After studying Chapter 12, 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 on-board computers.
List the various parts of an automotive computer.
List five input sensors.
List four devices controlled by the computer (output devices).
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.
THE FOUR BASIC COMPUTER FUNCTIONS
The operation of every computer can be divided into four basic functions.
Figure 12-1 All computer systems perform four basic functions: input, processing, storage, and output.
Input is the voltage signal from an input device to the computer. The device can be as simple as a button or a switch on an instrument panel, or a sensor on an automotive engine.
The computer receives these voltage signals, but before it can use them, the signals must undergo a process called input conditioning.
Processing is what a computer does to the input signals. Input voltage signals received by a computer are processed through a series of electronic logic circuits maintained in its programmed instructions.
Storage is where the program instructions for a computer are kept in electronic memory.
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; it cannot change the data stored in it. This 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.
Many chips are erasable, meaning that the program can be changed. These chips are called erasable programmable read-only memory or EPROM.
Chips are electronically erasable programmable read-only memory, abbreviated EEPROM or E2PROM. All vehicles equipped with on-board diagnosis second generation, called OBD II, are equipped with EEPROMs.
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 the data already in it.
Volatile RAM memory is lost whenever the ignition is turned off. However, a type of volatile RAM called keep-alive memory (KAM) can be wired directly to battery power.
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.
Output is when the computer has processed the input signals and sends voltage signals or commands to other devices in the system, such as system actuators. An actuator is an electrical or electro-mechanical device that converts electrical energy into heat, light, or motion, such as adjusting engine idle speed, altering suspension height, or regulating fuel metering.
Almost all outputs work electrically in one of three ways:
Pulse width modulated
Most computer circuits cannot handle a lot of current, therefore a relay is used.
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, are transistors that complete the ground path in the circuit.
High-side drivers, often abbreviated HSD, control the power side of the circuit. In these applications when the transistor is switched on, voltage is applied to the device.
Pulse Width Modulation
Pulse width modulation (PWM) is a method of controlling an output using a digital signal.
A PWM signal is a digital signal, usually 0 volts and 12 volts, that is cycling at a fixed frequency.
The ratio of on-time relative to the period of the cycle is referred to as duty cycle.
A good example of pulse width modulation is the cooling fan speed control. The speed of the cooling fan is controlled by varying the amount of on-time that the battery voltage is applied to the cooling fan motor.
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
Figure 12-2 A potentiometer uses a movable contact to vary resistance and send an analog voltage signal to the PCM.
Figure 12-3 A replaceable PROM used in an older General Motors computer. Notice that the sealed access panel has been removed to gain access.
Figure 12-4 A typical output driver. In this case, the PCM applies voltage to the fuel pump relay coil to energize the fuel pump.
Figure 12-5 A typical low-side driver (LSD), which uses a control module to provide the ground path for the relay coil.
Figure 12-6 A typical module controlled high-side driver (HSD) where the module itself supplies the electrical power to the electrical 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.
Figure 12-7 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.
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.
The signal is called “di gital ” because the on and off signals are processed by the computer as the digits or numbers 0 and a binary system contains only these two digits.
A digital computer changes continuously variable signals (voltage) called analog to digital bits (binary digits) of information through an analog-to-digital (AD) converter circuit.
Parts of a Computer
The hardware is the mechanical and electronic parts of a computer.
Central processing unit (CPU).
Microprocessor is the central processing unit (CPU) of a computer.
Figure 12-8 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.
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 .
Each combination is mapped in this manner to produce a PROM.
Older vehicle computers used a single PROM that plugged into the computer.
Clock Rates and Timing
The microprocessor communicates by transmitting long strings of 0s and 1s in a language called binary code. But the microprocessor must have some way of knowing when one signal ends and another begins. That is the job of a crystal oscillator called a clock generator.
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.).
The computer ’s processing speed is called the baud rate, or bits per second.
Control Module Locations
Most other engine computers are installed in the passenger compartment either under the instrument panel or in a side kick panel where they can be shielded from physical damage caused by temperature extremes, dirt, and vibration, or interference by the high currents and voltages of various underhood systems.
Figure 12-9 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.
Figure 12-10 The calibration module on many Ford computers contains a system PROM.
Figure 12-11 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.
Figure 12-12 This powertrain control module (PCM) is located under the hood on this Chevrolet pickup truck.
Figure 12-13 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.
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 air flow) sensor.
ECT (engine coolant temperature) sensor.
O2S (oxygen sensor).
TP (throttle position) sensor.
VS (vehicle speed) sensor.
A vehicle computer can do just two things:
Turn a device on.
Turn a device off.
Typical output devices include the following:
Idle speed control.
Evaporative emission control solenoids.
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 means that data is transmitted by a series of rapidly changing voltage signals pulsed from low to high or from high to low. Most modules are connected together in a network because of the following advantages:
A decreased number of wires is needed, thereby saving weight and 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 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. This system of intercommunication of computers or processors is referred to as a network.
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 mileage.
Allows features to be changed with software upgrades instead of component replacement.
SAE COMMUNICATION CLASSIFICATIONS
The Society of Automotive Engineers (SAE) standards include three categories of in-vehicle network communications, including the following.
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 General Motors (8192 bps).
CCD (Chrysler Collision Detection) used by DaimlerChrysler (7812.5 bps).
DaimlerChrysler SCI (Serial Communications Interface) is used to communicate 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.
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).
DaimlerChrysler Programmable Communication Interface (PCI). Meets SAE standard J-1850 pulse width modulated (PWM).
Ford Standard Corporate Protocol (SCP). Meets SAE standard J-1850 pulse width modulated (PWM).
High-speed networks (125,000 to 1,000,000 bits per second [125,000 to 1,000,000 kbs]) are generally used for real time powertrain and vehicle dynamic control. Most high-speed bus communication is controller area network or CAN.
Figure 12-14 A typical bus system showing module CAN communications and twisted pairs of wire.
MODULE COMMUNICATION DIAGNOSIS
Always follow the recommended testing procedures, which usually require the use of a factory scan tool.
Some tests of the communication bus (network) and some of the service procedures require the service technician to attach a DMM, set to DC volts, to monitor communications. A variable voltage usually indicates that messages are being sent and received.
One method of configuring networks is the loop configuration. The loop configuration has two data lines attached to each controller. The controllers can communicate with other controllers in either direction. The advantage of this type of configuration is that an open in the data line will not affect the operation of the network.
The disadvantage to this type of configuration is that if the entire network stops communicating due to a ground in the data line, or a short-to-ground within a controller, each controller will have to be isolated individually by disconnecting to diagnose the fault.
Another more common method of configuring networks is the star configuration. In this configuration, each controller is connected to a single data line wire that is then routed back to a common junction. This junction may be in the form of a bus bar or a splice clip.
The disadvantage to this type of circuit is that any open in a data line will cause that controller to lose communication with the rest of the network.
OBD-II DATA LINK CONNECTOR
All OBD-II vehicles use a 16-pin connector that includes
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) or 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
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
DaimlerChrysler, European, and Asian vehicles use:
ISO 9141-2 standard, which uses pins 4, 5, 7, 15, and 16
Chrysler Group 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
Figure 12-15 Sixteen-pin OBD-II DLC with terminals identified. Scan tools use the power pin (16) and ground pin (4) for power so that a separate cigarette lighter plug is not necessary on OBD-II vehicles.