196 Auto Suspension and Steering
with cornering and to make parallel parking easier. The
total rear wheel movement is much less than that of the
front wheels. Figure 9-62 shows the relationship between
steering wheel rotation and wheel movement in a typical
four-wheel steering system.
A four-wheel steering system uses input from the front
steering system to operate the rear wheel steering
mechanism. There are several ways to operate the rear-
wheel steering mechanism. All involve electronic sensing
of the front wheel position, as well as vehicle speed.
Note: More information on electronically
controlled steering systems will be given
in Chapters 13 and 14.
Hydraulic/Electronic Four-Wheel
Steering
The hydraulic/electronic four-wheel steering system
uses hydraulic pressure from the front power steering to
operate the rear-wheel steering mechanism. A rear-
mounted steering unit is attached to the rear wheels
through a tie rod assembly that resembles the tie rod
assembly on a front rack-and-pinion system. The steering
unit is operated by hydraulic pressure developed by the
power steering pump. A flow control solenoid assembly
and an electronic module control the hydraulic pressure to
the steering unit. The module receives inputs from the
vehicle speed sensor and a steering wheel–mounted
steering angle sensor. A diagram showing the position of
the system components and the connecting hoses is shown
in Figure 9-63. If the hydraulic pressure is lost or the
control system malfunctions, a fail-safe valve will close to
Ball stud
Valve actuator assembly
Pressure inlet
from pump
Valve spool
Centering
spring
Nut
Spool bolt
Valve body
Figure 9-61. Cutaway of a power piston control valve assembly. This is a linkage type unit. It generally attaches to the steering
center link. (Moog)
Figure 9-62. Note tire positions during the various degrees of
steering wheel rotation in a four-wheel steering system. (Honda)
(Deg.)
30
20
10
0
(Deg.)
Steer
angle
0° Turn 140° Turn 240° Turn 450° Turn
200 400
Front wheel
Rear wheel
Steering wheel angle
Front
Rear
0°
8° 15.6° 30.3°
0°
1.5°
0°
5.3°
keep hydraulic pressure from reaching the rear steering
unit. With pressure removed, centering springs keep the
rear wheels in the straight ahead position. A dashboard
light will illuminate if the system malfunctions.