chap53 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 53, the reader should
be able to:

•
•
•

Prepare for ASE Electrical/Electronic Systems
(A6) certification test content area “A” (General
Electrical/Electronic Systems Diagnosis).

Explain the purpose and function of onboard
computers.
List the various parts of an automotive
computer.

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OBJECTIVES:
After studying Chapter 53, the reader should
be able to:

•

•

List five input sensors.
List four devices controlled by the computer
(output devices).

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
actuator • analog-to-digital (AD) converter
binary
central processing unit (CPU) • clock generator •
controller • controller area network (CAN)

digital • duty cycle
EEPROM • E2 PROM • electronic control assembly (ECA)
• electronic control module (ECM) • electronic control unit
(ECU) • engine mapping
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KEY TERMS:
high-side drivers (HSD)
input conditioning
keep-alive memory (KAM)
low-side drivers (LSD)
multiplexing • network • nonvolatile

output drivers

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KEY TERMS:
power train control module (PCM) • programmable readonly memory (PROM)
Random-access memory (RAM) • read-only memory
(ROM)
serial data • splice pack
terminating resistors • volatile

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

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COMPUTER CONTROL
Modern automotive control systems consist of a network of 
electronic sensors, actuators, and computer modules designed to 
regulate the power train and vehicle support systems. The power 
train control module (PCM) is the heart of this system. It 
coordinates engine and transmission operation, processes data, 
maintains communications, and makes the control decisions needed 
to keep the vehicle operating.
Automotive computers use voltage to send and receive information. 
It converts input or data into voltage signal combinations that 
represent a variety of information—temperature, speed, or even 
words and letters, and then delivers the data in computed or 

processed form.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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THE FOUR BASIC COMPUTER FUNCTIONS
Operation of a computer can be divided into four basic functions:
Input

Processing

Storage

Output 

Figure 53–1
All computer systems perform four basic functions: input, processing, storage, and output.
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Input  First, the computer receives a voltage signal (input) from an 
input device. The device can be as simple as a button or a switch on 
an instrument panel, or a sensor on an automotive engine. 
Vehicles use mechanical, electrical, and magnetic sensors to 
measure factors such as speed, engine RPM, air pressure, oxygen 
content of exhaust gas, airflow, and engine coolant temperature. 
Figure 53–2

A potentiometer uses a
movable contact to vary
resistance and send an
analog voltage to the
PCM.

The signals must 
undergo input 
conditioning. 
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Processing  Input voltage signals received by a computer are 
processed through a series of electronic logic circuits maintained in 

its programmed instructions. These logic circuits change the input 
voltage signals, or data, into output voltage signals or commands.
Storage  The program instructions for a computer are stored in 
electronic memory. Some programs may require that certain input 
data be stored for later reference or future processing. In others, 
output commands may be delayed or stored before they are 
transmitted to devices elsewhere in the system.
Computers have two types of memory: permanent and temporary. 
Permanent memory is called read­only memory (ROM) because 
the computer can only read the contents; data is retained even when 
power to the computer is shut off.
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Part of the ROM is built into the computer, and the rest is located in 
an IC chip called a programmable read­only memory (PROM) or 
calibration assembly. 
Many chips are erasable, meaning the 
program can be changed. These chips 
are called erasable programmable read­
only memory or EPROM. 

Figure 53–3
A replaceable PROM used in an
older GM computer. Notice the
sealed access panel has been
removed to gain access.

These chips are electrically erasable 
programmable read­only memory, 
abbreviated EEPROM or E2PROM. 
Onboard diagnosis second generation, 
OBD II, vehicles can be reprogrammed 
by using a scan tool and proper 
software, usually called reflashing.  
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Temporary memory is called random­access memory (RAM) because 
the microprocessor can write or store new data into it as directed by the 
computer program, as well as read data already in it.
Automotive computers use two types of RAM memory: volatile and 
nonvolatile. Volatile memory is lost when the ignition is turned off. 
However, a type of volatile RAM called keep­alive memory (KAM) can 
be wired directly to battery power, preventing data from being erased 
when the ignition is turned off.
Both RAM and KAM have the disadvantage of losing their memory when 
disconnected from their power source. One example of RAM and KAM is 
the loss of station settings in a programmable radio. Since all the settings 
are stored in RAM, they have to be reset when the battery is reconnected. 
Trouble codes are commonly stored in RAM and can be erased by 
disconnecting the battery.
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Nonvolatile RAM memory can retain its information even when the 
battery is disconnected. One use for this type of RAM is the storage 
of odometer information in an electronic speedometer.
The memory chip retains the mileage accumulated by the vehicle. 
When speedometer replacement is necessary, the odometer chip is 
removed and installed in the new speedometer unit. KAM is used 
primarily in conjunction with adaptive strategies.

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Output After the computer has processed the input signals, it sends 
voltage signals or commands to other devices in the system, such as 
system actuators.
An actuator is an electrical or mechanical device that converts 
electrical energy into heat, light, or motion, such as adjusting engine 
idle speed, altering suspension height, or regulating fuel metering.
Computers also communicate with, and control, each other through 
output and input functions. This means output signal from one 
computer system can be input signal for another system through a 
network.

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Most outputs work electrically in one of three ways:
Switched

Pulse width modulated

Digital

A switched output is either on or off. In many circuits, the PCM 
uses a relay to switch a device on or off.  By using a relay circuit as 
shown here, the PCM provides the output control to the relay, which 
in turn provides the output control to the device. 
These switches are 
actually transistors, often 
called output drivers.
Figure 53–4 A typical output

driver. In this case, the PCM
applies voltage to the fuel
pump relay coil to energize
the fuel pump.

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Low­Side Drivers  Often abbreviated LSD, low­side drivers are 
transistors that complete the ground path in the circuit. Ignition voltage is 
supplied to the relay as well as battery voltage.
The computer output is connected to the ground side of the relay coil. The 
computer energizes the fuel pump relay by turning the transistor on and 

completing the ground path for the relay coil. A relatively low current 
flows through the relay coil and transistor that is inside the computer. This 
causes the relay to switch and provides the fuel pump with battery voltage.
The majority of switched outputs have typically been low­side drivers. 
Low­side drivers can  perform a diagnostic circuit check by monitoring 
the voltage from the relay to check that the control circuit for the relay is 
complete. A low­side driver, however, cannot detect a short­to­ground. 
See Figure 53–5. 

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Figure 53–5 A typical low-side driver (LSD) which uses a control module to control the ground

side of the relay coil.

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High­Side Drivers  Abbreviated HSD, high­side drivers control the power side of the circuit. When 
the transistor is switched on, voltage is applied to the device. A ground has been provided to the device 
so when the high­side driver switches the device will be energized.
In some applications, high­side drivers are used instead of low­side drivers to provide better circuit 
protection. GM has used a high­side driver to control the fuel pump relay instead of a low­side driver.
In the event of an accident, should the circuit to the fuel pump relay become grounded, a high­side 
driver would cause a short circuit, which would cause the fuel pump relay to de­energize.
High­side drivers inside modules can detect faults such as a lack of continuity when the circuit is not 

energized. See 53–6.

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Figure 53–6 A typical module-controlled high-side driver (HSD) where the module itself
supplies the electrical power to the device. The logic circuit inside the module can detect circuit
faults including continuity of the circuit and if there is a short-to-ground in the circuit being
controlled.

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Pulse Width Modulation A method of controlling an output using 
a digital signal is called Pulse width modulation (PWM). Instead of 
just on or off, the computer can control output devices more 
precisely by using pulse width modulation.
A vacuum solenoid could be a pulse width modulated device, to 
control vacuum that flows through the solenoid. A PWM signal is a 
digital signal, usually 0 & 12 volts, cycling at fixed frequency.
Varying length of time the signal is on can vary the on­and­off time 
of an output. The ratio of on­time relative to the period of the cycle 
is referred to as duty cycle. 
See Figure 53–7. 
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Figure 53–7 Both the top and bottom pattern have the same frequency. However, the amount of
on-time varies. Duty cycle is the percentage of the time during a cycle that the signal is turned on.

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An example is the cooling fan speed control. The speed of the fan is 
controlled by varying amount of on­time battery voltage is applied.
100% duty cycle—the fan runs at full speed
75% duty cycle—the fan runs at 3∕4 speed
50% duty cycle—the fan runs at 1∕2 speed
25% duty cycle—the fan runs at 1∕4 speed
PWM may be used to control amount of purge of the evaporative 
purge solenoid, the speed of a fuel pump motor, control of a linear 
motor, or even the intensity of a light bulb.

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DIGITAL COMPUTERS
In a digital computer, the voltage signal or processing function is 
a simple high/low, yes/no, on/off signal.
The computer can process thousands of digital signals per second it 
is able to switch voltage signals on and off in billionths of a second. 
The digital signal voltage is limited to two voltage levels: high 
voltage and low voltage. Since there is no stepped range of voltage 
or current in between, a digital binary signal is a “square wave.”
The signal is “digital” because the on and off signals are processed 
by the computer as the numbers, or digits 0 and 1. This is called 
the binary system. Any number or letter from any number system 
or language alphabet can be translated into a combination of 
binary 0s and 1s for the digital computer.
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A digital computer changes analog input signals (voltage) to digital 
bits (binary digits [bits]) of information through an analog­to­
digital (AD) converter circuit. The binary digital number is used by 
the computer in its calculations or logic networks.
Figure 53–8 Many
electronic components are
used to construct a typical
vehicle computer. Notice the
quantity of chips, resistors,
and capacitors used in this
General Motors computer.

Output signals are 
usually are digital 
signals that turn
system actuators
on and off.

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Parts of a Computer  The software consists of the programs and 
logic functions stored in the computer’s circuitry. The hardware is 
the mechanical and electronic parts of a computer.
Central Processing Unit (CPU)  The microprocessor is the central 
processing unit (CPU) of a computer.  Since it performs essential 
mathematical operations and logic decisions the CPU can be 
considered the heart of a computer. 
Computer Memory  Other IC devices store the computer operating 
program, system sensor input data, and system actuator output data, 
information that is necessary for CPU operation.

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