Mass Air Flow Sensors

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Mass Air Flow Sensors
Automotive Fuel and Emission Control Systems
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Barometric Pressure
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• Airflow Sensors
• Port electronic fuel-injection systems that
  use airflow volume for fuel calculation usually
  have a movable vane in the intake stream.
• The vane is part of the vane airflow (VAF)
  sensor.

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• Airflow Sensors (continued)

Courtesy of Bosch
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• Mass Airflow Sensors
• One uses a metal foil sensing element.

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• Mass Airflow Sensors (continued)
• Air flowing past the screen or wire cools it.
  The degree of cooling varies with air velocity,
  temperature, density, and humidity.
• As the screen or wire cools, more current is
  required to maintain the specified temperature.
  As the screen or wire heats up, less current is
  required.
• More intake air volume cooler sensor, more
  current.
• Less intake air volume warmer sensor, less
  current.

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• Mass Airflow Sensors (continued)
• Another contains a platinum wire.

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HOT WIRE MAF SENSOR

TOYOTA COURSE 852 ENGINE CONTROL SYSTEMS
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MAF SENSOR CIRCUIT
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• Karman Vortex Sensors
• In 1912, a Hungarian scientist named Theodore
  Van Karman observed that when air passed over a
  pointed surface that vortexes were created.
• Ultrasonic This type of sensor uses
  ultrasonic waves to detect the vortexes that are
  produced and produce a digital (on and off)
  signal where frequency is proportional to the
  amount of air passing through the sensor.
• Pressure-type DaimlerChrysler uses a
  pressure-type Karman Vortex sensor that uses a
  pressure sensor to detect the vortexes. As the
  airflow through the sensor increases, so do the
  number of pressure variations increase. The
  electronics in the sensor converts these
  pressure variations to a square wave signal,
  whose frequency is in proportion to the airflow
  through the sensor.

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• Testing Mass Airflow Sensors (continued)
• Airflow sensors and mass airflow (MAF) sensors
  are designed to measure all the air entering the
  engine. If an air inlet hose was loose or had a
  hole, extra air could enter the engine without
  being measured. This extra air is often called
  false air.

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MAF Sensor Grams per Second/Voltage Chart
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MAF SENSOR SIGNAL VOLTAGE

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• MAF Sensor Tap Test
• With the engine 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.

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• MAF Sensor Digital Meter Test
• Digital multimeter can be used to measure the
  frequency (Hz) output of the sensor and compare
  the reading with specifications.

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• CHAPTER SUMMARY
• A mass airflow sensor actually measures the
  density and amount of air flowing into the
  engine, which results in accurate engine control.
• An air vane sensor measures the value of the air
  and the intake air temperature sensor is used by
  the PCM to calculate the mass of the air entering
  the engine.
• A hot-wire MAF sensor uses the electronics in the
  sensor itself to heat a wire 70 C above the
  temperature of the air entering the engine.

Automotive Fuel and Emission Control Systems