Electronic Control Units
Electronic Control Units
Electronic Control Units (ECUs)are small
computers programmed to perform specific
automotive functions.
ECUs use electronic components in integrated circuits to perform their functions.
What are some typical automotive ECUs?
In the 1970’s, the decreasing cost and increasing power of computerized microprocessorslaunched the personal computer industry. Because of their speed and flexibility in carrying out complex functions, microprocessors were adapted for hundreds of uses beyond personal computers.
The first microprocessors began appearing in automotive engine control systems in the early 1980s. In automotive applications, they became known as electronic control units (ECUs). Today, some vehicles may have dozens of ECUs controlling a wide variety of vehicle systems, including:
• engine controls
• transmission
• braking
• steering
• air conditioning
• door locks
• suspension
• cruise control
Electronic
Control Units
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ECU Logic Function
ECUs have electronic logic circuits that “make decisions”
by evaluating conditions according to predetermined rules.
Light Control SW
Light control switch is in the AUTO position and
Light control sensor detects LOW ambient light and
Ignition switch is ON THEN
Turn headlights ON Turn taillights ON
An ECU is a small computer programmed to perform a specialized function in the vehicle. As with any computer, it operates on the principle of input, processing, and output.
Input –Information about conditions is supplied to the ECU as input signals.
Input can be provided by:
• sensors
• switches
• other ECUs.
Processing– The ECU analyzes the input signals. Based on its programming, it determines what output signals to send, if any.
Output– Vehicle systems are controlled by the ECU output signals. These signals may cause a motor to operate, a light to come on, or some other operation of a vehicle component.
For an example of the ECU’s logic function, consider the lighting control system which is within the Body ECU. A simple lighting control system uses three inputs – the light control switch, the light control sensor, and the ignition switch.
When the condition of these three inputs matches the conditions
preprogrammed in the ECU, the ECU turns on the headlights and taillights.
Logic Function
How ECUs
Work
Simple ECU Inputs
Variable Resistance What are some other
types or examples of ECU inputs? Variable Voltage
Voltage Pulse Pattern
Active Speed Sensor
MRE A MRE B Sensor IC
Exhaust Gas
Signals from switches and sensors can supply information to the ECU in several ways.
Voltage ON/OFF– A simple switch opens or closes a circuit. It is the presence or absence of voltage in the circuit that signals the ECU.
Variable Voltage – Some sensors produce a voltage that changes
depending on the conditions the sensor is measuring. The amount of voltage produced at any given moment provides information about the condition at that time.
Variable Resistance– In other types of sensors, electrical resistance increases or decreases as external conditions change. Sensing the changing voltage as a result of changing resistance in the circuit signals the ECU what the conditions are.
Variable Pulse Pattern– Another method for signaling the ECU about changing conditions is to turn a circuit on and off rapidly at a particular frequency. This works especially well for signaling rotational speed. It is the
Simple ECU
Inputs
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Voltage ON/OFF (Switch) Input
12.6V 12.6V
Voltage can also be measured at the ECU terminal.
The ECU detects the state of a ground-side switch by reading the circuit voltage.
When switch ON is detected, the ECU performs a function, such as turning on a lamp.
B+ B+
12.6v 0.1v *
* Conceptual illustration only
* 5V
0V
The diagrams above illustrate a ground-side switch connected to an ECU.
The ECU supplies battery voltage to the switch circuit and provides the circuit’s load (a resistor). The ECU’s electronic circuits detect when the voltage after the load is high (near battery voltage) or low (near ground voltage).
While the switch is open, no current is flowing and the available voltage after the load is near battery voltage. When the switch is closed, current flows and most of the battery voltage is dropped across the load. The available voltage after the load is now near ground voltage.
In this example, the switch controls a lamp, but is not actually part of the lamp circuit. When the ECU senses a voltage drop in the switch circuit, it supplies five volts to the transistor. This in turn closes the lamp circuit, lighting the lamp.
You can detect the same high or low voltage the ECU is detecting by measuring voltage at the appropriate ECU terminal. If the switch is closed and the voltage remains high, you’ll know there is an open in the circuit between the ECU and the switch.
The actual wiring inside the ECU is extremely complex. The ECU circuit details shown in the diagrams above and the diagrams on the following pages are to illustrate concepts, not actual internal connections.
Voltage ON/OFF (Switch) Input
NOTE
SERVICE TIP
ECM
Variable Voltage Input
The oxygen sensor is a voltage generator.
V> 0.45v : air-fuel ratio too rich V= 0.45v : air-fuel ratio correct V< 0.45v : air-fuel ratio too lean Atmosphere
Voltage The engine control module
interprets the voltage to make corrections to the air-fuel ratio.
V
Exhaust Gas
An oxygen sensor is a voltage generator, producing between 0.1v and 0.9v depending on the oxygen content of the exhaust gas compared to the atmosphere.
The engine control module’s electronic circuits measure the amount of voltage generated by the oxygen sensor, and use that information to control the air-fuel ratio.
Variable Voltage
Input
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Variable Resistance Input
A temperature sensor is a type of variable resistor.
Its resistance changes with temperature.
12.6V or 5V
An ECU can detect the change in the sensor’s resistance by measuring voltage.
ECU
V
A temperature sensor is a type of variable resistor whose resistance changes with temperature. This type of sensor is often called a thermistor.
Two types of thermistor are:
Positive temperature coefficient (PTC) thermistor– resistance increases as temperature increases
Negative temperature coefficient (NTC) thermistor– resistance decreases as temperature increases
Thermistors are commonly used for engine coolant temperature sensors and ambient temperature sensors. Modern Toyota vehicles use NTC thermistors exclusively.
Variable
Resistance Input
Pulse Pattern Input
An active wheel speed sensor generates a series of voltage pulses as the wheel rotates.
As rotation speed increases, pulses are generated at a higher frequency.
Lower Rotation Speed Voltage
Time
Voltage
Time Higher Rotation Speed
The ECU measures the pulse frequency to calculate vehicle speed.
MRE A
MRE B Sensor IC
Another type of ECU input is a pulse pattern. When voltage rises
momentarily, then falls, the transient voltage reading is called a pulse. When a component creates multiple pulses, the result is a pulse pattern (or pulse train).
An active wheel speed sensor is a component that generates a pulse pattern.
A magnetic ring mounted on the wheel hub has alternating north-south fields that are detected by the sensor pickup. As the wheel rotates, the alternating magnetic fields are converted into a series of voltage pulses. The frequency of the pulses increases with the wheel rotation speed.
When the pulse pattern is provided as ECU input, the ECU’s circuits are able to measure the pulse frequency and calculate wheel RPM and vehicle speed.
Pulse Pattern
Input
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See Appendix for More Info
AA
Simple ECU Outputs
When the operating conditions are met, the ECU makes a connection to power or ground to energize a circuit
B+
How a Transistor Works (NPN)
When voltage
The simplest way for an ECU to control a vehicle function is to turn a circuit on or off. A circuit can be ground-side switched or power-side switched.
Electronic circuits use transistors for switching circuits on and off. A transistor is a solid-state electronic component having a base, collector and emitter. In the more commonly used NPN transistor, when sufficient voltage is applied to the base, current flows from the collector to the emitter.
One of the advantages of the transistor is that a low voltage at the base is able to control a large current flowing through the collector and emitter. In that respect, a transistor is similar to a relay.
Some transistors also regulate current flow based on the amount of voltage applied to the base. Within the transistor’s limits, a higher base voltage results in a greater flow of current through the collector/emitter. This feature is used in amplifier circuits where the low voltage signal from a microphone regulates current flow in higher power speaker circuits
Simple ECU Outputs
Transistors as
Switches
Pulse Width Modulation
The ECU can open and close a circuit rapidly to control component operation.
The process of varying the amount of time a circuit is ON is called pulse width modulation.
The ECM regulates the injector ON time by regulating the width of the voltage pulse to the injectors.
Notice the pulse width increases at higher load as the ECM increases the injector ON time.
Voltage Pulses
Example
Pulse Width
An ECU’s electronic circuits have the ability to open and close a circuit very rapidly. The ECU can switch a circuit on for a fraction of a second at very precise intervals.
When a circuit is switched ON and then OFF, the momentary change in voltage creates a voltage pulse. (The pulse can be either a momentary increase or decrease in voltage depending on whether the circuit is ground-side switched or power-side switched and where the voltage is measured.)
When the voltage is viewed on an oscilloscope, the voltage pulse’s width represents the amount of time the circuit is switched ON and can be as brief as 1 millisecond or less. In some circuits, the ECU uses the amount of ON time to regulate component operation.
When the ECU varies the width of the voltage pulse (the ON time) to control a component, the process is called pulse-width modulation.
In the above example, the frequency of the pulses changes as well as the
Pulse Width Modulation
NOTE
Measuring Duty Cycle
When the ECU modulates a circuit at a constant frequency, you can measure the circuit’s duty cycle. Duty cycle is the percentage of ON time compared to total cycle time.
Varying the duty cycle can vary the brightness of a lamp or the speed of a motor.
12 V 0 V
If the percentage of ON time decreases, the lamp becomes dimmer.
In a ground-side controlled circuit, measure after the load.
5V
The terms pulse-width modulation and duty cycle are often confused or used incorrectly.
Pulse-width modulationis a function an ECU can perform to turn a circuit on and off rapidly to regulate the amount of ON time. As the pulse width changes, the frequency of the pulses might or might not change depending on the circuit design and intended operation.
When a circuit is switched on and off rapidly at a constant frequency, duty cyclemeasures the percentage of ON time compared to total cycle time. If the circuit is ON 75% of the time, it is operating at a 75% duty cycle. When a circuit is duty-cycle controlled, the pulse frequency does not change – only the percentage of ON time.
An ECU varies the duty cycle to control the speed of a motor or the brightness of a lamp by switching the circuit ON and OFF hundreds of times per second.
Human senses can’t perceive a lamp or motor being cycled on and off that quickly. Nonetheless, the amount of power to the component increases or decreases depending on how much of the time the circuit is ON versus OFF.
As OFF time increases, the net power supplied to a component decreases resulting in the lamp becoming dimmer or the motor running slower. As ON time increases, power increases and the lamp becomes brighter or the motor runs faster.
When the circuit is ground-side controlled, voltage before the load is always battery voltage, and voltage after the switch is zero, or near zero. To observe voltage modulation, place the positive probe between the load and the switch (which may be an ECU).
Duty Cycle
NOTE
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Measuring Duty Cycle
Signals in a power-side controlled circuit are the opposite of signals in a ground-side controlled circuit.
75% ON (powered)
In a power-side controlled circuit, measure before the load.
B+
If the percentage of ON time decreases, the lamp becomes dimmer.
25% ON (powered) 1 cycle (100%) 1 cycle
(100%) ECU
Most circuits in Toyota vehicles are ground-side controlled. When a pulse-width modulated circuit is power-side controlled, the voltage modulation is observable after the ECU and before the load. In this arrangement, the circuit is ON when the voltage rises.
Note that if voltage is measured after the load, a very minute change in voltage occurs as the circuit is modulated. At this point in the circuit, voltage is zero when the circuit is open. When the circuit is closed, ground voltage is present. The difference is usually less than 0.1V and may not be observable depending on your scope settings.
Power-Side
Control
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Self-Diagnosis
The ECU’s internal wiring can be arranged so it can detect when an input circuit is open or shorted to ground.
Throttle Position
Under normal conditions, the ECM senses more than 0V and less than 5V at VTA and VTA2.
5V
2002 Tundra V8 DTC P0120 Throttle/Pedal Position Sensor/Switch “A”
Circuit Malfunction
With either a short or an open in the input circuit, voltage at VTA and VTA2 becomes 0V and the ECU sets a DTC.
A significant reason ECUs have become so common in automobile systems is their ability to perform self-diagnosis. ECUs can identify faults in circuits, components, and even within the ECU itself. When a fault is detected, the ECU can:
• Illuminate a warning light
• Set a diagnostic trouble code
• Begin operating in a fail-safe mode by:
◦ Disabling a system that is working incorrectly
◦ Using sensor data from alternate sources
◦ Applying alternate rules for operating the vehicle or subsystem to maintain maximum safety
An ECU’s self-diagnosis capabilities can range from very simple to highly sophisticated. Each ECU has its own features and limitations, and very few work in exactly the same way.
The example above is a throttle position sensor circuit. The electronics inside the engine control module (ECM) are designed so that an open or a short to ground on VTA or VTA2 can be detected and a DTC set. The circuit arrangement inside the ECM is not able to distinguish a short from an open, however. In either case, the voltage the ECM is monitoring goes to 0V.
Self-Diagnosis
Differences in
Self-Diagnosis
Throttle Position Sensor
ECM
VTA1
VTA2 VC
E2
5V
Self-Diagnosis
ECUs can be wired so they can detect the difference between an open or short, and set a different DTC for each.
DTC P0122 Throttle/Pedal Position Sensor/Switch “A”
Circuit Low Input
DTC P0123 Throttle/Pedal Position Sensor/Switch “B”
Circuit High Input 2003 Tundra V8
What is the normal voltage at VTA2?
In this arrangement, what is the normal voltage at VTA1?
What is the voltage with a short in the circuit?
What is the voltage with an open in the circuit?
In this throttle position sensor circuit, the electronics inside the ECM are arranged slightly differently. In this arrangement, a short to ground on a VTA line causes the monitored voltage to go to 0V. An open in a VTA line, however, causes the monitored voltage to go to 5V.
Thus, this ECM can distinguish between an open or short on an input circuit and can set a DTC for one or the other. The additional data supplied by the ECM makes it easier and faster to diagnose and correct the problem.
Differences in
Self-Diagnosis (Cont’d)
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Like other computers, ECUs have internal memory. Besides storing DTCs, they can also store switch settings and component positions. Over time, ECUs can acquire and store information about the vehicle’s operating characteristics and driver/occupant preferences. The data stored in memory can have a direct affect on how well the vehicle operates and the driver’s perceptions of comfort and convenience.
Volatile memorychips are the type that require constant power to maintain what is stored in them. When the power is removed, their memory contents are erased. These types of memory chips are used for ordinary
microprocessor memory. (RAM for example.)
Non-volatile memorychips retain their contents even when the power is removed. These types of memory chips permanently store the
microprocessor’s operating instructions, or logic. (ROM for example.) Programmable Read Only Memory (PROM)– A memory chip that can be programmed once, but cannot be reprogrammed.
Erasable Programmable Read Only Memory (EPROM)– A programmable chip that can be removed from its circuit and reprogrammed.
Electrically Erasable Programmable Read Only Memory (EEPROM)– A programmable chip that can be electrically erased and reprogrammed without removing it from the circuit.
ECU Memory
• DTCs
• Driver preferences
• Vehicle operating characteristics ECUs have different types of memory.
B+ ECU Memory ECU program logic
ECU program logic, data (reflash)
Types of ECU Memory
ECU Memory
ECU Customization
Because ECUs have memory, they can be programmed with owner/driver preferences.
Main Body ECU Would you like the interior
light turned ON when the doors are unlocked?
Would you like the interior light turned ON when the ignition is turned OFF?
How long would you like the interior lights to be left ON?
No matter how carefully automobile manufacturers analyze the features that new car buyers want, there will always be those who want a feature to work differently. ECUs have made it much easier for owners to customize many of the vehicle’s convenience features to suit their own preferences.
The settings for customizable features are stored in ECU memory. Needless to say, if the memory is lost then any preferences the owner has chosen are also lost. Memory can be lost when the ECU loses its connection to the battery, and also when the ECU is replaced.
Before disconnecting the battery, make note of the owner’s customized settings and restore those settings when service is complete.
When one driver changes a customized setting without informing other drivers, another driver may view the change in operation as a malfunction. Be sure to consider the potential role of customized settings on a customer’s concern before beginning a problem diagnosis.
Customization
SERVICE TIP
NOTE
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ECU Initialization
Initialization procedures can be very different depending on the ECU.
• Unload the vehicle
• Jumper terminals 4 and 8 of DLC3
• Flash the headlights 3 times Headlamp Leveling ECU Initialization
Driver’s Door Power Window Initialization (Body ECU)
• Turn ignition ON
• Hold the switch to open the window
• Hold the switch to close the window
• Keep holding the switch until the switch stops blinking
Examples
Completely Closed
ECUs may need to be initialized when:
• A new ECU is installed
• Key components related to the ECU’s operation have been replaced
• Loss of power erases critical memory settings.
Initializing an ECUsimply means preparing it for operation. If an ECU is not initialized when required:
• The system may be inoperable or operate incorrectly
• Some system features may be disabled.
• Some system features may be disabled.