chap89 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 89, the reader should
be able to:

• Prepare for ASE Suspension and Steering (A4)
•
•
•

certification test content area “A” (Steering
System Diagnosis and Repair).
Discuss steering columns and intermediate
shafts.
Explain how a recirculating ball-nut and worm
gear steering gear system works.
Describe how a rack-and-pinion steering gear
works.

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

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KEY TERMS:
collapsible column • flexible coupling • gear lash
inflator module • kickback • lock plate • overcenter
adjustment
pinion torque • pitman arm • pitman shaft • preload
rack and pinion • rack support
sector gear • sector shaft • steering shaft • stub shaft
telescoping steering column • tilt steering column
universal joint • variable ratio • worm gear
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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When the driver turns the steering wheel in a circle, or
in a rotary motion, the steering linkage moves side to
side, or in a lateral motion, to steer the wheels.

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STEERING WHEELS
The steering wheel, which consists of a rigid rim and a number
of spokes connecting the rim to a center hub, attaches to the
top of the steering shaft at its center.
Most steering wheel hubs have internal splines that fit over 

external splines on the steering shaft. A bolt or nut at the
center of the hub secures the wheel to the shaft. 
The steering wheel may also contain controls for the cruise 
control and audio controls, as well as the driver’s airbag.

5

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Horn Operation The horn circuit is in a series circuit in which 
current has one path that it can follow when the circuit is complete.
A normally open switch in an electrical circuit is inside the horn 

button. When the driver pushes the horn button, the contacts on the 
switch close, allowing electrical current through the circuit to 
operate the horn.
A relay circuit is a more common method of wiring the horn. In a 
relay, closing a switch in a low­current series circuit triggers the 
high­current circuit that powers the horn.
See Figure 89–1. 

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Figure 89–1 The horn button is a normally open (NO) switch. When the button is depressed, the
switch closes, which allows electrical current to flow from the battery to sound the horn. Most
horn circuits use a relay to conduct the horn current.

Continued
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Airbags  An airbag is made of nylon cloth covered with neoprene, 
folded and stored in the center of the steering wheel. In a front­end 
collision, the airbag inflates to provide a cushion between the 
driver and the steering wheel and dashboard. 

Figure 89–2 The airbag inflates at
the same time the driver moves
toward the steering wheel during a
front-end collision and supplements
the protection of the safety belt.

The part of the steering 
wheel where the airbag 
is stored is called the 
inflator module. 

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

The module also contains 
an igniter, a canister of 
flammable gas, and 
sodium azide pellets. 
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The sodium azide pellets burn quickly, and rapidly release nitrogen 
gas, which fills the airbag. As it inflates, the airbag tears open the 
module cover and spreads out across the steering wheel, 
windshield, and dashboard. 
The entire process, from sensor reaction to full airbag deployment, 
takes 30 to 65 milliseconds. Within one second of inflating, the bag 
deflates partially. Once deployed, an airbag cannot be reused.
Electrical current is provided to the airbag through the spiral cable, 
which is also known as a coil. The spiral cable is a tightly coiled 
metal strip that connects to the airbag module with two wire leads 
and allows steering wheel rotation while maintaining electrical 
continuity.
See Figure 89–3.
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CAUTION: Whenever working on the steering column, consult service 
information for the recommended airbag disabling procedure.

The module fits in front of the nut that secures the steering wheel to 
the steering shaft. Bolts at the back of the steering wheel fasten the 
airbag module to the steering wheel.
Figure 89–3 The airbag module attaches to the
steering wheel and is removed as an assembly
to service the steering wheel
and column.

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By James D. Halderman

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STEERING COLUMNS
The steering shaft transmits rotary motion from the steering wheel 
to the steering gear, while the column jacket that encases it 
attaches to the vehicle body and offers a stationary mounting 
point for a number of switches and mechanisms.

Figure 89–4
The steering shaft links the steering wheel to the
steering gear while the column jacket, which surrounds
part of the shaft, holds support brackets and switches.
This steering shaft has a small intermediate section
between the main section and the steering gear.
Continued
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Steering Shaft  The steering shaft extends from steering wheel to 
steering gear. A bolt or nut secures the shaft to the steering wheel, 
and a flexible coupling joins it to the steering gear input shaft. 
Figure 89–5 A pot joint is a flexible coupling used
to join two shafts that allow plunging motion.

The coupling can be a rubber or fabric insert, a universal joint
(U­joint), or a pot joint. A In pot joint permits a directional
change of the shaft, and a limited plunging motion. 
Continued
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Universal Joint  A U­joint or universal joint consists of two 
yokes with a steel crosspiece joining them together. 
Universal joints allow changes in the angle between two rotating 
shafts. In a steering shaft, U­joints allow rotary motion transfer 
between the steering wheel and the steering gear even though the 
steering shaft meets the steering gear input shaft at an angle.
On some models the steering shaft itself is assembled in sections 
that are connected by U­joints. This permits the steering shaft
to bend around obstacles between the steering wheel and the 
steering gear.
See Figure 89–6. 

13

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Figure 89–6
A typical intermediate steering shaft assembly showing a U-joint and related components.

Continued
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Flexible Coupling  A flexible coupling is a simple device made 
of rubber, or rubber reinforced with fabric, that is placed between 
two shafts to allow for a change in angle between them.
The rubber in a steering shaft flexible coupling absorbs vibrations 
and helps keep the steering wheel from shaking. A fail­safe 
connection between the steering shaft and the steering gear
input shaft keeps the two shafts linked should the rubber
coupling wear out or break.
This allows the driver to maintain steering control, although the 
steering feels loose when this happens.
See Figure 89–7. 

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Figure 89–7
A flexible coupling is used to isolate road noise and vibration from the steering shaft.

Continued
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Column Cover  To keep wiring from jacket­
mounted switches out of sight, the part of
the steering column extending into the
passenger compartment is shrouded. 
Figure 89–8 Steering column covers are often
part of the interior trim.

Continued

A knee bolster under the column is a 
safety feature to keep the driver from 
sliding forward during a collision, 
required for U.S. safety standards.

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Collapsible Column  Federal 
law requires that all vehicles 
sold in the US have collapsible 
columns, steering columns and 
shafts that collapse during a 
collision to absorb crash energy 
and lessen danger of injury.
Figure 89–9 Collapsible steering columns
include a mesh design that crushes easily, a

bearing design that allows one section of the
column to slide into the other, and a breakaway
device that separates the steering column from
the body of the vehicle in the event of a frontend collision.

During a collision, the smaller 
section slides down into the 
larger one to collapse like a 
telescope.
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Do Not Pound On the Steering Column
Always use a steering wheel puller and/or the special tools recommended
by the vehicle manufacturer when servicing the steering column. If a
hammer is used on the steering shaft in an attempt to remove a steering
wheel, the shaft could collapse, requiring the replacement of the entire
steering column assembly.

One early method used a section of the steering column 
constructed out of mesh, which would collapse easily during a 
crash.
Another method is to use a two­piece column. One section of the 
column has a smaller diameter so that it fits inside the other and 
rides on a roller bearing. 

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Tilt Mechanisms  Many steering columns have tilt mechanisms, 
which allow the driver to adjust the angle of the steering wheel 
relative to the steering column.
In a typical tilt steering column, the steering shaft has a short 
section at the top joined to the rest of the steering shaft either by a 
U­joint or gears.
Most tilt mechanisms are some sort of ratchet device that enables 
the driver to lift the steering wheel and the top section of the shaft 
and place them in the desired position.
See Figure 89–10. 

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Usually, spring tension locks the steering wheel in place on the 
ratchet, and a release lever compresses the spring to allow tilt.

Figure 89–10 Tilt mechanisms vary by design and vehicle manufacturer, although most use a
ratchet to position the top portion of the steering column.

Telescoping Steering Columns  Some columns are designed to 
telescope, which means the top of the steering shaft and jacket
can be pulled out toward the driver or pushed in toward the 
dashboard, and then locked into the new position.
21

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Steering Column Construction  The steering shaft is at the center 
of the steering column. The top end of the steering shaft splines to 
the center of the steering wheel, and a large nut
fastens the steering wheel to the shaft.
The lock housing, which contains the ignition lock cylinder, 
encases the top part of the steering shaft. The steering column 
jacket covers the shaft under the ignition lock housing, and
the gear selector lever housing fits over a portion of the
column jacket.
A U­joint connects the lower end of the steering shaft to a
small intermediate shaft, often called a stub shaft. 
See Figures 89–11 and 89–12. 

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Because this steering column includes a gear selector lever, the 
lower end of the column also incorporates an attachment point that 
connects the shift tube to the gear shift rod. 

Figure 89–11 Typical steering column showing all of the
components from the steering wheel to the steering gear.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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The intermediate shaft extends through a hole in the floor where it 
is coupled to the steering gear input shaft by a U­joint. 
A toe plate bolts to the floor of the passenger compartment to 
cover the shaft opening and to protect the interior from noise, 
drafts, and dirt. See Figure 89­13. 

Figure 89–12
The steering shaft splines
onto the steering wheel.

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The toe plate has a tube for the intermediate shaft, and a seal and 
silencer fit on top of the tube.

Figure 89–13 The toe plate seals the
hole from the steering shaft and helps
seal out noise and moisture.
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