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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.).

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.

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.

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.

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.

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.

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.

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

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

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

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

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

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

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.

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

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.

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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