AT 261 - Chapter 9 - Electronic Engine Control Systems



AT 261 - Chapter 6 (B) A-Tech Trainer Boards - Electronic Engine Control Systems. (Part 1)

Name: _________________________________________________

Date: _______________________________________________________

TEMPERATURE SENSORS

One of the most common sensing devices found in automotive applications is the temperature sensor.

Temperature sensor circuits are used in electronic systems to monitor the temperature of various components, fluids, and even the air. Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), and Transmission Fluid Temperature (TFT) are all examples of this type of system. The circuit operation is basically the same for all three systems.

The circuit consists of a control module, temperature sensor, wiring and connectors. The control module contains a voltage regulator, current limiting resistor and a signal processing area that acts like a voltmeter.

The voltage regulator supplies a constant voltage level to the circuit. The control module interprets any voltage fluctuations a sensor change. The supply voltage must be regulated for the system to function properly.

The current limiting resistor is a fixed resistor that protects the circuit from an amperage overload. The resistor limits the amount of current flow if a short-to-ground condition exists between the control module and the temperature sensor.

The voltmeter portion of the control module measures the voltage level at point M. This voltage level depends on the resistance value of the temperature sensor.

The temperature sensor is a variable resistor in which the resistance values change as the temperature of the monitored medium changes. This type of sensor increases in resistance as the temperature decreases, and decreases as temperature increases. This sensor is called a thermistor.

The temperature sensor circuit is a type of voltage divider circuit. In this circuit a limiting resistor is in series with a variable resistor. This configuration creates a voltage drop across the thermistor that is directly proportional to the thermistor’s percentage of the total circuit resistance.

During normal operation, as the temperature being sensed increases, the resistance of the temperature sensor decreases and the voltage level M decreases. The reverse is true, as the sensed temperature decreases, temperature sensor resistance increases and voltage at point M increases. The control module uses the voltage value of M as an input to determine what type of changes should be made in the system. This circuit produces an analog voltage signal ranging from zero to five volts.

NORMAL OPERATION: Resistance Measurement - The Thermistor on the board. It is labeled Rt.

How many terminals does the thermistor have? ________________

( The thermistor is presently at room temperature (ambient). Measure the resistance of the thermistor.

What resistance do you measure at ambient? ______________________________

( With the Ohmmeter still connected, slowly warm up the thermistor with your finger.

What happens to the resistance of the thermistor as it is heated? __________________________________

( Now allow the thermistor to cool.

What happens to the resistance of the thermistor as it cools? ___________________________________

Therefore as temperature ____________, resistance ___________________, and as temperature

___________________, resistance __________________

The operation of this thermistor is similar to the ones used in the automobile. Most thermistors operate in this way. This is known as a negative coefficient thermistor.

Voltage Measurements build the circuit shown above.

A circuit breaker is included in the circuit to prevent possible damage to the components.

( Measure the voltage at the input to Voltage Regulator.

What is the input voltage? __________________________

( Measure the voltage at the output of the Voltage Regulator.

What is the output voltage? ________________________

The electronic control system in an automobile operates at 5 Volts. The job of the voltage regulator is to provide a CONSTANT (Stable) 5 volts that does not fluctuate. The regulator also provides a 5 Volt REFERENCE VOLTAGE to many of the sensors in the automobile, including the thermistor.

( Measure the thermistor voltage at point M in the circuit.

The voltmeter symbol indicates the point where the computer “MONITORS” the thermistor signal.

What is the thermistor voltage at ambient temperature?_________________________________

( Heat up the thermistor and measure the voltage again.

What happens to the thermistor voltage as the thermistor is heated? _____________________________

What happens to the voltage as the device cools down? ____________________________

Complete the following statement:

As temperature __________________, the thermistor voltage ____________________, and as temperature

____________________, this voltage ___________________________.

Does the thermistor produce an ANALOG signal (varying BETWEEN 0 and 5 volts) or a

DIGITAL signal (switched, 0 OR 5 volts)?

_____________________________________________________________________________________

ABNORMAL OPERATION:

These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault.

Open Ground:

( Monitor the voltage at point M in the circuit.

What voltage do you measure at ambient? _____________________________

( REMOVE the ground to the thermistor at point 1.

What voltage do you measure at point M? _________________________________

( REPLACE the ground at point 1.

Open Signal:

( REMOVE the signal wire at point 2, in the circuit.

What voltage do you measure at point M? _____________________________

( REPLACE the signal at point 2.

Short Signal:

( SHORT point 2 to ground with an additional wire.

What voltage do you measure at point M? ________________________________

REMOVE the short at point 2.

( Complete the following table to summarize your results:

Fault Voltage at M

Open Ground ____________________

Open Signal ____________________

Short Signal __________________

During normal operation, the thermistor voltage should be between approximately 0.5 and 4.5 volts. Signals out of this range indicate a fault.

During abnormal circuit conditions, such as an open or short, the circuit control cannot provide an accurate representation of the temperature that it is designed to sense. Any resistance value that exceeds the circuit design will affect the voltage level at point M, giving the control module inaccurate input. An open between the control module and the sensor ground will result in a five-volt reading at point M. A short-to-ground

between the control module and the sensor will result in a near zero voltage level at point M. A higher than normal voltage level will be at point M when there is too much resistance between the control module and the ground for the sensor. The input of the circuit will not represent the temperature being sensed when there is an abnormal circuit condition.

POSITION SENSOR CIRCUIT

Many electronically controlled systems require that a component’s position be monitored throughout its entire field of travel. One such application of this is in the electronic temperature control system where the control module needs to monitor the travel of the throttle. In this case, the module needs continuous feedback on the component’s position. This information is provided by a variable resistance position sensing circuit.

Like the temperature sensor circuit, the position sensor circuit contains a control module, sensor, wiring and connectors The control module has a voltage regulator, limiting resistor and a DC voltmeter function.

Although the position sensor is a variable resistor, it operates differently than the temperature sensor. The resistance of the position sensor changes mechanically. The position sensor contains a movable arm, or wiper, that slides across a fixed resistor. The wiper is mechanically connected to the component that requires monitoring. As the position of the component changes, the resistance of the position sensor changes. By using the voltmeter function, the control module determines the position of the component by the voltage at the wiper.

This circuit is also a voltage divider circuit, but unlike the temperature sensor circuit, it monitors voltage at the sensor by a sensor return line (M).

Although the temperature sensor and position sensor circuits are both voltage divider circuits, the total resistance of the position sensor circuit does not vary, therefore calculating the signal voltage is slightly different.

During normal operation, as the position being sensed moves to one end of its travel, the resistance of the position sensor will either increase or decreases depending on circuit design. The control module uses the monitored voltage as an input to determine what type of changes should be made in the system. If the resistance increases, the monitored voltage at the wiper will increase. The reverse is true as the sensor resistance decreases, the monitored voltage decreases. This circuit produces an analog voltage signal normally ranging from zero to five volts.

During excessively high or low resistance conditions, the circuit cannot give an accurate representation of the position that it is designed to sense. Any resistance value that is not within the circuit design limits will cause an inaccurate input. An open anywhere in either the reference voltage (VREF) or signal line will cause a zero volt reading. The same is true if there is an open in the sensor itself, or it the open is on the VREF side of the wiper. If the open is either in the sensor on the ground side of the wiper, or in the ground line, the module will sense five volts. A short-to-ground in either the reference line or signal line will also cause a zero volt level for monitored voltage. If the ground line to the module grounds prematurely, the input will not be affected.

NORMAL OPERATION: Resistance Measurement - Locate the symbol shows a resistor with an arrow. It is labeled 10K. (See Page 5)

How many terminals does the potentiometer have? ___________________

Measure the resistance across the outer two terminals. __________________

( Turn the potentiometer knob all the way clockwise and measure the resistance across the center terminal (3) (signal) and the bottom terminal (1) (ground).

What resistance do you measure? ___________________

( Begin to turn the knob (wiper) counter-clockwise and make the following measurements:

Knob Position Resistance

¼ Travel __________________

½ Travel __________________

¾ Travel __________________

Full Travel __________________

Complete the following statement:

A change in the knob (wiper) ______________, causes a change in the resistance measured at the wiper.

Voltage Measurements Build the circuit shown below.

What is the constant voltage supplied to this sensor? ____________________

( Turn the potentiometer knob all the way clockwise and measure the voltage at point M.

The voltmeter symbol indicates the point where the computer “MONITORS” the potentiometer signal.

What voltage do you measure? __________________________

( Begin to turn the knob (wiper) counter-clockwise and make the following measurements:

Knob Position Voltage at M

¼ Travel ______________________ V ½ Travel ______________________ V

¾ Travel ______________________ V Full Travel ______________________ V

Complete the following statement:

A change in the knob (wiper) position causes a change in the _________________ measured at the wiper.

Does the potentiometer produce an ANALOG signal (varying BETWEEN 0 and 5 volts) or a DIGITAL signal (switched, 0 OR 5 volts)?

___________________________________________________________________________

ABNORMAL OPERATION:

These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault.

Open Ground:

( Monitor the voltage at point M in the circuit.

( Adjust the knob until you get a reading of about 2.5 volts, which represents a TPS ½ throttle voltage.

( REMOVE the ground to the potentiometer at point 1.

What voltage do you measure at point M? ___________________________

REPLACE the ground at point 1.

Open Reference:

( REMOVE the reference voltage at point 2, in the circuit.

What voltage do you measure at point M? ____________________________

REPLACE the reference at point 2.

Open Signal:

( REMOVE the signal wire at point 3, in the circuit.

What voltage do you measure at point M? __________________________

( REPLACE the signal at point 3.

Short Signal:

( SHORT point 3 to ground with an additional wire.

What voltage do you measure at point M? ___________________________

( REMOVE the short at point 3.

( Complete the following table to summarize your results:

Fault Voltage at M

Open Ground ___________________ V

Open Reference _________________ V

Open Signal ____________________ V

Short Signal ________________ V

ON/OFF POSITION SENSORS

Certain applications require that a device or component be monitored to know only whether it is in one position or another. In these applications, it is not necessary to monitor the full range of the component’s position. In these applications, a switch can be used to provide this information. Almost every electronically controlled system contains at least one switched input circuit.

Unlike the variable resistor type position sensor which provides an analog DC voltage, a switch input circuit only provides a HI/LO, or ON/OFF signal. A switch position circuit produces a digital voltage signal that ranges from zero to the applied voltage. These switched circuits are normally referred to as either power side or ground side switching.

GROUND SIDE SWITCH NORMAL OPERATION — VOLTAGE MEASUREMENTS

Build the circuit shown above.

( Open the toggle switch (up position) and measure the voltage at point M.

The voltmeter symbol indicates the point where the computer “MONITORS” the switched signal.

What voltage do you measure? ___________________

( Now, close the toggle switch.

What voltage do you measure? __________________________

During normal operation, the measured voltage should be 5 volts (switch open) or 0 volts

(switch closed). Signals out of this range indicate a fault.

( Monitor the voltage at point M in the circuit and toggle the switch back and forth.

Describe the voltage at point M? ______________________________________________

What type of signal switches between two voltages (0 and 5 in this case)?___________________________

ABNORMAL OPERATION:

These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault.

Open Ground:

( REMOVE the ground to the switch at point 1.

( Again, monitor the voltage at point M and toggle the switch back and forth.

Describe the voltage at point M. ___________________________

( REPLACE the ground at point 1.

Open Signal:

( REMOVE the signal voltage wire at point 2, in the circuit.

Monitor the voltage at point M and toggle the switch back and forth.

Describe the voltage at point M. ______________________________

( REPLACE the signal at point 2.

Short Signal:

( SHORT point 2 to ground with an additional wire.

( Monitor the voltage at point M and toggle the switch back and forth.

Describe the voltage at point M. _______________________________

REMOVE the short at point 2.

( Complete the following table to summarize your results:

Fault Voltage at M

Open Ground ___________________

Open Signal __________________

Short Signal _____________________

Did the position of the toggle switch make any difference in these measurements? _______

A ground side switch position circuit is similar to a temperature sensor circuit. The most obvious difference is that a switch is connected in series with the limiting resistor instead of a temperature sensor. During normal operation, when the switch is open, there is a complete circuit consisting of the voltage regulator, R1 and the voltmeter. Since the voltmeter has more than ten times the resistance of R1, the voltage level at M is practically five volts.

HALL EFFECT DEVICES

Some Electronic Engine Control systems use an electronic switch position circuit called a Hall Effect Device Circuit.

The Hall effect circuit acts like a ground-side switch position circuit.

The difference is how the switching occurs. The switch circuit uses a mechanical switch; the Hall circuit uses an electronic switch.

The Hall effect circuit consists of a control module, Hall effect device, connectors and wiring.

The control module has a voltage regulator, limiting resistor and signal processor that acts like a voltmeter.

The voltage regulator in the control module supplies a constant voltage level. The limiting resistor is the load of the circuit. The voltmeter monitors the voltage level at the ground side of the resistor. The level switches between high and low as the Hall device opens and closes the circuit. The circuit produces a square wave voltage signal for the control module.

The Hall device has a voltage regulator, Hall element, amplifier, Schmitt trigger and switching transistor. The

voltage regulator in the Hall effect device powers the Hall element, amplifier and Schmitt trigger.

Lines of magnetism perpendicular to the applied voltage create a Hall voltage on the third plane.

The heart of the Hall effect device is the element itself. In 1897, E.H. Hall observed a voltage being created across a conductor carrying an electrical current whenever the conductor was exposed to a magnetic field whose flux lines were perpendicular to the direction of current flow. Later model devices use a semiconductor rather than a pure conductor because the latter creates a larger voltage. The Hall effect device actually consists of the semiconductor element and a permanent magnet.

As the element is exposed to the magnetic field, voltage begins to be induced across the element. Voltage increases as the element is exposed to more of the field. The voltage is at its peak when the element is engulfed in the field and will stay at that level until it begins to be shielded from the field. As more of the element is shielded from the field, the voltage decreases.

In summary, as long as a Hall element is exposed to a magnetic field and powered up, there will be voltage induced across the element. If the element is removed or shielded from the magnetic field, there will be no voltage across it.

Because the Hall element produces a very small voltage, the signal must be strengthened to be used by the rest of the sensor. The signal strength is increased by the amplifier, but the shape remains the same. The signal then must be sharpened by the Schmitt trigger before reaching the transistor. After passing through the trigger, the voltage signal is applied to the base of the transistor. This square wave signal turns the transistor ON and OFF.

The transistor acts like a switch that is turned ON and OFF by the trigger signals. It is the transistor that opens and closes the circuit from the control module.

During normal operation, when the transistor opens, there is still a complete circuit of the voltage regulator and voltmeter inside the control module. Since the voltmeter has more than

ten times the resistance of R1, the voltage level at M is practically five volts. When it closes, the transistor acts like a short-to-ground because it is in parallel with the voltmeter. Now M is at zero volts because all of the voltage is dropped across R1.

The Electronic Engine Control system uses a rotating vane cup to shunt the magnetic field. The vane is mounted on the distributor shaft. As the shaft rotates, the element is repeatedly exposed and then shielded from the magnetic field. This shielding and exposing creates an off/on/off/on voltage signal.

The Hall sensor in this system provides engine RPM and crankshaft position information to the control module.

An open circuit between the control module and the Hall device will result in a constant five-volt level at M. A short-to-ground condition in the same area will cause a constant zero voltage level at M.

HALL EFFECT SENSOR CIRCUIT - Locate the Hall Effect Sensor on the board.

( Place the ferrous metal (Hack Saw Blade) in the slot of the Hall Effect Sensor.

What do you observe? ______________________________________________

What side of the sensor is the magnet on? ______________________________________

( REMOVE the piece of metal.

NORMAL OPERATION:

Voltage Measurement - Build the circuit shown on page 11 using the board.

What is the SUPPLY voltage to the Hall Effect Sensor (voltage at point 2)? _________________

What is the REFERENCE voltage to the Hall Effect Sensor (voltage out of regulator)? _________________

( Using the DVOM, measure the voltage at point M.

The voltmeter symbol indicates the point where the computer “MONITORS” the Hall Effect signal.

What voltage do you measure? _________________

( Now, insert the Hack Saw Blade into the gap, and monitor the voltage at point M.

What do you observe about the voltage? _________________

Is the signal from this component varying between 0 and 5 volts (Analog) or switchedbetween 0 and 5 (Digital)?

_____________________________________________________________________________________

( Now, use a coin instead of the blade.

What do you observe?

_____________________________________________________________________________________

Can you explain why the Hall Effect Sensor behaves this way?

_____________________________________________________________________________________

The Hall element doesn’t switch the signal line on and off. It only supplies the signal, which triggers the electronics to turn the switching transistor on and off. It is the transistor that controls the signal line.

ABNORMAL OPERATION:

These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault.

Open Ground:

( REMOVE the ground to the Hall Effect Sensor at point 1.

Monitor the voltage at point M.

( Repeatedly pass the blade in and out of the gap.

Describe the signal voltage at point M.____________________________________________________

( REPLACE the ground at point 1.

( Open BAT(+):

( REMOVE the power at point 2, in the circuit.

( Monitor the voltage at point M.

( Repeatedly pass the blade in and out of the gap.

Describe the signal voltage. _________________________________________________________________

( REPLACE power at point 2.

Open Signal:

( REMOVE the signal voltage at point 3, in the circuit.

( Monitor the voltage at point M.

( Repeatedly pass the blade in and out of the gap.

Describe the signal voltage. ______________________________________________________________

( REPLACE the signal at point 3.

Short Signal:

( SHORT point 3 to ground with an additional wire.

( Monitor the voltage at point M and Repeatedly pass the blade in and out of the gap.

Describe the signal voltage. ___________________________________________________

( REMOVE the short at point 3.

( Complete the following table to summarize your results:

Fault Voltage at M

Open Ground ___________ V

Open Bat ___________ V

Open Signal ___________ V

Short Signal ___________ V

Did passing the blade in the gap make any difference with these measurements? ___________

Does this electronic ground side switch circuit behave any differently from a mechanical

ground side switch?. ___________

MAGNETIC PICKUP SENSORS

Magnetic pickup circuits are commonly used in any electronic system where rotational speed indication is a factor of system operation. Electronic Distributorless ignition Systems and Antilock Brake Systems both use magnetic pickup circuits. Magnetic pickup circuits operate basically the same regardless of the system.

The circuit consists of a control module, magnetic pickup sensor, reluctor, wiring and connectors. The control module contains a limiting resistor and a signal processing area that acts like an AC voltmeter. These two are in series with each other. The pickup sensor is a variable reluctance sensor. A variable reluctance sensor is a component whose magnetic field can be varied. This is done by passing a ferro magnetic material (reluctor) through the magnetic field of the sensor.

Any higher than normal resistance in the circuit will drop some voltage across it. This circuit condition will decrease the voltage level of the signals to the module. The same pattern may be caused by mispositioning the sensor in relation to the reluctor. If the distance between the two is too great, the magnetic field strength will not change as much as if the two were at the correct distance from each other. A smaller voltage will be included, resulting in weaker signals.

A short-to-ground or an open in the circuit will result in no signal to the control module.

Normal Operation:

Resistance Measurement

( - Locate the Variable Reluctance Sensor (VRS) on the board. It is the one mounted next to the DC motor in the bottom right-hand corner.

[pic]

How many terminals does the VR Sensor have? ________________________________

Measure the resistance of the VR Sensor across points 1 and 2. _____________________________ Ohms

Note: This is the resistance of the coil of wire inside the sensor.

( - Now take a paper clip and touch the threads on the front (motor end) of the sensor.

Is this sensor a permanent or electro-magnet? _______________________________________

Voltage Waveform Measurements:

Make sure the DC motor is switched off.

( - Set up the DVOM to read AC Volts.

( - Connect the DVOM probe tip to point M in the circuit, and connect the ground to point G.

( - Switch the DC motor on.

( - What is the AC voltage? ________________________________

( - Switch the motor off and observe the voltage signal as the motor slows down. Do this several times and answer the following questions.

( - What happens to the voltage? _____________________________________________

Abnormal Operation:

These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault. Switch the motor on, and monitor the voltage waveform at point M in the circuit.

Open/Weak Signal:

( - REMOVE the wire at point 2 and connect it to ground.

Describe the voltage on the DVOM. ______________________________________

( - REPLACE the wire at point 2.

Short Signal:

( - SHORT across points 1 and 2 in the circuit.

Describe the voltage on the DVOM. ______________________________________

( - REMOVE the short across points 1 and 2.

( - Switch the motor off.

Describe the voltage on the DVOM. ______________________________________

( - Switch the motor off.

How many notches does the reluctor wheel on the motor have? __________________________________

Therefore, how many pulses (or notches) are there in one revolution of the reluctor wheel? ____________________

This lab was partially constructed using text, and graphics from the following source;

Model 1835

Advanced Electricity/Electronics

Instructor Guide

ATech Training, Inc.

12290 Chandler Drive • P.O. Box 297

Walton, KY 41094 USA

Phone: (859) 485-7229 • Fax: (859) 485-7299

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