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There are two basic types of mass airflow sensors: hot wire and hot film. In the first type, a very thin wire (about 0.2 mm thick) is used as the heated element.



Components of a hot wire-type mass airflow sensor. Courtesy of Ford Motor Company.

The element temperature is set at 100° to 200°C above incoming air temperature. Each time the ignition switch is turned to the off position, the wire is heated to approximately 1,000°C for 1 second to burn off any accumulated dust and contaminants.

The second type uses a nickel foil sensor, which is kept 75°C above ambient air temperatures. It does not require a burn-off period and therefore is potentially longer lasting than the hot wire type.

A faulty MAF will cause driveability problems resulting from incorrect ignition timing and improper air/fuel ratios.

Vane-type MAF Sensors

A vane-type MAF sensor is found on many import and domestic vehicles with EFI. All intake air must flow through the sensor. Some MAF sensors are called volume air flow meters

* Begin checking a vane-type MAF sensor by checking the voltage supply wire and the ground wire to the MAF module before checking the sensor voltage signal.
* Always follow the recommended test procedure in the manufacturer's service manual and use the specifications supplied by the manufacturer.
* Typically, to test the sensor, a digital multimeter (DMM) is used and set on a DC voltage scale. The negative meter lead is connected to ground and the red lead to the MAF signal wire.




A voltmeter connected to measure the signal from a MAF sensor. Reproduced with permission from Fluke Corporation.

* Turn on the ignition switch and press the min/max button, if available, on the DMM.
* Slowly push the MAF vane from the closed to the wide-open position, and allow the vane to slowly return to the closed position.




Move the MAF sensor air vane from open to close to test it. Reproduced with permission from Fluke Corporation.

* Observe the maximum and minimum voltage readings as the vane was moved.
* If the minimum voltage signal is zero, there may be an open circuit in the MAF sensor variable resistor.
* When the voltage signal is not within the manufacturer's specifications, replace the sensor.

WARNING

While pushing the mass air flow sensor vane open and closed, be careful not to mark or damage the vane or sensor housing.

* Some vehicle manufacturers specify ohmmeter tests for the MAF sensor.
* With the MAF sensor removed, connect the ohmmeter across the sensor's output and input terminals.




Ohmmeter connections to a MAF sensor. Reprinted with permission.

* The resistance at these terminals is normally 200 to 600 ohms.
* Connect the ohmmeter leads to the specified MAF sensor terminals, and move the vane from the fully closed to the fully open position.
* With each specified meter connection and vane position, the ohmmeter should indicate the specified resistance.



Resistance specifications for a typical MAF sensor with door open and closed. Reprinted with permission.

* When the ohmmeter leads are connected to the sensor's input and output terminals, the ohmmeter reading should increase smoothly as the sensor vane is opened and closed.
* To check a vane-type MAF with a lab scope, connect the positive lead to the output signal terminal and the negative scope lead to a good ground.
* This type MAF should display an analog voltage signal when the engine is accelerated. A defective MAF will have sudden and erratic voltage changes.




The trace of a defective vane-type MAF sensor. Reproduced with permission from Fluke Corporation.

Hot-Wire-Type MAF Sensors

The test procedure for heated resistor and hot-wire MAF sensors varies depending on the vehicle make and year. Always follow the test procedure in the appropriate service manual. A frequency test may be performed on some MAF sensors, such as the AC Delco MAF on some General Motors' products.

* To check the MAF sensor's voltage signal and frequency, connect a voltmeter across the MAF voltage signal wire and ground wire.
* Start the engine and observe the voltmeter reading.
* On some MAF sensors, this reading should be 2.5 volts.
* Lightly tap the MAF sensor housing with a screwdriver handle and watch the voltmeter pointer.
* If the pointer fluctuates or the engine misfires, replace the MAF sensor.
* Some MAF sensors have experienced loose internal connections, which cause erratic voltage signals and engine misfiring and surging.
* Set the DMM so that it can read the frequency of DC voltage.
* With it still connected to the signal wire and ground, the meter should read about 30 Hz with the engine idling.
* Now, increase the engine speed, and record the meter reading at various speeds.
* Graph the frequency readings. The MAF sensor frequency should increase smoothly and gradually in relation to engine speed.
* If the MAF sensor frequency reading is erratic, replace the sensor.




Satisfactory and unsatisfactory MAF sensor frequency readings. Reproduced with permission from Fluke Corporation.

* When a scanner is used to diagnose a General Motors' vehicle, one test mode displays grams per second from the MAF sensor. This mode provides an accurate test of the MAF sensor.
* The grams per second reading should be 4 to 7 with the engine idling.
* This reading should gradually increase as the engine speed increases.
* When the engine speed is constant, the grams-per-second reading should remain constant.
* If the grams-per-second reading is erratic at a constant engine speed or if this reading varies when the sensor is tapped lightly, the sensor is defective.
* A MAF sensor fault code may not be present with an erratic grams-per-second reading, but the erratic reading indicates a defective sensor.
* Frequency-varying types of MAF sensors can be tested with a lab scope.
* The waveform should appear as a series of square waves.




A normal trace for a frequency-varying MAF sensor. Courtesy of Progressive Diagnostics--WaveFile AutoPro.

* When the engine speed and intake air flow increases, the frequency of the MAF sensor signals should increase smoothly and proportionately to the change in engine speed.
* If the MAF or connecting wires is defective, the trace will show an erratic change in frequency.




The trace of a defective frequency-varying MAF sensor. Courtesy of EDGE Diagnostics Systems.