Chapter 9 Steering Systems 169
Steering Wheel Size
The steering wheel size has an effect on the effort
expended by the driver to turn the vehicle. The larger the
wheel, the less effort needed to turn it. This is due to the
leverage exerted by the larger wheel.
Steering Wheel Attachments
Steering wheels contain other parts, which are
discussed below. While not directly related to the steering
system, these parts may require removal when the steering
wheel is serviced.
Horn Switch
The horn switch, or horn pad, is always mounted on
the steering wheel, Figure 9-2. When an emergency
occurs, the driver’s hands are already on the steering
wheel, and little movement is required to reach and press
the horn switch. Some horn switches are easily observable
buttons, while others are built into the steering wheel and
are operated by squeezing or pressing the vinyl cover over
the wheel or the spokes.
A horn switch has electrical contacts that connect the
rotating steering wheel components to the rest of the
vehicle’s electrical system. Stationary brass or copper rings
in the steering column touch sliding or rolling contacts in
the wheel assembly. The sliding or rolling contacts brush
against the rings as the wheel is turned, maintaining
electrical contact at all times.
Air Bag
In most late-model vehicles, the center of the steering
wheel also contains an air bag, Figure 9-3. Air bag is a
general term given to devices that inflate to protect the
vehicle’s occupants during a collision. In most manufac-
turers’ service literature, however, an air bag is referred to
as a supplemental inflatable restraint, or SIR. The air bag
inflates rapidly (within 1/10 of a second) if the vehicle is
involved in a collision. Air bag inflation is often called
deployment.
In addition to the steering wheel-mounted air bag,
many vehicles have an additional air bag mounted in the
dashboard on the passenger side of the vehicle. Some
modern vehicles are equipped with side air bags, which
will deploy if the vehicle is struck from the side.
Most air bag systems are designed to deploy only if
the vehicle is involved in a frontal collision (within a
certain number of degrees of its centerline). During an
impact, electronic impact sensors, which are often
mounted in the front portion of the vehicle, tell the air bag
control module that a collision has occurred. The control
module then sends a current to an air bag squib. The squib
generates heat, igniting a flammable substance in the bag.
As the flammable substance burns, it creates a large
amount of a harmless gas. The gas expands and causes the
bag to inflate. The sequence of events that results in air bag
inflation is shown in Figure 9-4.
To prevent unnecessary air bag deployment, at
least two impact sensors must indicate a collision. Most
vehicles have at least one primary impact sensor, as
well as a safing sensor, which confirms a collision has
taken place. On some newer vehicles, the impact sen-
sors are mounted in the air bag control module in the
passenger compartment.
Air bag control modules often contain a reserve
power supply that allows the air bag system to operate
even if the rest of the vehicle loses electrical power. The
reserve power supply can retain power for up to 30 min-
utes after the ignition is switched off.
Most steering wheel–mounted air bags use a
clockspring to provide an electrical connection
between the steering wheel and the steering column.
See Figure 9-5. The clockspring consists of a flat wire,
sometimes called a ribbon wire, wound into a coil. The
wire coil is called a clockspring because it resembles the
mainspring of a mechanical clock. One end of the clock-
spring is attached to the air bag system electrical
connector in the stationary steering column. The other end
of the clockspring is attached to the air bag electrical
connector in the steering wheel. The clockspring coils and
Horn pad/air bag
Air bag wire lead
Wire connector
Steering
wheel nut
Air
bag
Horn wire
Steering wheel
Figure 9-2. The horn buttons or pad is mounted on the steering
wheel. (Jeep)