag. First type used; still common. The squib heating
element ignites the gas-generating material. It takes about 2
amps to heat the element and ignite the inflator.
2. The Compressed gas type, commonly used in passenger-side
airbags and roof-mounted systems, this system uses a canister
filled with argon gas, plus a small percentage of helium at 3,000
psi(435 kPa). A small igniter ruptures a burst disc to release the
gas when energized. The compressed gas inflators are long
cylinders that can be installed inside the instrument panel, seat
back, door panel, or along any rail or pillar of the vehicle.
See Figures 46–7 and 46–8.
Continued
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Figure 46–7 Lifting the squib from the
airbag housing. The squib is the heating
element that ignites the pyrotechnic gas
generator that rapidly produces nitrogen
gas to fill the airbag.

Figure 46–8 This shows a
deployed side-curtain airbag on a
training vehicle.

Continued
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Once the inflator is ignited, the nylon bag quickly inflates (in about
30 milliseconds [ms] or 0.030 seconds) with nitrogen gas generated
by the inflator.
During an actual frontal collision accident, the driver is being
thrown forward by the driver’s own momentum toward the steering
wheel. The strong nylon bag inflates at the same time. Personal
injury is reduced by the spreading of the stopping force over the
entire upper-body region.
Sensors All three sensors are switches that complete an electrical
circuit when activated. The sensors are similar in construction and
operation, and the location of the sensor determines its name. All
airbag sensors are rigidly mounted to the vehicle and must be
mounted with the arrow pointing toward the front of the vehicle to
ensure that the sensor can detect rapid forward deceleration.
Continued
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There are three basic styles (designs) of airbag sensors:
1. The Magnetically retained gold-plated ball sensor. This
sensor uses a permanent magnet to hold a gold-plated steel
ball away from two gold-plated electrical contacts. See
Figure 46–9. If the vehicle (and the sensor) stops rapidly
enough, the steel ball is released from the magnet and
makes contact with the two gold-plated electrodes.
2. The Rolled up stainless-steel ribbon-type sensor. This
sensor is housed in an airtight package with nitrogen gas
inside to prevent harmful corrosion of the sensor parts. See
Figure 46–10. If the vehicle (and the sensor) stops rapidly,
the stainless-steel roll “unrolls” and contacts the two goldplated contacts.
Continued
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Figure 46–9 An airbag magnetic sensor.

Figure 46–10 Some vehicles
use a ribbon-type crash sensor.

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3.

The Integral sensor. Some vehicles use electronic
deceleration sensors built into the inflator module.
These sensors measure the rate of deceleration and,
through the computer logic, determine if the airbags
should be deployed.
See Figure 46–11 for a timeline that illustrates how quickly a
typical airbag system can react and inflate.

CAUTION: In the event of a collision that causes the airbag to deploy,
some vehicle manufacturers require that all sensors be replaced along with
the airbag assembly. The force of impact can cause unseen damage inside
the sensor; the sensor may not work correctly if used again.

Continued
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Figure 46–11 Notice that within 1/4 second of a collision, the sensors have closed, the airbag
has deployed, and the airbag has deflated.

Continued
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Wiring By worldwide agreement, all electrical wiring for airbags is
yellow. To ensure proper electrical connection to the inflator module
in the steering wheel, a coil assembly is used in the steering column.
This coil is a ribbon of copper wires that operates much like a
window shade when the steering wheel is rotated. This coil, called
an SIR coil or a clockspring, prevents the lack of continuity
between the sensors and the inflator assembly that might result from
a horn-ring type of sliding conductor.
See Figure 46–12.

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Figure 46–12
The airbag control module is
linked to the power train control
module (PCM) and the body
control module (BCM) on this
DaimlerChrysler system.
Notice the airbag wire
connecting the module to the
airbag through the clockspring.

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Most airbag systems also contain a diagnostic unit that often
includes an auxiliary power supply, which is used to provide the
current to inflate the airbag if the battery is disconnected from the
vehicle during a collision.
This auxiliary power supply usually uses capacitors that are
discharged through the squib of the inflation module.
See Figures 46–13 and 46–14.
Troubleshooting The electrical portion of most airbag systems is
constantly checked by the circuits within the airbag-energizing
power unit or through the vehicle’s computer system. If continuity
exists, a small voltage drop will be measured by the testing circuits.
If an open or short circuit occurs, a dash warning light is lighted and
a possible diagnostic trouble code (DTC) is stored.
Continued
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Figure 46–13 An airbag being deployed as part of a
demonstration in an automotive laboratory.

Continued

Figure 46–14 An airbag diagnostic tester. Included in the
plastic box are electrical connectors and a load tool that
substitutes for the inflator module during troubleshooting.

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