MASS AIR FLOW SENSOR (MAF)

Automotive Sensors

Communication Systems

Ignition Systems

Others

Diesel Systems

General description
      A mass air flow (MAF) sensor responds to the amount of air flowing through a chamber containing the sensor. It is intended to be insensitive to the density of the air.
      The sensor for volume consumption of air is used in many systems for management of engines for measuring value of the transitory cost of air. Air consumption is one of the basic parameters for calculating the necessary amount of fuel. MAF usually is placed after the air filter and before the throttle valve in the air flow sucked into the engine. 

Appearance 
Fig. 1 shows the MAF sensor made by BOSCH, and fig. 2 shows the MAF made by GM.

   
Fig. 1                                                    Fig. 2

Types of sensors
According to the principle of operation are:

  • With analog output signal. Sensor’s output signal voltage depends on the air consumption – VAF and Hot Wire sensors.
  • With a digital output. Sensor’s output signal frequency or duty cycle depends on the air consumption – HFM sensors.

Depending on the type of construction:

  • A sensor which measures the volume (l / h) of the air flow – Vane Meter Sensor (VAF and also known as LMM).
  • A sensor which measures the mass (kg/h) of the air flow – Hot Wire MAF sensor (also known as HLM).
  • A sensor which measures the mass (kg/h) of the air flow – Hot Film MAF (HFM).

      Nowadays the most common are MAF, because they don’t have mechanical moving parts and have great performance and accuracy. This type of sensor is not sensitive to the pulsations associated with opening and closing of intake valves and the output reading does not depend on the density of incoming air.

Working principle of the MAF sensor
MAF sensor which measures the mass of air flow – Hot Wire sensor
        Sensor of this type is shown in fig. 3.  Hot wire (2) with diameter of 70μm is mounted in a measuring tube located before the throttle valve.


Fig. 3

      Operation of MAF is based on the principle of constant temperature. Heated platinum wire, suspended in the engine’s air stream (3), is one of the legs of a Wheatstone bridge. Constant temperature of approximately 100 ºС is maintained by increasing or decreasing the electrical current flowing through the circuit while the incoming airflow cools the wire.
      By increasing the airflow, platinum wire gets cool and its resistance decreases. The Wheatstone resistor bridge is asymmetrical and a voltage appears which is submitted to an amplifier and directed to raise the wire temperature. This process continues until the temperature and resistance of the conductor does not lead to system balance. The current range is 0.5A – 1.2А.
      This current also flows through a calibration resistor and forms a voltage drop which enters the onboard controller for calculation of the quantity of the injected fuel. Changes of temperature are compensated by the resistor (4), which is a platinum ring, suspended into the air stream. The temperature changes simultaneously influence both the heated resistance conductor (2) and the temperature compensation resistor (4), and thus the Wheatstone resistor bridge remains balanced.
      During operation the platinum wire inevitably pollutes. To prevent the pollution after turning off the engine, the wire is heated to a temperature of 1000 º C for 1 sec. Thus all the dirt stick to the wire is burned. This process is controlled by the onboard controller.

MAF sensor which measures the mass of air flow – Hot Film sensor (HFM)


Fig. 4

      Hot-film MAF sensors function much like a hot wire sensor, and used a centrally-heated film or metallic grid-type element. One side of the film encounters cooling airflow, while the shielded backside maintains a consistent temperature, and the current differential between the two is measured and relayed as a square-wave digital frequency output, between around 30Hz at idle and 150Hz at wide-open throttle. Hot film sensors tend to be more robust and less susceptible to contamination than hot-wire types.

MAF sensor for volume consumption of air – VAF sensor
      Sensors for volume consumption of air (fig.5) have an air barrier (4) equipped with a return spring. This barrier is placed into the airflow consumed by the engine and is moved proportionately with increasing or decreasing the airflow.


Fig. 5

      The sensor is also equipped with an additional barrier (2), which serves not only for balance but as damper against fluctuations.
The barrier is mechanically connected to the potentiometer wiper (3). Supply voltage is fed to the potentiometer. Its output voltage depends on the position of the barrier position and the barrier position itself depends on the volume of the airflow.
Sensor’s measuring potentiometer is made on a ceramic substrate. The voltage divisor resistor’s terminals are made on the substrate and are arranged in a row and covered with a resistive layer.
      Potentiometer wiper is pressed to the contact resistive layer and due to the electrical contact between the wiper and the resistive layer wiper’s voltage is always equal to the voltage at the point of contact with the resistive layer. The potentiometer wiper is mechanically connected to the airflow movable barrier and each time the barrier position is changed it also moves in constant contact along the resistive layer, crawling on it. These shifts in constant contact along the resistive layer wears the potentiometer, which over time leads to damage of the measuring potentiometer. Consequently, the deterioration in some places of the contact, resistive layer disappears leaving only the ceramic substrate. Moving the wiper in such a worn area causes unstable or even lost electrical contact and the potentiometer’s output voltage will no longer correspond to the position of the movable barrier.
      In case of a serious air filter pollution or failure, the air channels of volume airflow sensor may get highly contaminated. Therefore, the movable barrier may get stuck from time to time or even stuck completely. Thus the output signal will no more correspond to the real air flow.
      Disadvantage of the volume air flow sensor is that it measures the volume of the incoming air. Therefore, it is necessary to calculate the amount of fuel to determine the mass of air and thus to adjust the sensor readings in accordance with the air density. Solution to this problem is placing an additional temperature sensor along with the air volume sensor.
      Output signal of MAF, made by BOSCH is a variable voltage in the range 1 - 5V, whose value depends on the mass of air flow through the sensor. At zero air flow (engine stopped) the sensor output voltage should be equal to 0.98V - 1.02V. Otherwise, the sensor is considered damaged. Increasing the air flow leads to increase of the sensor’s output voltage. This sensor can also detect the reverse air flows from the intake manifold to the air filter. Output voltage in this case reduced below 1V, proportional to the size of the return air flow.

General problems with MAF sensors:

  • Output signal does not change upon variation of the intake air flow.
  • Deviation of the output signal value from the correct one.
  • Reduction of the response speed of the sensor. In this case the engine significantly lost its "agility" and it is getting difficult to start the engine when cold. Reduction of the speed reaction in case of contamination of the heating resistor and the two temperature sensors.

NOTE: Self-test ECU will not register the MAF’s response speed reduction, as result of which this failure can not be found by reading the error codes with code reader. Reduced response speed can only be verified by using oscilloscope.

Principle of verification of the MAF sensor by using an oscilloscope

      When diagnose MAF with oscilloscope, speed of response of the sensor can be checked under a snap acceleration. At this point following happens: While the engine is idling (no load), the air, filling the intake manifold, is highly diluted because the air flow is almost completely restricted by the throttle valve and the idle control valve. Manifold absolute pressure is lower than the atmospheric pressure of 0.6-0.7 bar. The internal volume of the intake manifold is proportional to the operational volume of the engine, but the mass of the diluted air, filling the manifold during engine idling without load, is negligible.
      In case of a snap acceleration, the air immediately surges to the intake manifold and quickly fills the manifold volume until the absolute pressure in it becomes close to the atmospheric. This process happens very quickly whereat the flow of air through the MAF at that time reaches a level close to the air consumption of the engine during maximum load. Once the absolute pressure in the intake manifold gets close to the atmospheric pressure, air flow passing through the MAF becomes proportional to engine speed.
      The maximum value of output voltage signal of the MAF immediately after a snap acceleration should reach a value close to that in case of maximum engine load. For the sensors manufactured by BOSCH, the output voltage signal should increase briefly up to 4V.
      During the diagnosis it’s necessary to determine the value of the output signal of the sensor when engine is stopped and the average value of the signal with the engine is idling without load. 1V±0.02V value of the output voltage corresponds to zero air flow. The response speed can be assessed by monitoring the transition process when applying power to the sensor. Naturally, with increasing the pollution, the time of the transition process of the output signal increases rapidly.

Procedure to verify functionality of the MAF sensor

      Initially, the intake manifold has to be inspected for cracks, damages and its mounting position should be verified. Significant depressurization of the air collector can cause an engine explosion and depressurization in limited areas may affect the air/fuel mixture proportion.

— MAF SENSOR FOR VOLUME CONSUMPTION OF AIR (VAF TYPE)—

  • Connect the negative terminal of the voltmeter to the chassis ground.
  • Identify the voltage supply terminal and the ground terminal.
  • Connect the positive voltmeter terminal to the wire connected to the MAF sensor signal terminal.
  • Remove the air duct.
  • Remove the air filter assembly so that the valve (plate) of the MAF to be easily opened and closed.
  • Open and close the valve several times to check if it operates smoothly and does not stuck.
  • Turn on the ignition (engine is not running) - the voltage shown by the voltmeter must be between 0.2V ¸ 0.3V.
  • Open and close the MAF several times - the voltage shown by the voltmeter should gradually increase until it reaches 4.0V ¸ 4.5V.
  • Install the air duct. Start the engine and left it idling – you should read voltage in range 0.5V¸ 1.5V.
  • Open the throttle valve (press the accelerator pedal) so that the engine speed can grow up to 3000 rpm – voltage reading should be 2.0V ¸ 2.5V.
  • Open the throttle valve (press the accelerator pedal) briefly - in this case the voltage should be more then 3.0V.
  • If you take the measurements with oscilloscope, you should be observing the following waveform (fig. 6):


Fig. 6

-- Possible damages in volume sensor:
      Chaotic output signal

  • Chaotic output signal is present when the output voltage of the MAF sensor changes step by step, falls to zero or vanishes completely.
  • When the output signal of MAF is chaotic, the cause usually is in the resistive layer of the sensor or the valve (plate) is stuck. In this case you should replace the MAF sensor.
  • Sometimes when moving, the movable lever can move away from the conductive wire. This can also be a reason for producing a chaotic output signal.
  • Remove the top cover of the MAF sensor and check whether the lever touches the wire when moving from open to closed position. If the lever doesn’t touch the wire, in order to produce signal, it must be carefully folded until it touches the wire, or the wire should be cleaned thoroughly. This often helps to eliminate the causes of appearance of chaotic output signal.

      Missing voltage signal

  • Check the reference voltage of 5.0V on the supply terminal of the MAF sensor.
  • Check the condition of grounding in the ground terminal of the MAF.
  • If voltage is applied and grounding is correct, check the signal wire between the MAF sensor and the onboard controller.
  • If there is a problem with the supply voltage or with the grounding, you have to check the condition of wires between the MAF sensor and the onboard controller.
  • If all the wires are correct, you need to check all supply and ground terminals of the onboard controller. If supply voltages and grounds are good, the onboard controller itself falls under suspicion.
  • The signal or the reference voltage is equal to the voltage of the car battery.

      Check resistance

  • Connect an ohmmeter between the signal terminal of the MAF sensor and the supply voltage terminal or between the signal terminal of the MAF sensor and the ground terminal.
  • Open and close MAF valve several times – you should notice the smooth resistance change. When the plate of the flow is moving slowly from closed to fully open position, the MAF resistance can increase and decrease step by step, which is normal. If the resistance is equal to infinity or zero, this means that the MAF sensor is faulty.
  • You won’t see values of MAF resistance in this procedure – resistance varies in high range depending on the manufacturer of the MAF sensor. More important is the proper functioning of the sensor, rather than compliance of regulatory value of resistance.
  • Ohmmeter is connected between the MAF sensor ground terminal and the MAF sensor supply voltage terminals. The resulted resistance must be stable. If resistance inclines infinity or is equal to zero, the MAF sensor must be replaced.

— MAF SENSOR FOR MASS AIR CONSUMPTION —
(HOT WIRE)

-- Check output signal

  • Turn on the ignition - you should read voltage around 1.4V.
  • Start the engine and left it idling – voltage reading should be about 2V.
  • Open and close the throttle valve (press the accelerator pedal) rapidly several times. The voltage should increase significantly compared to the voltage measured at idle speed and no load.
  • The verification of the output signal of the Hot Wire MAF sensor is too complicated as it is impossible to simulate the condition of full load in service shop. This can only be performed with a dynamometer. But the procedure described below allows you to check the output signal for uninterruptedness (carrying out this procedure by using oscilloscope is considered significantly more reliable.)
  • Disconnect the air duct so that the hot wire can be accessed.
  • Turn on the ignition.
  • Use a piece of plastic tube to blow air over the hot wire. This should lead to a change in the sensor’s output voltage.

— MAF SENSOR FOR MASS AIR CONSUMPTION — 
      These are digital sensors so the output signal is a square-wave frequency dependable. Frequency depends on the throttle position - 30Hz at idle engine speed and 150Hz at wide-open throttle. Therefore output signal can be assessed only by using an oscilloscope.

-- Oscilloscope measurements
Hot Wire Sensor
      Connect the active oscilloscope probe to the signal terminal of the sensor and the ground probe - to the chassis ground.
Press the throttle pedal rapidly. Good operating sensor will have the following waveforms as in fig. 7.



Fig. 7
Note the value of the signal voltage at the first peak, it should be about 4.5V. 
Fig. 8 shows the voltage level in "half-dead" sensor and in fig. 9 and fig. 10 - a malfunctioning sensor.

Fig. 8 

Fig. 9 

Fig. 10

--- Possible damage to MAF sensor:
      Interrupted output signal

  • The signal will be interrupted when the voltage does not change smoothly, if falls to zero or if the circuit is broken.
  • The MAF resistance is checked as follows: ohmmeter is connected between terminals 2 and 3 of the MAF sensor’s connector - the resistance should be 2.5 – 3.1 Ω.
  • When the output signal of the MAF is chopped, with interruptions, and the supply the voltage and the ground are good, it is an indication for a damaged MAF sensor. In this case it must be replaced.

      Missing signal voltage

  • Check the power supply from the battery to the 5th terminal of the MAF sensor connector.
  • Check the connection of terminals 1 and 2 to the ground.
  • If the supply voltage and the ground are correct, you should check the connection between the MAF sensor and the onboard controller.
  • If the supply voltage and/or ground are bad, you should check conductivity of the power supply and/or the ground wires between the MAF sensor and the onboard controller.
  • If all the wires are correct, you should check the power supply and ground terminals of the onboard controller. If supply voltages and grounds are good, the onboard controller itself falls under suspicion.

Hot Film Sensor (HFM)
      Connect the active oscilloscope probe to the signal terminal of the sensor and the ground probe - to the chassis ground.
Press the throttle pedal rapidly. Good operating sensor will have the following waveform as in fig. 11. Frequency must vary from approximately 30Hz to 150Hz depending on the throttle position.


Fig. 11

Note the slight rounding of the square waveform edges. This is normal and should not be considered as a fault. 

-- Possible damage to MAF sensor:
      Interrupted output signal

  • The signal will be interrupted when the frequency does not change smoothly, if falls to zero or if the circuit is broken.
  • When the output signal has interruptions or is out of range, and the supply the voltage and the ground are good, it is an indication for a damaged MAF sensor. In this case it must be replaced.

      Missing signal voltage

  • Turn the ignition and check the power supply from the battery to the 2th terminal of the MAF sensor connector.
  • Turn the ignition on and check for +5V voltage on the 4th terminal.
  • If supply voltages are correct, you should check the connection between the MAF sensor and the onboard controller.
  • If supply voltages are bad, you should check conductivity of the power supply and/or the ground wires between the MAF sensor and the onboard controller.
  • If all wires are correct, you should check the power supply and ground terminals of the onboard controller. If supply voltages and grounds are good, the onboard controller itself falls under suspicion.
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