404 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
to keep the vehicle from rolling when parked. When the
driver pushes the emergency brake release button on the
dash, the servo motor reverses to allow the brake pads or
shoes to move away from the rotors or drums so the vehicle
can be driven.
Electric/Electronic Steering Systems
An electric steering assist system uses a small DC motor
to help rotate the steering gearbox and reduce steering effort
for the driver. The mechanical steering linkage is the same, but
a small electric motor instead of a hydraulic pump is used to
reduce the effort needed to turn the steering wheel.
As shown in Figure 21-14, the small motor often mounts
on the rack-and-pinion assembly. A small steering ECM con-
trols the amount of steering assist provided. Electric steering
assist is lighter and more sensitive than bulky hydraulic power
steering systems. An exploded view of an electric power steer-
ing assist mechanism is given in Figure 21-15.
Electric four-wheel steering systems use two rack-
and-pinion gearboxes that are driven by electric motors
to turn all four wheels. The front gearbox is controlled by
rotating the steering wheel. The rear gearbox is controlled
by an electronic control module (ECM). Electronic four-
wheel steering systems accurately control rear-wheel turn-
ing angles to improve vehicle handling.
Steering angle sensors, which are located at the gear-
boxes, provide the basic input signals to the ECU. In response
to these signals and the vehicle speed sensor signal, the ECU
directs the rear steering motor to turn the rear wheels.
In many designs, a sensor in the steering column pro-
vides the ECU with the same information sent by the steering
angle sensors. As a result, a fail-safe system is produced. The
two circuits continuously check on each other to prevent the
failure of the electronic steering system. Also, a mechanical
link to the front rack-and-pinion gearbox prevents a loss of
directional control if the electronic system fails.
Electronic four-wheel steering systems reduce the num-
ber of components needed in a four-wheel power steering
system. Also, hydraulic steering components, which tend
to develop leaks, are not used in these systems.
One type of steering sensor is in Figure 21-16. Note
that it uses light emitting diodes (LEDs), a slotted disc,
and photo transistors. The slotted disc is connected to the
steering shaft. It rotates with the steering wheel. As the disc
openings pass between the LEDs and the photo transistors,
a signal is produced. The photo transistors produce a small
voltage signal as they react to the light fl ashes from the disc
rotation and LEDs. The ECM uses this signal to detect steer-
ing direction and to control the shock system.
Electronic Vehicle Parking System
An electronic parking system uses the electric steering
assist motor and proximity sensors to automatically drive
the vehicle into a parking space. A park assist ECM can use
the proximity sensors to operate the transmission, engine,
and steering system to slowly parallel park the vehicle
automatically. See Figure 21-17.
Either ultrasonic sensors or video camera sensors are
used to detect distances between the vehicle and other
objects. These sensors feed information to the ECM so it
can operate the drive train and steering systems to quickly
park the vehicle or pull it into a garage.
A park assist ultrasonic sensor that works on a pulse/
echo principle is shown in Figure 21-18. The ultrasonic
sensor sends out a signal or series of light pulses about 120°
wide from the rear of the vehicle. Any obstacle behind the
car refl ects the high-frequency, invisible light waves back to
the sensor. The park assist ECM can then apply the brakes
to keep the vehicle from hitting any object.
Figure 21-13. Compare the two types of electric parking brake systems. A—This electric parking brake uses the ECM to operate the
electric wedge brakes to keep the wheels from rotating when engaged by a dash button. The servo motors and worm gears force
and hold the brake pads against the rotors. B—This electric parking brake system is designed to be used with conventional hydraulic
brakes. When the parking brake button in the dash is depressed, the ECM operates a small servo motor that turns a gear which
moves a lever arm to pull on the emergency brake cable to mechanically lock the emergency brakes. (Siemens)
A B