Mass Air Flow Sensors

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AIR FLOW SENSORS

• Port electronic fuel-injection systems that use
  air flow volume for fuel calculation usually have
  a movable vane in the intake stream.
• The vane is part of the vane air flow (VAF)
  sensor.
• The vane is deflected by intake air flow.

FIGURE 17-1 A vane air flow (VAF) sensor.
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AIR FLOW SENSORS
FIGURE 17-2 A typical air vane sensor with the
cover removed. The movable arm contacts a carbon
resistance path as the vane opens. Many air vane
sensors also have contacts that close to supply
voltage to the electric fuel pump as the air vane
starts to open when the engine is being cranked
and air is being drawn into the engine.
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MASS AIR FLOW SENSOR TYPES

• Hot Film Sensor
• The hot film sensor uses a temperature-sensing
  resistor (thermistor) to measure the temperature
  of the incoming air.
• Hot Wire Sensor
• The hot wire sensor is similar to the hot film
  type, but uses a hot wire to sense the mass air
  flow instead of the hot film.

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MASS AIR FLOW SENSOR TYPES
FIGURE 17-3 This five-wire mass air flow sensor
consists of a metal foil sensing unit, an intake
air temperature (IAT) sensor, and the electronic
module.
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MASS AIR FLOW SENSOR TYPES
FIGURE 17-4 The sensing wire in a typical hot
wire mass air flow sensor.
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KARMAN VORTEX SENSORS
FIGURE 17-5 A Karman Vortex air flow sensor uses
a triangle-shaped rod to create vortexes as the
air flows through the sensor. The electronics in
the sensor itself converts these vortexes to a
digital square wave signal.
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PCM USES FOR AIR FLOW SENSORS

• The PCM uses the information from the air flow
  sensor for the following purposes
• Air flow 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.
• Air flow 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).

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TESTING MASS AIR FLOW SENSORS

• Start the testing of a MAF sensor by performing a
  thorough visual inspection.
• Look at all the hoses that direct and send air,
  especially between the MAF sensor and the
  throttle body.
• Also check the electrical connector for
• Corrosion
• Terminals that are bent or pushed out of the
  plastic connector
• Frayed wiring

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TESTING MASS AIR FLOW SENSORS

• MAF Sensor Output Test
• Tap Test
• Digital Meter Test of a MAF Sensor

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HOT WIRE MAF SENSOR

TOYOTA COURSE 852 ENGINE CONTROL SYSTEMS
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MAF SENSOR CIRCUIT
TOYOTA COURSE 852 ENGINE CONTROL SYSTEMS
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WHAT IS FALSE AIR?
FIGURE 17-6 Carefully check the hose between the
MAF sensor and the throttle plate for cracks or
splits that could create extra (false) air into
the engine that is not measured by the MAF sensor.
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Chapter 15
Mass Air Flow Sensors
MAF Sensor Grams per Second/Voltage Chart
Automotive Fuel and Emission Control Systems
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MAF SENSOR SIGNAL VOLTAGE

TOYOTA COURSE 852 ENGINE CONTROL SYSTEMS
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Chapter 17
Mass Air Flow Sensors

• 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.

Automotive Fuel and Emission Control Systems
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Chapter 17
Mass Air Flow Sensors

• 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.

Automotive Fuel and Emission Control Systems
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Chapter 17
Mass Air Flow Sensors

• 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