Making Sense Out of Sensors

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Making Sense Out of Sensors

• Jim Halderman
• Dayton, Ohio

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We started communicating by writing on the cave
walls.
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As the years went by we progressed. We invented
tools to make communication better.
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Communication through Sensors

• The PCM uses sensors to learn what is
  happening in the engine.

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Sensor Basics

• How does a computer know what is going on under
  the hood?
• How does the voltage change inside the PCM if
  changes occur at the other end?

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Pull Up Resistors

• The voltage changes at the sense terminal inside
  the PCM after the resistor.

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Pull Down Resistor

• The voltage changes if the switch is open or
  closed.

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Resistor Network

• Resistances can be used to simplify inputs.
• Ford Manual
• Lever Position
• (MLP) is an
• example.

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What would happen if some resistance were added
to the circuit?
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If the resistance increases, what will happen if
you are driving at highway speeds in drive?
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Temperature Sensors

• Use a semiconductor material that becomes more
  conductive as the temperature increases.
• This is called a Negative Temperature Coefficient
  (NTC) thermistor.

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Temperature Sensors
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PCM

• What will the PCM read if unplugged?
• What will the PCM read if the wire going to the
  sensor is shorted to ground?

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ECT

• ECT should read the same at IAT at key on engine
  off (KOEO).

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Quick and Easy Metric Temperature Conversion

• Double the Celsius degree number
• Add 25
• Should be close to the Fahrenheit temperature
• Example 50 degrees X 2 10025125
• Actual 122

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A GM Stepped ECT
Switches at 1.0 volt (120F).
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ECT Sensor

• Is the sensor used in a stepped ECT circuit
  different from the sensor used in a one-step
  circuit?
• How many wires are used in a two-step ECT sensor?

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ECT Sensor Authority

• The ECT sensor is a high-authority sensor
  especially at engine start
• Helps to determine the base pulse-width
• Can account for up to 60 of the pulse-width
  determining factors

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(No Transcript)
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Intake Air Temperature (IAT)

• Similar to the ECT sensor except it has opening
  for the airflow.
• Is used by the PCM to modify the fuel and spark
  timing program based on the temperature of the
  air entering the engine

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IAT Sensor Authority

• The IAT is usually considered to be a
  low-authority sensor
• However it is usually capable of causing the PCM
  to add up to 20 to the injector pulse-width if
  the incoming air is cold
• The IAT can cause the PCM to reduce the injector
  pulse-width by as much as 20 if very hot air in
  entering the engine.

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High-Performance IAT Sensor Trick

• If a 10 K Ohm 1/2 watt resistor is used in the
  place of the sensor, the PCM will assume that
  the air temperature is about freezing (32
  degrees)
• This will cause the PCM to advance the timing
  compared to if the air temperature was warmer (4
  to 8 degrees).
• Will increase the pulse width up to 20.

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Throttle Position Sensors

• Three-wire potentiometer
• Five volts from PCM
• Signal return
• Ground

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Testing the TP sensor

• Great location to check for five-volt reference
  (Vref).
• General Motors recommends checking for PCM ground
  voltage drop at the TP sensor. (There should be
  less than 0.035 volts between the TP sensor
  ground terminal and the negative terminal of the
  battery.)

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TP Sensor Waveform(Defective)
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TP Sensor Authority

• The TP sensor can cause the PCM to command up to
  500 (5 times) the base pulse width if the
  accelerator is depressed rapidly to the floor
• Can cause the PCM to reduce the pulse width by up
  to 70 if the throttle is rapidly closed

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MAP versus Vacuum
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MAP Sensor

• Manifold Absolute Pressure (MAP)
• A decrease in manifold vacuum means an increase
  in manifold pressure.
• Compares manifold vacuum to a perfect vacuum.

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Silicon-Diaphragm Strain Gauge Design MAP Sensor

• Most commonly used.
• Silicon wafer is exposed to engine vacuum.
• This results in changes in resistance due to
  strain on the resistors attached to the wafer
• (called Piezo-resistivity).
• Resistors are connected to a Wheatstone bridge
  and then to a differential amplifier, which
  creates a voltage in proportion to the vacuum
  applied.

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Silicon-Diaphragm MAP Sensor
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MAP Voltage

• Normal engine Vacuum is 17-21 in. Hg.
• MAP sensor voltage is normally between 0.88 volts
  to 1.62 volts (GM).
• 17 in. Hg. is equal to about 1.62 volts.
• 21 in. Hg. is equal to about 0.88 volts.
• Therefore, a good reading should be about 1 volt.

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Capacitor-Capsule MAP Sensor

• Used by Ford.
• Uses two alumina plates with an insulating washer
  spacer to create a capacitor.
• The deflection due to engine vacuum changes the
  capacitance.
• The electronics in the sensor translate this into
  a frequency output.

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Ford Frequency versus Vacuum

• KOEO. 156-159 Hz (0 in. Hg.)
• Idle (sea level).. 102-109 Hz (17-21 in.
• 
  Hg.)
• WOT. 156-159 Hz (Almost 0
• in. Hg.)

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Ceramic Disc MAP

• Used by DaimlerChrysler.
• Ceramic disc converts manifold pressure into a
  capacitive discharge.
• The discharge controls the amount of voltage drop
  delivered by the sensor to the PCM.
• The output is the same as the previously used
  strain gauge/Wheatstone bridge design.

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Ceramic Disc MAP
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MAP versus BARO

• KOEO MAP should equal BARO.
• Will vary with altitude and weather conditions.
• The BARO reading is set at key on and updated if
  the throttle is detected to be at WOT and will
  update the BARO reading.

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Testing a MAP Sensor

• Key on engine off (KOEO). Voltage should be
  4.6 to 4.8 volts at sea level.
• Check for vacuum to the sensor.
• Check the hose.
• Replace the MAP sensor if anything comes out of
  the sensor.

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MAP Sensor Authority

• The MAP sensor is a high-authority sensor on an
  engine that uses the Speed-Density method of fuel
  control.
• If the exhaust is rich, try disconnecting the MAP
  sensor.
• If the engine now runs OK, then the MAP sensor is
  skewed or giving the PCM wrong information.

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High-Performance MAP Sensor Trick

• Insert a plastic vacuum fitting into the vacuum
  line to the MAP sensor
• Use a hot straight pin and burn a small hole in
  the plastic fitting creating a small vacuum leak
• Do not exceed 0.020 inch hole
• PCM assumes a higher engine load and increases
  the injector pulse-width

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Air Vane Sensor

• Usually contains an internal IAT sensor
• Works similar to a TP sensor where the air vane
  is used to move a potentiometer
• Airflow moves the vane, which causes a switch to
  close to power the fuel pump.

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Air Vane Sensor
This is not a mass air flow sensor.
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Karman-Vortex

• Named for Theodore Van Karman, a Hungarian
  scientist (1881 1963).
• He observed the vortex phenomenon in 1912.
• This type of sensor has proven to be very
  reliable and not subject to dirt.

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Karman-Vortex
This is not a mass air flow sensor.
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Ultrasonic Karman Vortex

• Used by Mitsubishi in many vehicles.
• Very reliable.
• Early versions used LEDs and phototransistors,
  which were subject
• to dirt.

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Pressure-Type Karman Vortex

• DaimlerChrysler uses a Karman Vortex sensor that
  uses a pressure sensor to detect the vortexes.
• As the flow increases, so do the number of
  pressure variations.
• The electronic circuitry in the sensor converts
  these pressure variations to a square wave signal
  that is proportional to the airflow through the
  sensor.

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Mass Air Flow

• A hot wire is used to measure the mass of the
  air entering the engine.
• The electronics, in the sensor itself, try to
  keep the wire 70 C above the temperature of the
  incoming air.
• The more current (amperes) needed to heat the
  wire, the greater the mass of air.
• The current is converted to a frequency.

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MAF Sensor
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Normal MAF Readings

• Use a scan tool to look at the grams per second.
• Warm the engine at idle speed with all
  accessories off. Should read 3 to 7 grams per
  second.
• GM 3800 V-6 should read 2.37 to 2.52 KHz.
• If not within this range, check for false air or
  contamination of the sensor wire.

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MAF Sensor Diagnosis

• If the MAF sensor wire were to become coated, it
  cannot measure all of the incoming air.
• A normal warm engine at idle should be 3 to 7
  grams per second.
• Rapidly depress the accelerator pedal to WOT. It
  should read over
• 100 grams per second or
• higher than 7 kHz or
• 4 volts

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MAF and Altitude Reading

• Barometric pressure (BARO) is determined by
  the Powertrain Control Module (PCM) software at
  WOT.
• At high airflows, a contaminated MAF sensor
  will under estimate airflow coming into the
  engine, and therefore, the PCM determines that
  the vehicle is operating at a higher altitude.

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Visual Inspection

• Look for a very dirty filter.
• Look for fuzz on the sensing wire from fibers
  coming off of the filter paper.

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Fuzzy MAF
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Cleaning a MAF Sensor

• Can clean the sensing wire using alcohol and a
  Q-tip, if you are careful.
• Brake clean ?
• Contact Cleaner ?
• Cable Tie ?

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False Air
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Mass Air Flow (MAF)-False Air
Usually affects operation in drive may run OK if
driving in reverse.
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MAF Sensor Authority

• High authority sensor
• If in Doubt-Take it Out
• If the MAF sensor is disconnected, the PCM
  substitutes a backup value.
• If the engine runs OK with the MAF disconnected,
  then the MAF has been supplying incorrect
  information.

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Oxygen Sensors

• Oxygen sensors react to the presence or absence
  of oxygen in the exhaust.
• The voltage signal is used by the PCM for fuel
  control.
• The mixture must switch from rich to lean for the
  three-way catalytic (TWC) converter to work.

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O2 Sensor
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Oxygen Sensors

• An absence of oxygen results in a voltage of
  higher than 450 mV.
• The presence of oxygen results in a voltage lower
  than 450 mV.

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Mud-Coated O2S
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Do Not Solder O2S Wires

• Universal oxygen sensors are often used when
  replacement becomes necessary.
• Many O2S breathe through the wiring itself.
• If the wires are soldered, then the sensor cannot
  detect outside oxygen.
• Use crimp and seal connectors if a universal
  sensor is used.

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Crimp and Seal Connectors
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Oxygen Sensor Diagnosis

• If the fuel system is functioning correctly, the
  oxygen sensor voltage should fluctuate from above
  800 mV to below 200 mV.
• As the oxygen sensor degrades, the range narrows.
• A sensor should at least be capable of reading
  above 600 mV and go below 300 mV.

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O2 Sensor Diagnosis (Continued)

• Use a digital multimeter (DMM) set to read DC
  volts and use the min/max feature.
• Back probe the signal wire and operate the engine
  normally while recording the readings using
  min/max.
• The maximum reading should be above 800 mV and
  the minimum reading below 200 mV.

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O2 Sensor Diagnosis (Continued)

• Check the average of the O2 sensor readings
• If the average is above 450 mV, then the engine
  is operating rich or the sensor is skewed high.
• If the average is below 450 mV, then the engine
  is operating lean or the sensor is skewed low.

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Rich to Lean
Should switch in less than 100 ms.
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False Lean Readings
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False Lean Readings

• A cracked exhaust manifold can cause oxygen to be
  drawn into the exhaust upstream from the oxygen
  sensor.
• An ignition misfire can also cause a false
  low-oxygen sensor reading.
• Remember that the oxygen sensor looks at the
  oxygen in the exhaust, not the unburned fuel!

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Antifreeze Contamination

• If the engine has had a blown head gasket, be
  sure to check or replace the oxygen sensor.
• The silicates can coat the sensor.
• Dexcool and other organic acid technology (OAT)
  coolants do not cause this problem.

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Antifreeze on an O2 Sensor
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Oxygen Sensor Authority

• The O2S is a high-authority sensor when the
  engine is operating in closed loop.
• Can add or subtract up to 25 from the base pulse
  width (Some vehicles even more)
• If the sensor is skewed, it can create a
  driveability problem.
• If in doubt, take it out.
• If the sensor voltage is not connected, the PCM
  will go into open loop.

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High/Low Authority Sensors

• High-Authority
• ECT especially when the engine starts and is
  warming up.
• O2S-while the engine is operating in closed loop
• MAF
• TP

• Low-Authority
• IAT
• TFT
• PRNDL
• KS

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Position Sensors

• Two Types
• Analog sensors - such as magnetic or
  variable-magnetic sensors
• Digital sensors - such as Hall-Effect
  magnetic-resistive or optical sensors.

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The first magnetic sensors were called pulse
generators (pickup coils).
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Magnetic Sensor Operation
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Magnetic Sensors

• Used for Crankshaft Position (CKP)
• Used for Camshaft Position (CMP)
• Used for wheel speed sensors (WSS)

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Magnetic Sensors

• First,
• be sure they are magnetic.
• A cracked magnet becomes two weak magnets.

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Magnetic Sensor

• The sensor housing should not be cracked or
  melted.

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Ford Probe Story (2.5 liter V-6)

• Problem occurred at highway speed.
• Stopped running.
• Was towed to shop.
• Runs, but at idle only.
• Dies as soon as accelerator is depressed.
• Everything checks out OK.

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Magnetic Sensor

• Found a loose CKP sensor.

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Magnetic Sensors

• Produce an AC voltage signal when the magnetic
  field strength changes.

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Crankshaft Position (CKP) Sensor
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Camshaft Position (CMP) Sensor

• Can be magnetic
• Look at the sequence to determine application
• Notice that this is a 2,3,2 sequence sensor
  wheel.

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Wheel Speed Sensor
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The Soldering Gun Trick

• Hold a soldering gun near a magnetic sensor and
  the changing magnetic field around the soldering
  gun will induce a voltage into the windings of
  the magnetic sensor,
• The frequency will be 60 Hz.
• If used on a VSS and the ignition is on, the
  speedometer will read 54 mph (GM).

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Hall-Effect Sensors

• Effect discovered by Edwin H. Hall in 1879.
• He discovered that a voltage is created if a
  magnetic field is exposed to an element.
• The voltage goes to zero if the magnetic field is
  shunted or blocked.
• Very accurate.

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Hall-Effect

• Three-wire sensor
• Power
• Ground
• Signal
• Output is a square wave.
• Very accurate and will work at lower speeds than
  a magnetic sensor.

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Hall-Effect
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Hall-Effect
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Optical Sensor
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Optical Sensor

• Produces a square wave signal.
• Very accurate can be used to show every 1
  degree of crankshaft rotation.
• Optical sensors do not like light must be
  shielded (sparks inside the distributor can cause
  problems if the sensor is not shielded).

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Optical Sensor
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Magnetic-Resistive Sensors

• Generates a digital signal using two magnets.
• Electronics in the sensor generate a square-wave
  output signal.

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CKP and CMP Waveforms
What type of sensor is used for the CKP? What
type of sensor is used for the CMP?
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Toyota Wheel Speed Sensor

• Can detect forward or reverse motion.

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Questions?
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Jim Haldermanjim_at_jameshalderman.com