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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
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.
THE FOUR BASIC COMPUTER FUNCTIONS
Operation of a computer can be divided into four basic functions:
Figure 53–1 All computer systems perform four basic functions: input, processing, storage, and output.
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 .
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.
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
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.
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.
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.
Most outputs work electrically in one of three ways:
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. Continued
Pulse width modulated
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 .
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.
Figure 53–5 A typical low-side driver (LSD) which uses a control module to control the ground side of the relay coil. Continued
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.
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
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.
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
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 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.
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.
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 ( bi nary digi ts [ 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
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.
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 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. Continued 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.
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
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.
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. Continued 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.
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.
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. O nboard 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.
Figure 53–12 This power train control module (PCM) is located under the hood on this Chevrolet pickup truck.
Most computers are installed in the passenger compartment under the instrument panel or in a side kick panel…
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. … where they can be shielded from physical damage, dirt, vibration, or interference by the high voltages.
COMPUTER INPUT SENSORS
The vehicle computer uses the signals (voltage levels) from the following engine sensors:
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.
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.
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.
A vehicle computer can do just two things.
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.
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.
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.
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.
Multiplexing Sending multiple signals at the same time over a signal wire and separating the signals at the receiving end is called multiplexing .
Figure 53–14 A network allows all modules to communicate with other modules. Continued 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.
Multiplexing has a number of advantages, including:
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:
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
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.
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.
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.
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.
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.
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.
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).
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
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?
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.
Figure 53–18 Checking the terminating resistors using an ohmmeter at the DLC.
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)
Vehicles may use one of two major standards including:
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 )
General Motors vehicles use:
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
Chrysler, European, and Asian vehicles use:
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
Ford vehicles use:
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
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.
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.