Chapter 53

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start

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

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•List five input sensors.
•List four devices controlled by the computer
(output devices).
After studying Chapter 53, the reader should
be able to:
OBJECTIVES:

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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
KEY TERMS:
Continued

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high-side drivers (HSD)
input conditioning
keep-alive memory (KAM)
low-side drivers (LSD)
multiplexing • network • nonvolatile
output drivers
KEY TERMS:
Continued

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

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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.

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THE FOUR BASIC COMPUTER FUNCTIONS
Operation of a computer can be divided into four basic functions:
Continued
Input ProcessingStorage Output
Figure 53–1
All computer systems perform four basic functions: input, processing, storage, and output.

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Figure 53–2
A potentiometer uses a
movable contact to vary
resistance and send an
analog voltage to the
PCM.
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.
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 read-only memory (ROM) because
the computer can only read the contents; data is retained even when
power to the computer is shut off.
Continued

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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 read-only memory (PROM) or
calibration assembly.
Figure 53–3
A replaceable PROM used in an
older GM computer. Notice the
sealed access panel has been
removed to gain access.
Many chips are erasable, meaning the
program can be changed. These chips
are called erasable programmable read-
only memory or EPROM.
These chips are electrically erasable
programmable read-only 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.
Continued

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Temporary memory is called random-access 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 keep-alive 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.
Continued

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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.
Continued

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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.
Continued
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:
Continued
Switched Pulse width modulated•Digital
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.
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.

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Low-Side Drivers Often abbreviated LSD, low-side 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 low-side
drivers. Low-side 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 low-side driver, however, cannot
detect a short-to-ground. See Figure 53–5.
Continued

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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.
Continued

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High-Side Drivers Abbreviated HSD, high-side 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 high-side driver switches the device will be
energized.
In some applications, high-side drivers are used instead of low-side
drivers to provide better circuit protection. GM has used a high-side
driver to control the fuel pump relay instead of a low-side driver.
In the event of an accident, should the circuit to the fuel pump relay
become grounded, a high-side driver would cause a short circuit,
which would cause the fuel pump relay to de-energize.
High-side drivers inside modules can detect faults such as a lack of
continuity when the circuit is not energized. See 53–6.
Continued

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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.
Continued

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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 on-and-off time
of an output. The ratio of on-time relative to the period of the cycle
is referred to as duty cycle.
See Figure 53–7.
Continued

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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.
Continued

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An example is the cooling fan speed control. The speed of the fan is
controlled by varying amount of on-time battery voltage is applied.
100% duty cycle—the fan runs at full speed
75% duty cycle—the fan runs at 34
⁄
speed
50% duty cycle—the fan runs at 12
⁄
speed
25% duty cycle—the fan runs at 14
⁄
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.
Continued
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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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.
A digital computer changes analog input signals (voltage) to digital
bits (binary digits [bits]) of information through an analog-to-
digital (AD) converter circuit. The binary digital number is used by
the computer in its calculations or logic networks.
Output signals are
usually are digital
signals that turn
system actuators
on and off.
Continued

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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.
Continued
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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Computer Programs By operating a vehicle on a dynamometer
and manually adjusting the variable factors such as speed, load, and
spark timing, it is possible to determine the optimum output settings
for the best driveability, economy, and emission control. This is
called engine mapping.
Continued
Figure 53–9 Typical ignition timing map developed from testing and used by the vehicle
computer to provide the optimum ignition timing for all engine speeds and load combinations.
Engine mapping creates a
3D graph that applies to a
given vehicle and power
train combination.
Each combination mapped
individualizes the computer
for a particular model.

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Figure 53–10
The calibration module on many Ford
computers contains a system PROM.
Many older vehicle computers used a single PROM that plugged
into the computer. Some Ford computers used a larger “calibration
module” that contained the system PROM.
NOTE: If the onboard computer needs to be replaced, the PROM or
calibration module must be removed from the defective unit and installed
in the replacement computer. Since the mid-1990s, computers must be
programmed or flashed before being put into service.
Continued

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Clock Rates and Timing The microprocessor receives sensor
input voltage signals, processes them by using information from
other memory units, and sends voltage to the appropriate actuators.
Continued
Figure 53–11
The clock generator produces a series of
pulses that are used by the
microprocessor and other components to
stay in step with each other at a steady
rate.
The microprocessor communicates by transmitting long strings
of 0s and 1s in binary code. A crystal oscillator called the clock
generator tells the processor when one signal ends and another
begins.

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Computer Speeds Some computers are faster than others. The
speed at which a computer operates is specified by cycle time, or
clock speed, required to perform certain measurements. This value
is measured in megahertz (4.7 MHz, 8.0 MHz, 15 MHz, etc.).
Baud Rate The computer transmits bits of a serial data stream at
precise intervals called baud rate, or expressed in bits-per-second.
Continued
Storage of a single character requires eight bits per byte, plus an
additional two bits to indicate stop and start. Transmission of one
character thus requires 10 bits.
Dividing baud rate by 10 gives the maximum number of words
per second that can be transmitted. At a baud rate of 600, about
60 words can be received or sent per minute.

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Control Module Locations The onboard automotive computer has
many names. It may be called an electronic control unit (ECU),
electronic control module (ECM), electronic control assembly
(ECA), or a controller, depending on the manufacturer and the
computer application. The SAE bulletin J-1930 standardizes the
name as a power train control module (PCM).
The computer hardware is all mounted on one or more circuit
boards and installed in a metal case to help shield it from
electromagnetic interference (EMI). The wiring harnesses that link
the computer to sensors and actuators connect to multipin
connectors or edge connectors on the circuit boards.
Onboard computers range from single-function units that control a
single operation to multifunction units that manage all of the
separate (but linked) electronic systems in the vehicle.
Continued

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Figure 53–12 This power train control module
(PCM) is located under the hood on this
Chevrolet pickup truck.
Figure 53–13 This PCM on a DaimlerChrysler
vehicle can only be seen by hoisting the vehicle
because it is located next to the radiator, and in
the airflow to help keep it cool.
Most computers are installed in
the passenger compartment
under the instrument panel or
in a side kick panel…
…where they can be shielded
from physical damage, dirt,
vibration, or interference by
the high voltages.

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COMPUTER INPUT SENSORS
The vehicle computer uses the signals (voltage levels) from the
following engine sensors:
Continued
•Engine speed (RPM or revolutions per minute) sensor This
signal comes from the primary signal in the ignition module.
•MAP (manifold absolute pressure) sensor This sensor
detects engine load. The computer uses this information for
fuel delivery and for onboard diagnosis of other sensors and
systems such as the exhaust gas recirculation (EGR) system.
•MAF (mass airflow) sensor This sensor measures the mass
(weight and density) of the air entering the engine. The
computer uses this information to determine the amount of fuel
needed by the engine.

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•ECT (engine coolant temperature) sensor This sensor
measures the temperature of the engine coolant needed by the
computer to determine the amount of fuel and spark advance.
This is a major sensor, especially when the engine is cold and
when the engine is first started.
•O
2
S (oxygen sensor) This sensor measures the oxygen in the
exhaust stream. These sensors are used for fuel control and to
check other sensors and systems.
•TP (throttle position) sensor This sensor measures the
throttle opening and is used by the computer to control fuel
delivery as well as spark advance and the shift points of the
automotive transmission/transaxle.
Continued

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•VS (vehicle speed) sensor This sensor measures the vehicle
speed using a sensor located at the output of the
transmission/transaxle or by monitoring sensors at the wheel
speed sensors.
•Knock sensor The voltage signal from the knock sensor (KS) is
sent to the PCM. The PCM retards the ignition timing until the
knocking stops.

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COMPUTER OUTPUTS
A vehicle computer can do just two things.
Continued
Turn a device on.
The computer can turn devices such as fuel injectors on and off very
rapidly or keep them on for a certain amount of time. Typical output
devices include:
Turn a device off.
•Fuel injectors The computer can vary the amount of time the
injectors are held open, thereby controlling the amount of fuel
supplied to the engine.
•Ignition timing The computer can trigger the signal to the
ignition module to fire the spark plugs based on information
from the sensors. The spark is advanced when the engine is
cold and/or when the engine is operating under light load
conditions.

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•Transmission shifting The computer provides a ground to the
shift solenoids and torque converter clutch solenoid. The
operation of the automatic transmission/transaxle is optimized
based on vehicle sensor information.
•Idle speed control The computer can pulse the idle speed
control (ISC) or idle air control (IAC) device to maintain engine
idle speed and to provide an increased idle speed when needed,
such as when the air-conditioning system is operating.
•Evaporative emission control solenoids The computer can
control the flow of gasoline fumes from the charcoal canister to
the engine and seal off the system to perform a fuel system leak
detection test as part of the OBD II onboard diagnosis.

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MODULE COMMUNICATION AND NETWORKS
Since the 1990s, vehicles use modules to control most of the
electrical component operation. A typical vehicle will have 10 or
more modules and they communicate with each other over data
lines or hard wiring, depending on the application.
Continued

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Serial Data Data transmitted by a series of rapidly changing
voltage signals pulsed from low to high or from high to low is
called serial data.
Continued
Most modules are connected together in a network because of the
following advantages:
•A decreased number of wires is needed, thereby saving
weight, cost, as well as helping with installation at the factory,
and decreased complexity, making servicing easier.
•Common sensor data can be shared with those modules that
may need the information, such as vehicle speed, outside air
temperature, and engine coolant temperature.

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Multiplexing Sending multiple signals at the same time over a
signal wire and separating the signals at the receiving end is called
multiplexing.
Continued
Figure 53–14 A network allows all
modules to communicate with other
modules.
This intercommunication of
computers or processors is
referred to as a network.
By connecting computers
together on a communications
network, they can share
information back and forth.

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Multiplexing has a number of advantages, including:
Continued
•The elimination of redundant sensors and dedicated wiring for
these multiple sensors.
•The reduction of the number of wires, connectors, and circuits.
•Addition of more features and option content to new vehicles.
•Weight reduction, increasing fuel economy.
•Allows features to be changed with software upgrades instead
of component replacement.
Ring link networks. In a ring-type network, all modules are
connected to each other by a serial data line in a line until all are
connected in a ring. See Figure 53–15.
The three most common types of networks used on GeneralMotors
vehicles include:

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Figure 53–15
A ring link network reduces the number of wires it takes to interconnect all of the modules.
See the chart on Page 612 of your textbook.
Continued

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Star link. A serial data line attaches to each module and then each
is connected to a central point. This central point is called a splice
pack, abbreviated SP such as in “SP 306” and uses a bar to splice
all of the serial lines together.
Some GM vehicles use two or more splice packs to tie the modules
together and a serial data line connects one splice pack to the others.
In most applications the bar used in a splice pack can be removed.
When the bus bar is removed a special tool (J 42236) can be
installed in place of the removed bus bar. Using this tool, the serial
data line for each module can be isolated and tested for a possible
problem. Using the special tool at the splice pack makes diagnosing
this type of network easier than many others.
See Figure 53–16.
Continued

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Figure 53–16
A star-link-type network where all of the modules are connected together using splice packs.
See the chart on Page 613 of your textbook.

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Ring/Star hybrid. In a ring/star network, the modules are
connected using both types of network configuration.
Check service information (SI) for details on how this network is
connected on the vehicle being diagnosed and always follow the
recommended diagnostic steps.
Continued

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SAE COMMUNICATION CLASSIFICATIONS
The Society of Automotive Engineers (SAE) standards include
three categories of in-vehicle network communications, including:
Continued
Class A Low-speed networks (less than 10,000 bits per second [10
kbs]) are generally used for trip computers, entertainment, and other
convenience features. Most low-speed Class A communication
functions are performed using the following:
•UART standard (Universal Asynchronous Receive/Transmit)
used by GM (8192 bps).
•CCD (Chrysler Collision Detection) by Chrysler (7812.5 bps).
NOTE: The “collision” in CCD-type bus communication refers to the
program that avoids conflicts of information exchange within the bus, and
does not refer to airbags or other accident-related circuits of the vehicle.

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Continued
•Chrysler SCI (Serial Communications Interface) is used
tocommunicate between the engine controller and a scan tool
(62.5 kbps).
•ACP (Audio Control Protocol) is used for remote control of
entertainment equipment (twisted pairs) on Ford vehicles.

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Class B Medium-speed networks (10,000 to 125,000 bits per
second [10 to 125 kbs]) are generally used for information transfer
among modules, such as instrument clusters, temperature sensor
data, and other general uses.
Continued
•General Motors GMLAN; both low-and medium-speed and
Class 2, which uses 0-to 7-volt pulses with an available pulse
width. Meets SAE 1850 variable pulse width (VPW).
•Chrysler Programmable Communication Interface (PCI). Meets
SAE standard J-1850 pulse width modulated (PWM).
•Ford Standard Corporate Protocol (SCP). Meets SAE
standardJ-1850 pulse width modulated (PWM).

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Figure 53–17
A typical bus system showing
module CAN communications
and twisted pairs of wire.
Class C
High-speed networks
(125,000+ bits per second)
are generally used for real-
time power train and
vehicle dynamic control.
Most high-speed bus
communication is
controller area network
or CAN.
Continued

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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A bus is a term used to describe a communication network. Therefore,
there are connections to the bus and bus communications, both of which
refer to digital messages being transmitted among electronic modules or
computers.
What is a Bus?

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MODULE COMMUNICATION DIAGNOSIS
Most vehicle manufacturers specify a scan tool be used to
diagnose modules and communications. Some tests of the
communication bus (network) and some of the service procedures
require a DMM, set to DC volts, to monitor communications.
Variable voltage indicates messages are being sent and received.
Most high-speed bus systems use resistors at each end called
terminating resistors to help reduce interference into other
systems in the vehicle. Usually two 120-ohm resistors are
installed at each end and connected electrically in parallel. Two
120-ohm resistors connected in parallel would measure 60 ohms if
being tested using an ohmmeter.
See Figure 53–18.
Continued

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Figure 53–18 Checking the terminating resistors using an ohmmeter at the DLC.

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OBD II DATA LINK CONNECTOR
.All OBD II vehicles use a 16-pin connector that includes:
Continued
Figure 53–19
Sixteen-pin OBD II DLC with terminals identified. Scan tools use the power pin (16) ground pin (4)
for power so that a separate cigarette lighter plug is not necessary on OBD II vehicles.
Pin 4 = chassis ground
Pin 5 = signal ground
Pin 16 = battery power
(4A max)

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Vehicles may use one of two major standards including:
Continued
•ISO 9141-2 Standard
(ISO = International Standards Organization)
Pins 7 and 15
(or wire at pin 7 and no pin at 2 or a wire at 7 and at 2
and/or 10)
•SAE J-1850 Standard
(SAE = Society of Automotive Engineers)
Two types:
VPW (variable pulse width)
PWM (pulse width modulated)
Pins 2 and 10 (no wire at pin 7)

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General Motors vehicles use:
Continued
•SAE J-1850 (VPW—Class 2—10.4 kb) standard, which
uses pins 2, 4, 5, and 16 and not 10
•GM Domestic OBD II
Pin 1 and 9—CCM (Comprehensive Component Monitor) slow
baud rate—8192 UART
Pins 2 and 10—OEM Enhanced—Fast Rate—40,500 baud rate
Pins 7 and 15—Generic OBD II—ISO 9141—10,400 baud rate

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Chrysler, European, and Asian vehicles use:
Continued
•ISO 9141-2 standard
uses pins 4, 5, 7, 15, and 16
•Chrysler OBD II
Pins 2 and 10—CCM
Pins 3 and 14—OEM Enhanced—60,500 baud rate
Pins 7 and 15—Generic OBD II - ISO 9141—10,400 baud rate

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Ford vehicles use:
Continued
•SAE J-1850(PWM) (PWM—41.6 kb) standard, which
uses pins 2, 4, 5, 10, and 16
•Ford Domestic OBD II
Pins 2 and 10—CCM
Pins 6 and 14—OEM Enhanced—Class C—40,500 baud rate
Pins 7 and 15—Generic OBD II—ISO 9141—10,400 baud rate

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SUMMARY
Continued
1.The Society of Automotive Engineers (SAE) standard J-1930
specifies that the term power train control module (PCM) be
used for the computer that controls the engine and
transmission in a vehicle.
2.The four basic computer functions include input, processing,
storage, and output.
3.Read-only memory (ROM) can be programmable (PROM),
erasable (EPROM), or electrically erasable (EEPROM).

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SUMMARY
1.Computer input sensors include engine speed (RPM), MAP,
MAF, ECT, O2S, TP, and VS.
2.A computer can only turn a device on or turn a device off, but
it can do the operation very rapidly.
(cont.)

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end