- A computer is an electronic device that stores and processes data and is capable of operating other devices.
- The operation of the computer is divided into four basic functions: input, processing, storage, and output.
- Binary numbers are represented by the numbers 1 and 0. A transistor that operates as a relay is the basis of the digital computer. As the input signal switches from off to on, the transistor output switches from cutoff to saturation. The on and off output signals represent the binary digits 1 and 0.
- Logic gates are the thousands of field effect transistors that are incorporated into the computer circuitry. The FETs use the incoming voltage patterns to determine the pattern of pulses that leave the gate. The most common logic gates are NOT, AND, OR, NAND, NOR, and XOR gates.
- There are several types of memory chips used in the body computer; ROM, RAM, and PROM are the most common types.
- ROM (read only memory) contains a fixed pattern of Is and O’s representing permanent stored information used to instruct the computer on what to do in response to input data.
- RAM (random access memory) will store temporary information that can be read from or written to by the pP.
- PROM (programmable read only memory) contains specific data that pertains to the exact vehicle in which the computer is installed.
- EPROM (Erasable PROM) is similar to PROM except its contents can be erased to allow new data to be installed.
- EEPROM (Electrically Erasable PROM) allows changing the information electrically one bit at a time.
- NVRAM (Nonvolatile RAM) is a combination of RAM and EEPROM into the same chip.
- Actuators are devices that perform the actual work commanded by the computer. They can be in the form of a motor, relay, switch, or solenoid.
- A servomotor produces rotation of less than a full turn. A feedback mechanism is used to position itself to the exact degree of rotation required.
- A stepper motor contains a permanent magnet armature with two, four, or more field coils. It is used to move the controlled device to whatever location is desired by applying voltage pulses to selected coils of the motor.
Once the computer’s programming instructs that a correction or adjustment must be made in the controlled system, an output signal is sent to an actuator. This involves translating the electronic signals into mechanical motion.
An output driver is used within the computer to control the actuators. The circuit driver usually applies the ground circuit of the actuator. The ground can be applied steadily if the actuator must be activated for a selected amount of time. For example, if the BCM inputs indicate that the automatic door locks are to be activated, the actuator is energized steadily until the locks are latched. Then the ground is removed.
Other systems require the actuator to be turned either on and off very rapidly or for a set amount of cycles per second. It is duty cycled if it is turned on and off a set amount of cycles per second. Most duty cycled actuators cycle ten times per second. To complete a cycle it must go from off to on to off again. If the cycle rate is ten times per second, one actuator cycle is completed in one tenth of a second. If the actuator is turned on for 30% of each tenth of a second and off for 70%, it is referred to as a 30% duty cycle.
FIGURE. Duty cycle is the percentage of on time per cycle. Duty cycle can be changed; however total cycle times remains constant.
If the actuator is cycled on and off very rapidly, the pulse width can be varied to provide the programmed results. For example, the computer program will select an illumination level of the digital instrument panel based on the intensity of the ambient light in the vehicle. The illumination level is achieved through pulse width modulation of the lights. If the lights need to be bright, the pulse width is increased, which increases the length of on-time. As the light intensity needs to be reduced, the pulse width is decreased.
FIGURE. Pulse width is the duration of on time. (A) Pulse width modulation to achieve dimmer dash lights. (B) Pulse width modulation to achieve brighter dash illumination.
Most computer-controlled actuators are electromechanical devices that convert the output commands from the computer into mechanical action. These actuators are used to open and close switches, control vacuum flow to other components, and operate doors or valves, depending on the requirements of the system.
Although they do not fall into the strict definition of an actuator, the BCM can also control lights, gauges, and driver circuits.
FIGURE. The computer’s output driver applies the ground for the relay coil.
Relays. A relay allows control of a high-current draw circuit by a very low current draw circuit. The computer usually controls the relay by providing the ground for the relay coil. The use of relays protects the computer by keeping the high current from passing through it. For example, the motors used for power door locks require a high current draw to operate them. Instead of having the computer operate the motor directly, it will energize the relay. With the relay energized, a direct circuit from the battery to the motor is completed.
FIGURE. The computer controls the operation of the door lock motors by controlling the relays.
Solenoids. Computer control of the solenoid is usually provided by applying the ground through the output driver. A solenoid is commonly used as an actuator because it operates well under duty-cycling conditions.
One of the most common uses of the solenoid is to control vacuum to other components. Many automatic climate control systems use vacuum motors to move the blend doors. The computer can control the operation of the doors by controlling the solenoid.
Motors. Many computer-controlled systems use a stepper motor to move the controlled device to whatever location is desired. A stepper motor contains a permanent magnet armature with two, four, or more field coils. By applying voltage pulses to selected coils of the motor, the armature will turn a specific number of degrees. When the same voltage pulses are applied to the opposite coils, the armature will rotate the same number of degrees in the opposite direction.
FIGURE. Typical stepper motor.
Some applications require the use of a permanent magnet field servomotor. A servomotor produces rotation of less than a full turn. A feedback mechanism is used to position itself to the exact degree of rotation required. The polarity of the voltage applied to the armature windings determines the direction the motor rotates. Hie computer can apply a continuous voltage to the armature until the desired result is obtained.
FIGURE. Reversible permanent magnet motor.
The air charge temperature (ACT) sensor input will be used as an example of how the computer processes information. If the air temperature is low, the air is denser and contains more oxygen per cubic foot. Warmer air is less dense and therefore contains less oxygen per cubic foot. The cold, dense air requires more fuel compared to the warmer air that is less dense. The microprocessor must supply the correct amount of fuel in relation to air temperature and density. An ACT sensor is positioned in the intake manifold where it senses air temperature. This sensor contains a resistive element that has an increased resistance when the sensor is cold. Conversely, the ACT sensor resistance decreases as the sensor temperature increases. When the ACT sensor is cold, it sends a high-analog voltage signal to the computer, and the A/D converter changes this signal to a digital signal.
When the microprocessor receives this ACT signal, it addresses the tables in the ROM. The look-up tables list air density for every air temperature. When the ACT sensor voltage signal is very high, the look-up table indicates very dense air. This dense air information is relayed to the microprocessor, and the microprocessor operates the output drivers and injectors to supply the exact amount of fuel the engine requires.
FIGURE. The microprocessor addresses the lookup tables in the ROM, retrieves air density information, and issues commands to the output devices.
FIGURE. The NOT gate symbol and truth table. The NOT gate inverts the input signal.
FIGURE. The AND gate symbol and truth table. The AND gate operates similar to switches in series.
FIGURE. The AND gate circuit.
FIGURE. OR gate symbol and truth table. The OR gate is similar to parallel switches.
FIGURE. Symbols and truth tables for NAND and NOR gates. The small circle represents an inverted output on any logic gate symbol.
FIGURE. XOR gate symbol and truth table. A XOR gate is a combination of NAND and NOR gates.
Logic gates are the thousands of field effect transistors (FETs) incorporated into the computer circuitry. These circuits are called logic gates because they act as gates to output voltage signals dependingon different combinationsof input signals. The FETs use the incomingvoltage patterns to determine the pattern of pulses leaving the gate. The following are some of the most common logic gates and their operations. The symbols represent functions and not electronic construction:
- NOT gate: A NOT gate simply reverses binary l’s to 0′s and vice versa. A high input results in a low output and a low input results in a high output.
- AND gate: The AND gate will have at least two inputs and one output. The operation of the AND gate is similar to two switches in series to a load. The only way the light will turn on is if switches A and В are closed. The output of the gate will be high only if both inputs are high. Before current can be present at the output of the gate, current must be present at the base of both transistors.
- OR gate: The OR gate operates similarly to two switches that are wired in parallel to a light. If switch A or В is closed, the light will turn on. A high signal to either input will result in a high output.
- NAND and NOR gates: A NOT gate placed behind an OR or AND gate inverts the output signal.
- Exclusive-OR (XOR) gate: A combination of gates that will produce a high-output signal only if the inputs are different.
FIGURE. Simplified temperature sensing circuit that will turn on the air conditioning compressor when inside temperatures reach a predetermined value.
FIGURE. Selection at inputs D, С, B, A will determine which data input will be processed.
FIGURE. Block diagram representation of the MUX and DEMUX circuit.
FIGURE. (A) RS flip-flop symbol. (B) Truth table. (C) Logic diagram. Variations of the circuit may include NOT gates at the inputs, if used, the truth table outputs would be reversed.
FIGURE. Clocked RS flip-flop symbol.
These different gates are combined to perform the processing function. The following are some of the most common combinations:
- Decoder circuit: A combination of AND gates used to provide a certain output based on a given combination of inputs. When the correct bit pattern is received by the decoder, it will produce the high-voltage signal to activate the relay coil.
- Multiplexer (MUX): The basic computer is not capable of looking at all of the inputs at the same time. A multiplexer is used to examine one of many inputs depending on a programmed priority rating. This process is called sequential sampling. This means the computer will deal with all of the sensors and actuators one at a time.
- Demultiplexer (DEMUX): Operates similar to the MUX except that it controls the order of the outputs.
- RS and clocked RS flip-flop circuits: Logic circuits that remember previous inputs and do not change their outputs until they receive new input signals. The illustration shows a basic RS flip-flop circuit. The clocked flip-flop circuit has an inverted clock signal as an input so that circuit operations occur in the proper order. Flip-flop circuits are called sequential logic circuits because the output is determined by the sequence of inputs. A given input affects the output produced by the next input.
- Driver circuits: A driver is a term used to describe a transistor device that controls the current in the output circuit. Drivers are controlled by the microprocessor to operate high-current circuits. The high currents handled by a driver are not really that high; they are just more than what is typically handled by a transistor. Several types of driver circuits are used on automobiles, such as Quad, Discrete, Peak and Hold, and Saturated Switch driver circuits.
- Registers: A register is a combination of flip-flops that transfer bits from one to another every time a clock pulse occurs. It is used in the computer to temporarily store information.
- Accumulators: Registers designed to store the results of logic operations that can become inputs to other modules.
FIGURE. It takes four clock pulses to load 4 bits into the register.
The computer requires a means of storing both permanent and temporary memory. The memories contain many different locations. These locations can be compared to file folders in a filing cabinet, with each location containing one piece of information. An address is assigned to each memory location. This address may be compared to the lettering or numbering arrangement on file folders. Each address is written in a binary code, and these codes are numbered sequentially beginning with 0.
While the engine is running, the engine computer receives a large quantity of information from a number of sensors. The computer may not be able to process all this information immediately. In some instances, the computer may receive sensor inputs that the computer requires to make a number of decisions. In these cases, the microprocessor writes information into memory by specifying a memory address and sending information to this address. When stored information is required, the microprocessor specifies the stored information address and requests the information. When stored information is requested from a specific address, the memory sends a copy of this information to the microprocessor. However, the original stored information is still retained in the memory address.
The memories store information regarding the ideal air-fuel ratios for various operating conditions. The sensors inform the computer about the engine and vehicle operating conditions. The microprocessor reads the ideal air-fuel ratio information from memory and compares this information with the sensor inputs. After this comparison, the microprocessor makes the necessary decision and operates the injectors to provide the exact air-fuel ratio the engine requires.
FIGURE. EPROM memory is erased when the ultra-violet ray contact the microcircuitry.
Several types of memory chips may be used in the computer:
- Read only memory (ROM) contains a fixed pattern of l’s and O’s that represent permanent stored information. This information is used to instruct the microprocessor on what to do in response to input data. The microprocessor reads the information contained in ROM but it cannot write to it or change it. ROM is permanent memory that is programmed in. This memory is not lost when power to the computer is lost. ROM contains formulas, calibrations, and so on.
- Random access memory (RAM) is constructed from flip-flop circuits formed into the chip. The RAM will store temporary information that can be read from or written to by the pR RAM stores information that is waiting to be acted upon and it stores output signals that are waiting to be sent to an output device. RAM can be designed as volatile or nonvolatile. In volatile RAM, the data will be retained as long as current flows through the memory. RAM that is connected to the battery through the ignition switch will lose its data when the switch is turned off (see number 7, nonvolatile RAM).
- Keep alive memory (KAM) is a version of RAM. KAM is connected directly to the battery through circuit protection devices. For example, the microprocessor can read and write information to and from the KAM and erase KAM information. However, the KAM retains information when the ignition switch is turned off. KAM will be lost when the battery is disconnected, the battery drains too low, or the circuit opens.
- Programmable read only memory (PROM) contains specific data that pertains to the exact vehicle in which the computer is installed. This information may be used to inform the microprocessor of the accessories that are equimicroprocessored on the vehicle. The information stored in the PROM is the basis for all computer logic. The information in PROM is used to define or adjust the operating perimeters held in ROM.
- Erasable PROM (EPROM) is similar to PROM except that its contents can be erased to allow new data to be installed. A piece of Mylar tape covers a window. If the tape is removed, the microcircuit is exposed to ultraviolet light that erases its memory.
- Electrically erasable PROM (EEPROM) allows changing the information electrically one bit at a time. Some manufacturers use this type of memory to store information concerning mileage, vehicle identification number, and options. The flash EEPROM may be reprogrammed through the data link connector (DLC) using the manufacturer’s specified diagnostic equipment.
- Nonvolatile RAM (NVRAM) is a combination of RAM and EEPROM in the same chip. During normal operation, data is written to and read from the RAM portion of the chip. If the power is removed from the chip, or at programmed timed intervals, the data is transferred from RAM to the EEPROM portion of the chip. When the power is restored to the chip, the EEPROM will write the data back to the RAM.
Adaptive Strategy and Memory
If a computer has adaptive strategy capabilities, the computer can actually learn from past experience. For example, the normal voltage input range from an ambient temperature sensor may be 0.6 volt to 4.5 volts. If the sensor sends a 0.4-volt signal to the computer, the microprocessor interprets this signal as an indication of component wear and stores this altered calibration in the RAM. The microprocessor now refers to this new calibration during calculations and normal system performance is maintained. If a sensor output is erratic or considerably out of range, the computer may ignore this input. When a computer has adaptive strategy, a short learning period is necessary under the following conditions:
- After the battery has been disconnected.
- When a computer system component has been replaced or disconnected.
- On a new vehicle.
Adaptive memory is the ability of the computer system to store changing values in order to correct operating characteristics. For example, a transmission control module may monitor the transmission’s input and output shaft speeds to determine gear ratio. If the input speed sensor indicates a speed of 1,000 rpms and the output speed sensor indicates a speed of 333 rpms then the controller determines that the ratio is 3.00 to 1 (1st gear). When the controller determines that it will make the shift to second gear (2.00 to 1 ratio) it will monitor the sensors to see how long it takes to achieve the ratio change from 3.00 to 1 to 2.00 to 1. The length of time required represents the amount of fluid needed to stroke the clutch piston and lock up the clutch element. This value is learned so the timing of the shifts can be altered as the clutch elements wear, yet the quality of the shifts will not deteriorate over the life of the transmission.
FIGURE. Main components of the computer and the Microprocessor.
The microprocessor is the brain of the computer. The microprocessor is constructed of thousands of transistors that are placed on a small chip. The microprocessor brings information into and out of the computer’s memory. Hie input information is processed in the microprocessor and checked against the program in memory. The microprocessor also checks memory for any other information regarding programmed parameters. The information obtained by the microprocessor can be altered according to the program instructions. The program may have the microprocessor amicroprocessorly logic decisions to the information. Once all calculations are made, the microprocessor will deliver commands to make the required corrections or adjustments to the operation of the controlled system.
The program guides the microprocessor in decision making. For example, the program may inform the microprocessor when sensor information should be retrieved and then tell the microprocessor how to interpret this information. Finally, the program guides the microprocessor regarding the activation of output control devices such as relays and solenoids. Hie various memories contain the programs and other vehicle data that the microprocessor refers to as it performs calculations. As the microprocessor performs calculations and makes decisions, it works with the memories by either reading or writing information to them.
The microprocessor has several main components. The registers used include the accumulator, the data counter, the program counter, and the instruction register. The control unit implements the instructions located in the instruction register. The arithmetic logic unit (ALU) performs the arithmetic and logic functions.
Remembering the basics of electricity, voltage does not flow through a conductor; current flows and voltage is the pressure that “pushes” the current. However, voltage can be used as a signal; for example, difference in voltage levels, frequency of change, or switching from positive to negative values can be used as a signal.
The computer is capable of reading only voltage signals. A program is a set of instructions the computer must follow to achieve desired results. The program used by the computer is “burned” into integrated circuit (1С) chips using a series of numbers. These numbers represent various combinations of voltages that the computer can understand. The voltage signals to the computer can be either analog or digital. Many of the inputs from the sensors are analog variables. For example, ambient temperature sensors do not change abruptly. The temperature varies in infinite steps from low to high. The same is true for several other inputs such as engine speed, vehicle speed, fuel flow, and so on.
FIGURE. Analog voltage signals are constantly variable. Digital voltage patterns are either on or off. Digital signals are referred to as a square sine wave.
FIGURE. Simplified voltage sensing circuit that indicates if the switch is opened or closed.
Compared to an analog voltage representation, digital voltage patterns are squareshaped because the transition from one voltage level to another is very abrupt. A digital signal is produced by an on/off or high/low voltage. The simplest generator of a digital signal is a switch. If 5 volts are applied to the circuit, the voltage sensor will read 5 volts (a high-voltage value) when the switch is open. Closing the switch will result in the voltage sensor reading close to 0 volts. This measuring of voltage drops sends a digital signal to the computer. The voltage values are represented by a series of digits, which create a binary code. Binary code is represented by the numbers 1 and 0. Any number and word can be translated into a combination of binary l’s and 0′s.
A transistor that operates as a relay is the basis of the digital computer. As the input signal switches from off to on, the transistor output switches from cutoff to saturation. The on and off output signals represent the binary digits 1 and 0.
The computer converts the digital signal into binary code by translating voltages above a given value to 1 and voltages below a given value to 0. As shown, when the switch is open and 5 volts are sensed, the voltage value is translated into a 1 (high voltage). When the switch is closed, lower voltage is sensed and the voltage value is translated into a 0. Each 1 or 0 represents one bit of information.
FIGURE. Each binary 1 and 0 is one bit of information. Eight bits equal one byte.
FIGURE. Binary number code conversion to base 10 numbers.
In the binary system, whole numbers are grouped from right to left. Because the system uses only two digits, the first portion must equal a 1 or a 0. To write the value of 2, the second position must be used. In binary, the value of 2 would be represented by 10 (one two and zero ones). To continue, a 3 would be represented by 11 (one two and one one). Figure illustrates the conversion of binary numbers to digital base ten numbers. If a thermistor is sensing 150 degrees, the binary code would be 10010110. If the temperature increases to 151 degrees, the binary code changes to 10010111.
The computer contains a crystal oscillator or clock circuit that delivers a constant time pulse. Hie clock is a crystal that electrically vibrates when subjected to current at certain voltage levels. As a result, the chip produces a very regular series of voltage pulses. The clock maintains an orderly flow of information through the computer circuits by transmitting one bit of binary code for each pulse. In this manner, the computer is capable of distinguishing between the binary codes such as 101 and 1001.
FIGURE. Interaction of the main components of the computer. All of the components monitor clock pulses.
Signal Conditioning and Conversion
The input and/or output signals may require conditioning in order to be used. This conditioning may include amplification and/or signal conversion.
Some input sensors produce a very low-voltage signal of less than 1 volt. This signal also has an extremely low-current flow. Therefore, this type of signal must be amplified, or increased, before it is sent to the microprocessor. This amplification is accomplished by the amplification circuit in the input conditioning chip inside the computer.
FIGURE. Amplification and interface circuits in the computer. The amplification circuit boosts the voltage and conditions it. The interface converts analog inputs into digital signals. The digital-to-analog converter changes the output from digital to analog.
For the computer to receive information from the sensor and give commands to actuators, it requires an interface. Hie computer will have two interface circuits: input and output. An interface is used to protect the computer from excessive voltage levels and to translate input and output signals. The digital computer cannot accept analog signals from the sensors and requires an input interface to convert the analog signal to digital. The analog to digital (A/D) converter continually scans the analog input signals at regular intervals. For example, if the A/D converter scans the TPS signal and finds the signal at 5 volts, the A/D converter assigns a numeric value to this specific voltage. Then the A/D converter changes this numeric value to a binary code.
FIGURE. The A/D converter assigns a numeric value to input voltages and changes this numeric value to a binary code.
Also, some of the controlled actuators may require an analog signal. In this instance, an output digital to analog (D/A) converter is used.
A computer processes the physical conditions that represent information (data). Hie operation of the computer is divided into four basic functions:
- Input: A voltage signal sent from an input device. This device can be a sensor or a switch activated by the driver or technician.
- Processing: The computer uses the input information and compares it to programmed instructions. Hie logic circuits process the input signals into output demands.
- Storage: The program instructions are stored in an electronic memory. Some of the input signals are also stored for later processing.
- Output: After the computer has processed the sensor input and checked its programmed instructions, it will put out control commands to various output devices. These output devices may be the instrument panel display or a system actuator. The output of one computer can also be used as an input to another computer.
Understanding these four functions will help today’s technician organize the troubleshooting process. When a system is tested, the technician will be attempting to isolate the problem to one of these functions.