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CHAPTER 22 OBJECTIVES

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CHAPTER 22 OBJECTIVES. Explain the purpose and function of the ECT and IAT temperature sensors. ... Hot Wire Sensor. The hot wire sensor is similar to the hot film ... – PowerPoint PPT presentation

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Title: CHAPTER 22 OBJECTIVES


1
  • CHAPTER 22 OBJECTIVES
  • Explain the purpose and function of the ECT and
    IAT temperature sensors.
  • Describe how to test temperature sensors.
  • Discuss how throttle position sensors work.
  • List the methods that can be used to test TP
    sensors.
  • Discuss how MAP sensors work.
  • List how the operation of the MAP sensor affects
    vehicle operation.
  • Discuss how MAF sensors work.
  • Discuss how O2S sensors work.
  • List how the operation of the O2S sensor affects
    vehicle operation.

2
  • Engine Coolant Temperature Sensors
  • The ECT sensor is also used as an important input
    for the following
  • Idle air control (IAC) position
  • Oxygen sensor closed loop times
  • Canister purge on/off times
  • Idle speed
  • Engine coolant temperature sensors are
    constructed of a semiconductor material that
    decreases in resistance as the temperature of the
    sensor increases.

3
  • Engine Coolant Temperature Sensors (continued)
  • Coolant sensors have very high resistance when
    the coolant is cold and low resistance when the
    coolant is hot.
  • This is referred to as having a negative
    temperature coefficient (NTC).

4
  • Testing The Engine Coolant Temperature Sensor
  • Testing the ECT using a multimeter
  • Both the resistance (in ohms) and the voltage
    drop across the sensor can be measured and
    compared with specifications.

5
  • Testing The Engine Coolant Temperature Sensor
    (continued)
  • General Motors ECT sensor

6
  • Testing The Engine Coolant Temperature Sensor
    (continued)
  • Always consult the manufacturers recommended
    procedures for checking this wiring.
  • Normal operating temperature varies with vehicle
    make and model.
  • Some vehicles are equipped with a thermostat with
    an opening temperature of 180 F (82 C), whereas
    other vehicles use a thermostat that is 195 F
    (90 C) or higher.
  • Before replacing the ECT sensor, be sure that the
    engine is operating at the temperature specified
    by the manufacturer.
  • Most manufacturers recommend checking the ECT
    sensor after the cooling fan has cycled twice,
    indicating a fully warmed engine.
  • Testing the ECT sensor using a scan tool
  • Comparing the temperature of the engine coolant
    as displayed on a scan tool with the actual
    temperature of the engine is an excellent method
    to test an engine coolant temperature sensor.
  • Record the scan tool temperature of the coolant
    (ECT).
  • Measure the actual temperature of the coolant
    using an infrared pyrometer or contact-type
    temperature probe.

7
  • Intake Air Temperature Sensor
  • The intake air temperature (IAT) sensor is a
    negative temperature coefficient (NTC) thermistor
    that decreases in resistance as the temperature
    of the sensor increases.
  • The IAT sensor can be located in one of the
    following locations
  • In the air cleaner housing
  • In the air duct between the air filler and the
    throttle body
  • Built into the mass air flow (MAF) or air flow
    sensor
  • Screwed into the intake manifold where it senses
    the temperature of the air entering the cylinders
  • The IAT sensor information is used for fuel
    control (adding or substituting fuel) and spark
    timing, depending on the temperature of incoming
    air.
  • If the air temperature is cold, the PCM will
    modify the program amount of fuel delivery and
    add fuel.
  • If the air temperature is hot, the PCM will
    subtract the calculated amount of fuel.

8
  • Intake Air Temperature Sensor (continued)
  • Spark timing is also changed, depending on the
    temperature of the air entering the engine. The
    timing is advanced if the temperature is cold and
    retarded from the base programmed timing if the
    temperature is hot.
  • Cold air is more dense and contains more oxygen
    and therefore requires a richer mixture to
    achieve the proper air-fuel mixture. Air at 32
    F (0 C) is 14 denser than air at 100 F (38
    C).
  • Hot air is less dense and contains less oxygen
    and therefore requires a leaner mixture to
    achieve the proper air-fuel mixture.
  • The IAT sensor is a low-authority sensor and is
    used by the computer to modify the amount of fuel
    and ignition timing as determined by the engine
    coolant temperature sensor.
  • The IAT sensor is used by the PCM as a backup in
    the event that the ECT sensor is determined to be
    inoperative.

9
  • Throttle Position Sensors
  • Engines use a throttle position (TP) sensor to
    signal to the computer the position of the
    throttle.

10
  • Throttle Position Sensors (continued)
  • The TP sensor consists of a potentiometer
    variable resistor.
  • A potentiometer is a variable-resistance sensor
    with three terminals.
  • One end of the resistor receives reference
    voltage, while the other end is grounded.
  • The third terminal is attached to a moveable
    contact that slides across the resistor to vary
    its resistance.
  • Depending on whether the contact is near the
    supply end or the ground end of the resistor,
    return voltage is high or low.

11
  • Throttle Position Sensors (continued)
  • A typical sensor has three wires
  • A 5-volt reference feed wire from the computer
  • A ground wire back to the computer
  • A voltage signal wire back to the computer as
    the throttle is opened, the voltage to the
    computer changes
  • Normal throttle position voltage on most vehicles
    is about 0.5 volt at idle (closed throttle) and
    4.5 volts at wide-open throttle (WOT).

12
  • TP Sensor Computer Input Functions
  • The computer senses this change in throttle
    position and changes the fuel mixture and
    ignition timing.
  • The throttle position (TP) sensor used on
    fuel-injected vehicles acts as an electronic
    accelerator pump. This means that the computer
    will pulse additional fuel from the injectors
    when the throttle is depressed.
  • The PCM supplies the TP sensor with a regulated
    voltage that ranges from 4.8 to 5.1 volts. This
    reference voltage is usually referred to as a
    5-volt reference or Vref. The TP output signal
    is an input to the PCM and the TP sensor ground
    also flows through the PCM.

13
  • Ford Throttle Position (TP) Sensor Chart
  • NOTE Generally, any reading higher than 80
    represents wide-open throttle to the computer.

14
  • PCM Uses For The TP Sensor
  • Clear Flood Mode
  • If the throttle is depressed to the floor engine
    cranking, the PCM will either greatly reduce or
    entirely eliminate any fuel injector pulses to
    aid in cleaning a flooded engine.
  • Torque Converter Clutch Engagement and Release
  • The torque converter clutch will be released if
    the PCM detects rapid acceleration to help the
    transmission deliver maximum torque to the drive
    wheels.
  • Rationality Testing for MAP and MAF Sensors
  • As part of the rationality tests for the MAP
    and/or MAF sensor, the TP sensor signal is
    compared to the reading from other sensors to
    determine if they match.
  • For example, if the throttle position sensor is
    showing wide open throttle (WOT), the MAP and/or
    MAF reading should also indicate that this engine
    is under a heavy load.

15
  • PCM Uses For The TP Sensor (continued)
  • Automatic Transmission Shift Points
  • The shift points are delayed if the throttle is
    opened wide to allow the engine speed to
    increase, thereby producing more power and to aid
    in the acceleration of the vehicle.
  • Target Idle Speed (Idle Control Strategy)
  • When the TP sensor voltage is at idle, the PCM
    then controls idle speed using the idle air
    control (IAC) and/or spark timing variation, to
    maintain the commanded idle speed.
  • Air-Conditioning Compressor Operation
  • The TP sensor is also used as an input sensor for
    traction control and air-conditioning compressor
    operation.
  • If the PCM detects that the throttle is at or
    close to wide open, the air-conditioning
    compressor is disengaged.
  • Backs up Other Sensors
  • The TP sensor is used as a backup to the MAP
    sensor and/or MAF in the even the PCM detects
    that one or both are not functioning correctly.
  • The PCM then calculates fuel needs and spark
    timing based on the engine speed (RPM) and
    throttle position.

16
  • Testing The Throttle Position Sensor
  • A TP sensor can be tested using one or more of
    the following tools
  • A digital voltmeter with three test leads
    connected in series between the sensor and wiring
    harness connector or back probing using T-pins.
  • A scan tool or a specific tool recommended by the
    vehicle manufacturer.
  • An oscilloscope.
  • The procedure for testing the sensor using a
    digital multimeter is as follows
  • Turn the ignition switch on (engine off)).
  • Measure the voltage between the signal wire and
    ground (reference low) wire. The voltage should
    be about 0.5 volt.

17
Chapter 22
Computer Sensors
TP Sensor Diagnosis Photo Sequence
Diagnosis and Troubleshooting of Automotive
Electrical, Electronic and Computer Systems
18
  • Testing The Throttle Position Sensor (continued)
  • With the engine still not running (but with the
    ignition still on), slowly increase the throttle
    opening. The voltage signal from the TP sensor
    should also increase. Look for any dead spots
    or open circuit readings as the throttle is
    increased to the wide-open position.

19
Testing The Throttle Position Sensor
(continued) Use the accelerator pedal to depress
the throttle because this applies the same force
on the TP sensor as the drive does during normal
driving. Moving the throttle by hand under the
hood may not accurately test the TP sensor.
20
  • Manifold Absolute Pressure (MAP) Sensors
  • The manifold absolute pressure (MAP) sensor is
    used by the engine computer to sense engine load.

21
  • Manifold Absolute Pressure (MAP) Sensors
    (continued)
  • The relationship among barometer pressure, engine
    vacuum, and MAP sensor voltage includes
  • Absolute pressure is equal to barometric pressure
    minus intake manifold vacuum.
  • A decrease in minimum vacuum means an increase in
    manifold pressure.

22
  • Manifold Absolute Pressure (MAP) Sensors
    (continued)
  • The MAP sensor compares manifold vacuum to a
    perfect vacuum.
  • Barometric pressure minus MAP sensor readings
    equals intake manifold vacuum. Normal engine
    vacuum is 17 21 in. Hg.
  • Supercharged and turbocharged engines require a
    MAP sensor that is calibrated for pressures above
    atmospheric, as well as for vacuum.

23
  • Silicon-Diaphragm Strain Gauge MAP Sensor
  • This is the most commonly used design for a MAP
    sensor and the output is an analog variable
    voltage.
  • One side of a silicon wafer is exposed to engine
    vacuum and the other side is exposed to a perfect
    vacuum.
  • There are four resistors attached to the silicon
    wafer which changes in resistance when strain is
    applied to the wafer.

24
Silicon-Diaphragm Strain Gauge MAP Sensor
(continued)
25
  • Capacitor Capsule MAP Sensor
  • A capacitor-capsule is a type of MAP sensor used
    by Ford and it uses two ceramic (alumina) plates
    with an insulating washer spacer in the center to
    create a capacitor.
  • Changes in engine vacuum cause the plates to
    deflect, which changes the capacitance.

26
  • Capacitor Capsule MAP Sensor (continued)
  • The electronics in the sensor then generates a
    varying digital frequency output signal, which is
    proportional to the engine vacuum.

27
Capacitor Capsule MAP Sensor (continued)
28
Capacitor Capsule MAP Sensor (continued)
29
  • PCM Uses Of The MAP Sensor
  • The PCM uses the MAP sensor to determine the
    following
  • The load on the engine.
  • Altitude, fuel, and spark control calculations.
  • EGR system operation.
  • Detect deceleration (vacuum increases).

30
  • PCM Uses Of The MAP Sensor (continued)
  • Monitor engine condition.
  • Load detection for returnless-type fuel
    injection.
  • Altitude and MAP sensor values.
  • Barometric pressure and altitude are inversely
    related
  • As altitude increase barometric pressure
    decreases
  • As altitude decrease barometric pressure
    increases

31
  • PCM Uses Of The MAP Sensor (continued)
  • Altitude and MAP Sensor Values

32
  • Testing The MAP Sensor Using A DMM
  • Four different types of test instruments can be
    used to test a pressure sensor
  • A digital voltmeter with three test leads
    connected in series between the sensor and the
    wiring harness connector
  • A scope connected to the sensor output, power,
    and ground

33
  • Testing The MAP Sensor Using A DMM (continued)
  • A scan tool or a specified tool recommended by
    the vehicle manufacturer
  • A breakout box connected in series between the
    computer and the wiring harness connection(s). A
    typical breakout box includes test points at
    which pressure sensor values can be measured with
    a digital voltmeter (or frequency counter, if a
    frequency-type MAP sensor is being tested)

34
  • Testing The MAP Sensor Using A DMM (continued)
  • Most pressure sensors use three wires
  • A 5-volt wire from the computer
  • A variable-signal wire back to the computer
  • A ground or reference low wire

35
  • Testing The MAP Sensor Using A DMM (continued)
  • The procedure for testing the sensor is as
    follows
  • Turn the ignition on (engine off)
  • Measure the voltage (or frequency) of the sensor
    output
  • Using a hand-operated vacuum pump (or other
    variable vacuum source), apply vacuum to the
    sensor

36
  • Testing The MAP Sensor Using A DMM (continued)
  • A good pressure sensor should change voltage (or
    frequency) in relation to the applied vacuum.
  • If the signal does not change or the values are
    out of range according to the manufacturers
    specifications, the sensor must be replaced.

37
  • Airflow Sensors
  • The vane airflow sensor used in Bosch L-Jetronic,
    Ford, and most Japanese electronic port
    fuel-injection systems is a moveable vane
    connected to a laser-calibrated potentiometer.
  • The vane is mounted on a pivot pin and is
    deflected by intake airflow proportionate to air
    velocity.
  • As the vane moves, it also moves the
    potentiometer.

38
  • Airflow Sensors (continued)
  • There is a special dampening chamber built into
    the VAF to smooth out vane pulsations which would
    be created by intake manifold air-pressure
    fluctuations caused by the vane opening and
    closing.
  • Many vane airflow sensors include a switch to
    energize the electric fuel pump.
  • This is a safety feature that prevents the
    operation of the fuel pump if the engine stalls.

39
  • Mass Airflow Sensor
  • Hot Film Sensor
  • The hot film sensor uses a temperature-sensing
    resistor (thermistor) to measure the temperature
    of the incoming air.
  • Through the electronics within the sensor, a
    conductive film is kept at a temperature 70 C
    above the temperature of the incoming air.

40
  • Mass Airflow Sensor
  • Hot Film Sensor (continued)
  • Because the amount and density of the air both
    tend to contribute to the cooling effect as the
    air passes through the sensor, this type of
    sensor can actually produce an output based on
    the mass of the airflow.
  • Mass equals volume times density.
  • Therefore, a mass airflow sensor is designed to
    measure the mass, not the volume of the air
    entering the engine.

41
  • Mass Airflow Sensor
  • Hot Film Sensor (continued)
  • Most of these types of sensors are referred to as
    mass airflow (MAF) sensors because unlike the air
    vane sensor, the MAF sensor takes into account
    relative humidity, altitude, and temperature of
    the air.
  • The denser the air, the greater the cooling
    affect on the hot film sensor and the greater the
    amount of fuel required for proper combustion.

42
  • Mass Airflow Sensor (continued)
  • Hot Wire Sensor
  • The hot wire sensor is similar to the hot film
    type, but uses a hot wire to sense the mass
    airflow instead of the hot film.
  • Like the hot film sensor, the hot wire sensor
    uses a temperature-sensing resistor (thermistor)
    to measure the temperature of the air entering
    the sensor.

43
  • Mass Airflow Sensor (continued)
  • Hot Wire Sensor (continued)
  • The electronic circuitry within the sensor keeps
    the temperature of the wire at 70 C above the
    temperature of the incoming air.
  • The operating principle can be summarized as
    follows
  • More intake air volume cooler sensor, more
    current.
  • Less intake air volume warmer sensor, less
    current.
  • The computer constantly monitors the change in
    current and translates it into voltage signals
    that is used to determine injector pulse width.

44
  • Mass Airflow Sensor (continued)
  • Burn-off circuit
  • Some MAF sensors use a burn-off circuit to keep
    the sensing wire clean of dust and dirt.
  • A high current is passed through the sensing wire
    for a short time, but long enough to cause the
    wire to glow due to the heat.
  • The burn-off circuit is turned on when the
    ignition switch is switched off after the engine
    has been operating long enough to achieve normal
    operating temperature.

45
  • PC Uses For Airflow Sensors
  • The PCM uses the information from the airflow
    sensor for the following purposes
  • Airflow sensors are used mostly to determine the
    amount of fuel needed and base pulse-width
    numbers. The greater the mass of the incoming
    air, the longer the injectors are pulsed on.
  • Airflow sensors back up the TP sensor in the
    event of a loss of signal or an inaccurate
    throttle position sensor signal. If the MAF
    sensor fails, then the PCM will calculate the
    fuel delivery needs of the engine based on
    throttle position and engine speed (RPM).

46
  • Testing Mass Airflow Sensors
  • Start the testing of a MAF sensor by performing a
    thorough visual inspection.
  • Check the electrical connector for
  • Corrosion
  • Terminals that are bent or pushed out of the
    plastic container
  • Frayed wiring

47
  • MAF Sensor Output Test
  • A digital multimeter can also be used to check
    the MAF sensor.
  • See the chart that shows the voltage output
    compared with the grams per second of airflow
    through the sensor.
  • Normal airflow is 3 to 7 grams per second.
  • MAF sensor grams per second/voltage chart

48
  • Tap Test
  • With the engine running at idle speed, gently tap
    the MAF sensor with the fingers of an open hand.
  • If the engine stumbles or stalls, the MAF sensor
    is defective.
  • This test is commonly called the tap test.

49
  • Digital Meter Test Of A MAF Sensor
  • A digital multimeter can be used to measure the
    frequency (Hz) output of the sensor and compare
    the reading with specifications.
  • The frequency output and engine speed in RPM can
    also be plotted on a graph to check to see if the
    frequency and RPM are proportional, resulting in
    a straight line on the graph.

50
  • Contaminated Sensor Test
  • Dirt, oil, silicon, or even spider webs can coat
    the sensing wire.
  • Tests for a contaminated MAF sensor include
  • At WOT, the grams per second, as read on a scan
    tool, should exceed 100.
  • At WOT, the voltage, as read on a digital
    voltmeter, should exceed 4.
  • At WOT, the frequency, as read on a meter or scan
    tool, should exceed 7 kHz.
  • If the readings do not exceed these values, then
    the MAF sensor is contaminated.

51
  • Oxygen Sensors
  • In a zirconia oxygen sensor, the tip contains a
    thimble made of zirconium dioxide (ZrO2), an
    electrically conduction material capable of
    generating a small voltage in the presence of
    oxygen.
  • Exhaust from the engine passes through the end of
    the sensor where the gases contact the outer side
    of the thimble.
  • Atmospheric pressure enters through the other end
    of the sensor or thorough the wire of the sensor
    and contacts the inner side of the thimble.

52
  • Oxygen Sensors (continued)
  • The inner and outer surfaces of the thimble are
    plated with platinum.
  • The inner surface becomes a negative electrode
    the outer surface is a positive electrode.
  • The atmosphere contains a relatively constant 21
    percent of oxygen.

53
  • Oxygen Sensors (continued)
  • Rich exhaust gases contain little oxygen.
  • Exhaust from a lean mixture contains more oxygen.
  • Negatively charged oxygen ions are drawn to the
    thimble where they collect on both the inner and
    outer surfaces.

54
  • Oxygen Sensors (continued)
  • Because the oxygen present in the atmosphere
    exceeds that in the exhaust gases, the air side
    of the thimble draws more negative oxygen ions
    than the exhaust side.
  • The difference between the two sides creates an
    electrical potential, or voltage.
  • When the concentration of oxygen on the exhaust
    side of the thimble is low, a high voltage (0.60
    to 1.0 volts) is generated between the electrodes.

55
  • Oxygen Sensors (continued)
  • As the oxygen concentration on the exhaust side
    increases, the voltage generated drops low (0.00
    to 0.3 volts).

56
  • Oxygen Sensors (continued)
  • An O2S does not send a voltage signal until its
    tip reaches a temperature of about 572 F (300
    C).
  • Also, O2 sensors provide their fastest response
    to mixture changes at about 1,472 F (800 C).
  • When the engine starts and the O2S is cold, the
    computer runs the engine in the open loop mode,
    drawing on prerecorded data in the PROM for fuel
    control on a cold engine, or when O2S output is
    not within certain limits.

57
  • Oxygen Sensors (continued)
  • There are several different designs of oxygen
    sensors, including
  • One-wire oxygen sensor
  • The one wire of the one-wire oxygen sensor is the
    O2S signal wire.
  • The ground for the O2S is through the shell and
    threads of the sensor and through the exhaust
    manifold.

58
  • Oxygen Sensors (continued)
  • Two-wire oxygen sensor
  • The two-wire sensor has a signal wire and a
    ground wire for the O2S.
  • Three-wire oxygen sensor
  • The three-wire sensor design uses an electric
    resistance heater to help get the O2S up to
    temperature more quickly and to help keep the
    sensor at operating temperature even at idle
    speeds.
  • The three wires include the O2S signal, the
    power, and ground for the heater.
  • Four-wire oxygen sensor
  • The four-wire sensor is a heated O2S (HO2S) that
    uses an O2S signal wire and signal ground.
  • The other two wires are the power and ground for
    the heater.

59
  • Zirconia Oxygen Sensors
  • The greater the differences between the oxygen
    content between the inside and outside of the
    sensor.
  • Rich mixture
  • A rich mixture results in little oxygen in the
    exhaust stream.
  • Compared to the outside air, this represents a
    large difference and the sensors create a
    relatively high voltage of about 1.0 volt (1000
    mV).

60
  • Zirconia Oxygen Sensors (continued)
  • Lean mixture
  • A lean mixture leaves some oxygen in the exhaust
    stream and did not combine with the fuel.
  • This left over oxygen reduces the difference
    between the oxygen content of the exhaust
    compared to the oxygen content of the outside
    air.
  • As a result, the sensor voltage is low or almost
    zero volt.

61
  • Zirconia Oxygen Sensors (continued)
  • O2S voltage above 450 mV is produced by the
    sensor when the oxygen content in the exhaust is
    low. This is interpreted by the engine computer
    (PCM) as being a rich exhaust.
  • O2S voltage below 450 mV is produced by the
    sensor when the oxygen content is high. This is
    interpreted by the engine computer (PCM) as being
    a lean exhaust.

62
  • Titania Oxygen Sensor
  • The titania (titanium dioxide) oxygen sensor does
    not produce a voltage but rather the presence of
    oxygen in the exhaust.
  • All titania oxygen sensors use a four-terminal
    variable resistance unit with a heating element.
  • A titania sensor samples exhaust air only and
    uses a reference voltage from the PCM.

63
  • Titania Oxygen Sensor (continued)
  • Titania oxide oxygen sensors use a 14-mm thread
    and are not interchangeable with zirconia oxygen
    sensor.
  • One volt is applied to the sensor and the
    changing resistance of the titania oxygen sensor
    changes the voltage of the sensor circuit.
  • As with a zirconia oxygen sensor, the voltage
    signal is above 450 mV when the exhaust is rich
    and low (below 450 mV) when the exhaust is lean.

64
  • Wide-Band Oxygen Sensors
  • A wide-band oxygen sensor, also called a lean
    air-fuel (LAF) ratio sensor or a linear air-fuel
    ratio sensor, allows engines to operate as lean
    as 231 and still maintain closed-loop operation.
  • This type of sensor usually uses five wires.
  • One power wire
  • One ground wire for the electric heater
  • Three sensor wires
  • When the air-fuel mixture is perfectly balanced
    at 14.71, the sensor produces no output current.

65
  • Wide-Band Oxygen Sensors (continued)
  • When the air-fuel mixture is rich, the sensor
    produces a negative current ranging from zero to
    about 2 milliamps, which represents an air-fuel
    ratio of about 121.
  • When the air-fuel ratio is lean, the sensor
    produces a positive current that ranges from zero
    to 1.5 milliamperes as the mixture gets leaner up
    to about 221.

66
  • Closed Loop And Open Loop
  • When the PCM alone is determining the amount of
    fuel needed, it is called open-loop operation.
  • As soon as the oxygen sensor (O2S) is capable of
    supplying rich and lean signals, adjustments by
    the computer can be made to fine tune the correct
    air-fuel mixture.
  • This checking and adjusting of the computer is
    called closed-loop operation.

67
  • PCM Uses Of The Oxygen Sensor
  • Fuel Control
  • The upstream oxygen sensors are one of the main
    sensor(s) used for fuel control while operating
    in closed loop.
  • Fuel Trim
  • Diagnosis
  • The oxygen sensors are used for diagnosis of
    other systems and components.
  • For example, the exhaust gas recirculation (EGR)
    system is tested by the PCM by commanding the
    valve to open during the test.
  • Some PCMs determine whether enough exhaust gas
    flows into the engine by looking at the oxygen
    sensor response (fuel trim numbers).

68
  • PCM Uses Of The Oxygen Sensor (continued)
  • Diagnosis (continued)
  • The upstream and downstream oxygen sensors are
    also used to determine the efficiency of the
    catalytic converter.

69
  • Testing An Oxygen Sensor Using A Digital
    Voltmeter
  • The oxygen sensor can be checked for proper
    operation using a digital high-impedance
    voltmeter.
  • With the engine off, connect the red lead of the
    meter to the oxygen sensor signal wire.

70
  • Testing An Oxygen Sensor Using A Digital
    Voltmeter (continued)
  • Start the engine and allow it to reach
    closed-loop operation.
  • In closed-loop operation, the oxygen sensor
    voltage should be constantly changing as the fuel
    mixture is being controlled.
  • The results should be interpreted as follows
  • If the oxygen sensor fails to respond, and its
    voltage remains at about 450 millivolts, the
    sensor may be defective and require replacement.
    Before replacing the oxygen sensor, check the
    manufacturers recommended procedures.

71
  • Testing An Oxygen Sensor Using A Digital
    Voltmeter (continued)
  • If the oxygen sensor reads high all the time
    (above 550 millivolts), the fuel system could be
    supplying too rich a fuel mixture or the oxygen
    sensor may be contaminated.
  • If the oxygen sensor voltage remains low (below
    350 millivolts), the fuel system could be
    supplying too lean a fuel mixture. Check for a
    vacuum leak or partially clogged fuel
    injector(s). Before replacing the oxygen sensor,
    check the manufacturers recommended procedures.

72
Testing The Oxygen Sensor Using The Min/Max Method
73
  • Testing The Oxygen Sensor Using The Min/Max
    Method
  • Min/max oxygen sensor test chart

74
Testing The Oxygen Sensor Using The Min/Max
Method (contd)
75
  • Testing An Oxygen Sensor Using A Scan Tool
  • A good oxygen sensor should be able to sense the
    oxygen content and change voltage outputs rapidly.

76
  • Testing An Oxygen Sensor Using A Scan Tool
    (continued)
  • To test an engine using a scan tool, follow these
    steps
  • Connect the scan tool to the DLC and start the
    engine.
  • Operate the engine at a fast idle (2500 RPM) for
    2 minutes to allow time for the oxygen sensor to
    warm to operating temperature.
  • Observe the oxygen sensor activity on the scan
    tool to verify closed-loop operation. Select
    snapshot mode and hold the engine speed steady
    and start recording.

77
  • Testing An Oxygen Sensor Using A Scan Tool
    (continued)
  • Playback snapshot and place a mark beside each
    reach of oxygen sensor voltage for each frame of
    the snapshot.
  • A good oxygen sensor and computer system should
    result in most snapshot values at both ends (0 to
    300 and 600 to 1000 mV).
  • If most of the readings are in the middle, the
    oxygen sensor is not working correctly.

78
  • Testing An Oxygen Sensor Using A Scope
  • A scope can also be used to test an oxygen
    sensor.
  • Connect the scope to the signal wire and ground
    for the sensor (if it is so equipped).

79
  • Testing An Oxygen Sensor Using A Scope
    (continued)
  • With the engine operating in closed loop, the
    voltage signal of the sensor should be constantly
    changing.

80
  • Testing An Oxygen Sensor Using A Scope
    (continued)
  • Check for rapid switching from rich to lean and
    lean to rich and change between once every 2
    seconds and 5 times per second (0.5 to 5.0 Hz).

81
Testing An Oxygen Sensor Using A Scope (continued)
82
  • False O2S Readings
  • False Lean
  • False lean indications (low O2S readings) can be
    attributed to the following
  • Ignition misfire. An ignition misfire due to a
    defective spark plug wire, fouled spark plug,
    etc., causes no burned air and fuel to be
    exhausted past the O2S. The O2S sees the
    oxygen (not the unburned gasoline) and the O2S
    voltage is low.

83
  • False O2S Readings
  • False Lean (continued)
  • Exhaust leak in front of the O2S. An exhaust
    leak between the engine and the oxygen sensor
    causes outside oxygen to be drawn into the
    exhaust and past the O2S. This oxygen is read
    by the O2S and produces a lower than normal
    voltage. The computer interrupts the lower than
    normal voltage signal from the O2S as meaning
    that the air-fuel mixture is lean. The computer
    will cause the fuel system to deliver a richer
    air-fuel mixture.

84
  • False O2S Readings (continued)
  • False Lean (continued)
  • A spark plug misfire represents a false lean
    signal to the oxygen sensor. The computer does
    not know that the extra oxygen going past the
    oxygen sensor is not due to a lean air-fuel
    mixture. The computer commands a richer mixture,
    which could cause the spark plugs to foul,
    increasing the rate of misfirings.

85
  • False O2S Readings (continued)
  • False Rich
  • False rich indication (high O2S readings) can be
    attributed to the following
  • Contaminated O2S due to additives in the engine
    coolant or due to silicon poisoning
  • A stuck open EGR valve (especially at idle)

86
  • False O2S Readings (continued)
  • False Rich (continued)
  • A spark plug wire too close to the oxygen sensor
    signal wire, which can induce a higher than
    normal voltage in the signal wire thereby
    indicating to the computer a false rich condition
  • A loose oxygen sensor ground connection, which
    can cause a higher than normal voltage and a
    false rich signal

87
  • False O2S Readings (continued)
  • False Rich (continued)
  • A break or contamination of the wiring and its
    connectors, which could prevent reference oxygen
    from reaching the oxygen sensor resulting in a
    false rich indication (All oxygen sensors require
    an oxygen supply inside the sensor itself for
    reference to be able to sense exhaust gas oxygen.)

88
  • Post Catalytic Converter Oxygen Sensor Testing
  • A changing air-fuel mixture is required for the
    most efficient operation of the converter.
  • If the converter is working correctly, the oxygen
    content after the converter should be fairly
    constant.

89
  • What types of sensors are the CTS and IAT?
  • Where are these sensors found?
  • How many wires are used for these sensors and
    what are they used for?
  • How are these sensors tested?

90
  • What type of sensor is a TPS?
  • How many wires are found on a TPS and what are
    they used for?
  • What is the voltage range of the TPS signal?
  • How can a TPS be tested?
  • What information does the TPS provide to the PCM?
  • What does the PCM use this information for?

91
  • What is the purpose of the MAP sensor?
  • Where is it located?
  • How is this sensor tested?
  • How does a MAF differ from a MAP sensor?
  • How is a MAF sensor tested?

92
  • What are the main types of oxygen sensors
    currently used?
  • How do they differ?
  • How many wires are found on oxygen sensors and
    what are they used for?
  • What tools should be used for testing oxygen
    sensors?
  • How do oxygen levels in the exhaust affect sensor
    output?
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