Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

350 Chapter Fifteen
Elastomeric
Seal
Igniter and
Housing Assembly
Filter/Cooling
Media
Figure 15-54.
Operations)
Intensifier
Assembly
Generant
Generant
Housing
Igniter assembly. (GM Service and Parts
Air Bag
Module
Steering Wheel
Vibration Damper
FWD Automatic
Transmission Only
Clock Spring
Electrical
Connector
Steering
Column
Figure 15-55. Liner and steering wheel trim cover.
(GM Service and Parts Operations)
Igniter Assembly (Figure 15-55)
Inflation of the air bag is caused by an explosive
release of gas. For the explosion to occur, a
chemical reaction must be started. The igniter
assembly does this when it receives a signal
from the air bag monitor. Actually, the igniter is
a two-pin bridge device: When the electrical
current is applied, it arcs across the two pins,
creating a spark that ignites a squib (canister
of gas) that generates zirconic potassium perchlorate
(ZPP). This material ignites the propellant.
Some newer model air bags now use solid
propellant and argon. This gas has a stable
structure, cools more quickly, and is inert as
well as non-toxic.
Inflator Module
The inflator module contains the ZPP. Once it
triggers the igniter, the propellant charge is progressive,
burning sodium azide, which converts
to nitrogen gas as it burns. It is the nitrogen gas
that fills the air bag.
Almost as soon as the bag is filled, the gas is
cooled and vented, deflating the assembly as the
collision energy is absorbed. The driver is cradled
in the envelope of the supplemental restraint
bag instead of being propelled forward to strike
the steering wheel or be otherwise injured by
follow-up inertia energy from seat belt restraint
systems. In addition, a certain degree of facial
protection against flying objects is obtained just
when it is needed.
It is important to remember that only the tandem
action of at least one main sensor and a safing sensor
initiates safety restraint system activation. The
micro-controller also provides failure data and
trouble codes for use in servicing various aspects
of most systems.
Mounting Plate and Retainer Ring
The mounting plate and retainer ring attach the
air bag assembly to the inflator. They also keep
the entire air bag module connected to the steering
wheel.
Liner and Steering Wheel Trim Cover
(Figure 15-56)
The liner houses the air bag; the trim cover goes
over the exterior of the steering wheel hub.
Passenger-side air bags are very similar in design
to the driver’s unit. The actual capacity of gas
required to inflate the bag is much greater because
the bag must span the extra distance between the
occupant and the dashboard at the passenger seating
location. The steering wheel and column
make up this difference on the driver’s side.
WARNING: When the air bag is deployed, a great deal of heat is generated. Although the heat
is not harmful to passengers, it may damage the clock spring electrical connector.When replacing
a deployed air bag module, examine all of the electrical connections for signs of scorching
or damage. If damage exists, it must be repaired.