516 Auto Fundamentals
provide inputs on vehicle speed. The vehicle speed
inputs come from the same sensors that provide
inputs used to control the automatic transmission
shifting and anti-lock brake (ABS) operation. The
computer processes these inputs and sends cur-
rent to the electric motor. The amount and direc-
tion of the current depends on the inputs received
from the sensors. At low speeds or during park-
ing, the computer sends relatively high current to
the motor to provide maximum steering assist. At
high speeds, the computer sends a relatively small
current to the motor. This reduces assist to increase
road feel.
Summary
Steering linkage is provided to transfer the
force from the steering gear to the steering arms.
The two most popular linkage arrangements are
the parallelogram arrangement and the rack-and-
pinion arrangement. The various linkage parts
are connected by ball-and-socket connections,
which allow swiveling movement without end
play. Some vehicles are equipped with four-
wheel steering systems. In these systems, the
rear wheels turn with the front wheels.
The steering wheel and steering shaft
deliver the turning effort to the steering gear.
Vehicles can use either a manual steering gear
or a power steering unit. Some popular types of
gearing used in both manual and power systems
include the recirculating ball, worm-and-sector,
worm-and-taper pin, and worm-and-roller
steering gears. Most new cars use rack-and-
pinion steering.
Both conventional and rack-and-pinion
steering gears can be power assisted. Both
types of power steering systems are actuated
by oil pressure. The pressurized oil is admitted
to either side of the power cylinder by a rotary
control valve.
Road feel is maintained by the use of a
torsion bar, balanced oil pressure, and return
springs. The power steering system uses oil
pressure to assist the driver only when the pull
on the steering wheel exceeds 1 to 9 lbs. When
the turning effort on the steering wheel falls
below this amount, oil pressure is equalized on
both sides of the power piston.
Oil pressure is provided by a high-pressure
oil pump. The system can use the vane, rotor,
roller-vane, or slipper-vane pump unit. The
pump is connected to the power unit by high-
pressure hoses. The correct fl uid must always be
used in the power steering system.
The power steering system is operated by
an electric motor in some late-model vehicles.
In these systems, a computer is used to control
system operation based on inputs from several
sensors.
C D B A
Figure 25-33.
Four main types of electric power steering. A—An electrohydraulic system with a hydraulic pump that is driven by an
electric motor instead of a belt from the engine. B—An electromechanical system with the electric motor installed in
the steering gear. C—An electromechanical system with the electric motor mounted on the steering column. D—An
electromechanical system with the electric motor installed next to and meshing with the pinion gear.
TRW