2. OBJECTIVES After studying Chapter 2, the reader will 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.
List five input sensors.
List four devices controlled by the computer (output devices).

3. COMPUTER CONTROL
Modern automotive control systems consist of a network of electronic sensors, actuators, and computer modules designed to regulate the powertrain and vehicle support systems.
The powertrain 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.

4. THE FOUR BASIC COMPUTER FUNCTIONS
The operation of every computer can be divided into four basic functions.
Input
Processing
Storage
Output
FIGURE 2-1 All computer systems perform four basic functions: input, processing, storage, and output.

5. THE FOUR BASIC COMPUTER FUNCTIONS
FIGURE 2-2 A potentiometer uses a movable contact to vary resistance and send an analog voltage right to the PCM.

6. THE FOUR BASIC COMPUTER FUNCTIONS
FIGURE 2-3 A replaceable PROM used in an older General Motors computer. Notice that the sealed access panel has been removed to gain access.

7. THE FOUR BASIC COMPUTER FUNCTIONS Low-Side Drivers
Low-side drivers, often abbreviated LSD, are transistors that complete the ground path in the circuit.
FIGURE 2-4 A typical output driver. In this case, the PCM applies voltage to the fuel pump relay coil to energize the fuel pump.

8. THE FOUR BASIC COMPUTER FUNCTIONS Low-Side Drivers
FIGURE 2-5 A typical low-side driver (LSD) which uses a control module to control the ground side of the relay coil.

9. THE FOUR BASIC COMPUTER FUNCTIONS High-Side Drivers
High-side drivers, often abbreviated HSD, control the power side of the circuit.
FIGURE 2-6 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.

10. THE FOUR BASIC COMPUTER FUNCTIONS Pulse Width Modulation
Pulse width modulation (PWM) is a method of controlling an output using a digital signal.
Instead of just turning devices on or off, the computer can control output devices more precisely by using pulse width modulation.
FIGURE 2-7 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.

11. DIGITAL COMPUTERS
In a digital computer, the voltage signal or processing function is a simple high/low, yes/no, on/off signal.
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.”
FIGURE 2-8 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.

12. DIGITAL COMPUTERS 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).
Computer Memory.

13. DIGITAL COMPUTERS 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.
FIGURE 2-9 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.

14. DIGITAL COMPUTERS Computer Programs
FIGURE 2-10 The calibration module on many Ford computers contains a system PROM.

15. DIGITAL COMPUTERS Clock Rates and Timing
The microprocessor receives sensor input voltage signals, processes them by using information from other memory units, and then sends voltage signals to the appropriate actuators.
FIGURE 2-11 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.

16. DIGITAL COMPUTERS Computer Speeds
Not all computers operate at the same speed; some are faster than others.
The speed at which a computer operates is specified by the cycle time, or clock speed, required to perform certain measurements.
Cycle time or clock speed is measured in megahertz (4.7 MHz, 8.0 MHz, 15 MHz, 18 MHz, etc.).

17. DIGITAL COMPUTERS Baud Rate
The computer transmits bits of a serial data stream at precise intervals.
The computer’s processing speed is called the baud rate, or bits per second.

18. DIGITAL COMPUTERS 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.
FIGURE 2-12 This powertrain control module (PCM) is located under the hood on this Chevrolet pickup truck.

19. DIGITAL COMPUTERS Control Module Locations
FIGURE 2-13 This PCM on a Chrysler 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.

20. COMPUTER INPUT SENSORS
The vehicle computer uses the signals (voltage levels) from the following engine sensors:
Engine speed (RPM or revolutions per minute) sensor.
MAP (manifold absolute pressure) sensor.
MAF (mass airflow) sensor.
ECT (engine coolant temperature) sensor.
O2S (oxygen sensor).
TP (throttle position) sensor.
VS (vehicle speed) sensor.
Knock sensor.

21. COMPUTER OUTPUTS A vehicle computer can do just two things.
Turn a device on.
Turn a device off.
Typical output devices include the following.
Fuel injectors.
Ignition timing.
Transmission shifting.
Idle speed control.
Evaporative emission control solenoids.

22. MODULE COMMUNICATION AND NETWORKS
Serial Data
Serial data is data that is transmitted by a series of rapidly changing voltage signals pulsed from low to high or from high to low.
Multiplexing
Multiplexing is the process of sending multiple signals of information at the same time over a signal wire and then separating the signals at the receiving end.

23. MODULE COMMUNICATION AND NETWORKS
FIGURE 2-14 A network allows all modules to communicate with other modules.

24. MODULE COMMUNICATION AND NETWORKS
The three most common types of networks used on General Motors vehicles include:
Ring link networks
Star link
Ring/star hybrid

25. MODULE COMMUNICATION AND NETWORKS
FIGURE 2-15 A ring link network reduces the number of wires it takes to interconnect all of the modules.

26. MODULE COMMUNICATION AND NETWORKS
FIGURE 2-16 A star-link-type network where all of the modules are connected together using splice packs.

27. SAE COMMUNICATION CLASSIFICATIONS
The Society of Automotive Engineers (SAE) standards include three categories of in-vehicle network communications, including the following.
Class A
Class B
Class C

28. SAE COMMUNICATION CLASSIFICATIONS
FIGURE 2-17 A typical bus system showing module CAN communications and twisted pairs of wire.

29. MODULE COMMUNICATION DIAGNOSIS
Most vehicle manufacturers specify that a scan tool be used to diagnose modules and module communications.
Always follow the recommended testing procedures, which usually require the use of a factory scan tool.

30. MODULE COMMUNICATION DIAGNOSIS
FIGURE 2-18 Checking the terminating resistors using an ohmmeter at the DLC.

31. OBD II DATA LINK CONNECTOR
FIGURE 2-19 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.

32. SUMMARY
The Society of Automotive Engineers (SAE) standard J-1930 specifies that the term powertrain control module (PCM) be used for the computer that controls the engine and transmission in a vehicle.
The four basic computer functions include input, processing, storage, and output.
Read-only memory (ROM) can be programmable (PROM), erasable (EPROM), or electrically erasable (EEPROM).
Computer input sensors include engine speed (RPM), MAP, MAF, ECT, O2S, TP, and VS.
A computer can only turn a device on or turn a device off, but it can do the operation very rapidly.

33. REVIEW QUETSIONS
What part of the vehicle computer is considered to be the brain?
What is the difference between volatile and nonvolatile RAM?
List four input sensors.
List four output devices.

34. CHAPTER QUIZ
What unit of electricity is used as a signal for a computer?
Volt
Ohm
Ampere
Watt

35. CHAPTER QUIZ
What unit of electricity is used as a signal for a computer?
Volt
Ohm
Ampere
Watt

36. CHAPTER QUIZ 2. The four basic computer functions include _____.
Writing, processing, printing, and remembering
Input, processing, storage, and output
Data gathering, processing, output, and evaluation
Sensing, calculating, actuating, and processing

37. CHAPTER QUIZ 2. The four basic computer functions include _____.
Writing, processing, printing, and remembering
Input, processing, storage, and output
Data gathering, processing, output, and evaluation
Sensing, calculating, actuating, and processing

38. CHAPTER QUIZ 3. All OBD II vehicles use what type of read-only memory?
ROM
PROM
EPROM
EEPROM

39. CHAPTER QUIZ 3. All OBD II vehicles use what type of read-only memory?
ROM
PROM
EPROM
EEPROM

40. CHAPTER QUIZ 4. The “brain” of the computer is the _____. PROM RAM CPU
AD converter

41. CHAPTER QUIZ 4. The “brain” of the computer is the _____. PROM RAM CPU
AD converter

42. CHAPTER QUIZ 5. Computer processing speed is measured in _____.
Baud rate
Clock speed (Hz)
Voltage
Bytes

43. CHAPTER QUIZ 5. Computer processing speed is measured in _____.
Baud rate
Clock speed (Hz)
Voltage
Bytes

44. CHAPTER QUIZ Which item is a computer input sensor? RPM
Throttle position angle
Engine coolant temperature
All of the above

45. CHAPTER QUIZ Which item is a computer input sensor? RPM
Throttle position angle
Engine coolant temperature
All of the above

46. CHAPTER QUIZ 7. Which item is a computer output device? Fuel injector
Transmission shift solenoid
Evaporative emission control solenoid
All of the above

47. CHAPTER QUIZ 7. Which item is a computer output device? Fuel injector
Transmission shift solenoid
Evaporative emission control solenoid
All of the above

48. CHAPTER QUIZ 8. The SAE term for the vehicle computer is _____. PCM
ECM
ECA
Controller

49. CHAPTER QUIZ 8. The SAE term for the vehicle computer is _____. PCM
ECM
ECA
Controller

50. CHAPTER QUIZ
9. What two things can a vehicle computer actually perform (output)?
Store and process information
Turn something on or turn something off
Calculate and vary temperature
Control fuel and timing only

51. CHAPTER QUIZ
9. What two things can a vehicle computer actually perform (output)?
Store and process information
Turn something on or turn something off
Calculate and vary temperature
Control fuel and timing only

52. CHAPTER QUIZ
10. Analog signals from sensors are changed to digital signals for processing by the computer through which type of circuit?
Digital
Analog
AD converter
PROM

53. CHAPTER QUIZ
10. Analog signals from sensors are changed to digital signals for processing by the computer through which type of circuit?
Digital
Analog
AD converter
PROM