chap88 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 88, the reader should
be able to:

•
•
•

Prepare for ASE Suspension and Steering (A4) certification
test content area “B” (Suspension System Diagnosis and
Repair).

Describe how suspension height sensors function.
Explain the use of the various sensors used for electronic
suspension control.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 88, the reader should
be able to:

•
•

Discuss the steering wheel position sensor.
Explain how solenoids and actuators are used to control the
suspension.

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

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KEY TERMS:
actuator • air suspension • armature • AS • automatic level
control (ALC)
CCVRTMR • computer command ride (CCR)
desiccant • driver select switch • EBCM • ECU • electromagnet
handwheel position sensor • height sensor

input • lateral accelerometer sensor • LED
magneto-rheological (MR) • mode select switch • motor •
MRRTD • output
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
output • perform ride mode • photocell • phototransistor • pulse
width • pulse-width modulation
real-time dampening (RTD) • RPO • RSS
selectable ride (SR) • solenoid • solenoid controlled damper •
stabilitrack • steering wheel position sensor
touring ride mode

vehicle stability enhancement system (VSES)
VS sensor • yaw rate sensor
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Since the mid-1980s, many vehicle manufacturers have been
introducing models with electronic suspension controls that
provide a variable shock stiffness or spring rate.
The main advantage of electronic controls is that the
suspension can react to different conditions.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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ELECTRONICALLY-CONTROLLED
SUSPENSION
Electronically-controlled suspension provides a firm suspension
feel for fast cornering and quick acceleration and braking, with a
soft ride for cruising.

Figure 88–1 An electronically controlled suspension system can help reduce body roll and
other reactions better than most conventional suspension systems.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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ELECTRONIC CONTROLS AND SENSORS
Sensors and switches provide input to the
electronic control module (ECM), or computer.

Also referred to as the electronic control unit
(ECU), the ECM is a small computer that
receives input in the form of electrical signals
from the sensors and switches and provides
output electrical signals to the system actuators.
Figure 88–2 Input devices monitor conditions and provide information
to the electronic control module, which processes the information and
operates the actuators to control the movement of the suspension.

The electrical signal causes an actuator to
perform some type of mechanical action.
Continued
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Height Sensors A height sensor senses the vertical relationship
between the suspension component and the body. Its signal
indicates to the ECM how high the frame or body is, or how
compressed the suspension is. A number of sensor designs are
used to determine ride height, including a photocell sensor.

Figure 88–3 A typical electronic
suspension height sensor, which
bolts to the body and connects to
the lower control arm through a
control link and lever.

Continued
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Four height sensors, one at each wheel, deliver an input signal to
the ECM. All use a control link, lever, slotted disc, and photo
interrupters to transmit a signal. Each photo interrupter consists
of a light-emitting diode (LED) and a phototransistor, which
reacts to the LED.
The LEDs and phototransistors are positioned opposite each
other on each side of the slotted disc. The ECM applies voltage
to the LEDs, which causes them to illuminate.
Light from an LED shining on the phototransistor causes the
transistor to generate a voltage signal. Signals generated by the

phototransistors are delivered to the ECM as an input that
reflects ride height.
See Figure 88–4.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 88–4 When suspension action moves the lever, it rotates the slotted disc and varies how
much of the photo transistor is exposed to the LEDs, which vary the input signal.

Continued
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General Motors Electronic Suspension Sensors There are five
different sensors found on electronic suspension systems, and GM
vehicles can have between one and four of these sensors.
Depending on the vehicle, the suspension position sensor may be
called by a different name. It can be called:
An automatic level control sensor
An electronic suspension position sensor
A position sensor
An air suspension sensor
The sensor provides the control module with information regarding
the relative position and movement of suspension components. The
common mounting location is between the vehicle body and the
suspension control arm. See Figure 88–5.
Continued

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NOTE: Some systems may require sensor learning or “reprogramming”
after replacement. For instance, the sensor used on the Tahoe or Suburban
needs to be programmed if replaced.

Figure 88–5 Typical
suspension position sensor.

Continued
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The sensor is either an air suspension sensor two-wire type or a
potentiometer three-wire type. The air suspension sensor is called a
linear Hall-effect sensor, and consists of a moveable iron core
linked to the components. As the core moves, it varies inductance of
the internal sensor coil relative to suspension position.
The suspension control module energizes and de-energizes the coil
approximately 20 times a second, thereby measuring sensor
inductance as it relates to suspension position.
The potentiometer three-wire sensor requires reference and ground
voltage. Similar to the throttle position (TP) sensor, it produces a
variable analog voltage signal.
See Figure 88–6.
Continued
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Suspension position sensor voltage changes relative to this
movement. The sensor receives a 5-volt reference signal from the
control module. The position sensor returns a voltage signal
between 0 and 5 volts depending on suspension arm position.

Figure 88–6 A three-wire
suspension position sensor
schematic.

Continued
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As the suspension moves up or down, an arm moves on the
suspension position sensor through a ball-and-cup link.

Figure 88–7
A suspension
height sensor.

Continued
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Steering Wheel Position Sensor Depending on the vehicle, the
steering wheel position sensor may also be called a hand wheel
position sensor.
The function of this sensor is to provide the control module with
signals relating to steering wheel position, the speed and direction
of handwheel position.
The sensor is found on most real-time dampening (RTD) and
road-sensing suspension (RSS) applications. The sensor is
typically located at the base of the steering column.
Always refer to service information for vehicle-specific
information.
See Figure 88–8 and 88–9.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 88–8 The steering wheel position (handwheel position) sensor wiring schematic and
how the signal varies with the direction that the steering wheel is turned.

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The handwheel sensor produces
two digital signals, which are
used by the electronic brake
control module (EBCM).

Figure 88–9 The handwheel position sensor
is located at the base of the steering column.

These signals are produced as
the steering wheel is rotated.
The sensor uses a 5-volt signal
reference.
Analog signal voltage values
increase or decrease, between
0 and 5 volts, as the steering
wheel is moved left and right
of center.
Continued
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The digital signal is also a standard power-to-ground circuit as
shown in the schematic.
There are three possible digital signals:
Phase A
Phase B
Index pulse
These signals provide the suspension control module with steering
wheel speed and direction information. Digital signals are either
high or low, 5 volts or 0 volts.
See Figure 88–10.

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Figure 88–10 Steering wheel (handwheel) position sensor schematic.

Continued
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Vehicle Speed Sensor The vehicle speed (VS) sensor is used by the
EBCM to help control the suspension system. The vehicle speed
sensor is a magnetic sensor and generates an analog signal whose
frequency increases as the speed increases.
Figure 88–11 The VS sensor information
is transmitted to the EBCM by Class 2
serial data.

The ride is made firmer at
high speeds and during
braking and acceleration
and less firm at cruise
speeds.

Continued
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Pressure Sensor A pressure transducer (sensor) is typically
mounted on the compressor assembly. This sensor is typically
found on suspension systems that use a compressor assembly.
The main function of the pressure sensor is to provide feedback
to the suspension control module about the operation of the
compressor. The sensor assures both that a minimum air
pressure is maintained in the system and that a maximum
value is not exceeded.
A pressure transducer (sensor) is typically mounted on the
compressor assembly. This sensor is typically found on systems
such as air suspension, real-time damping, and road-sensing
suspension that use a compressor assembly.
See Figure 88–12
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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The pressure sensor requires a 5-volt reference, a ground, and a
signal wire to provide feedback to the control module. A high
voltage indicates high pressure; low voltage, a low pressure.
Figure 88–12 An air pressure sensor.

Continued
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Lateral Accelerometer Sensor The lateral accelerometer
sensor is also called a G-sensor, with the letter “G” representing
the force of gravity. It provides the suspension control module
with feedback regarding vehicle cornering forces.
This information is processed by the suspension control module
to provide appropriate damping on the inboard and outboard
dampers during cornering events.
The lateral accelerometer sensor is found on the more complex
suspensions systems, such as RTD and RSS systems that
incorporate the vehicle stability enhancement system (VSES).

Continued
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