# Mass Air Flow sensors (MAF)

The mass air flow sensor has become the standard technique used by vehicle manufacturers to accurately measure the mass of the air the engine is consuming. Having an accurate measure of the intake air mass it is possible to calculate fuel delivery and other engine operating parameters very accurately. These sensors can also be known as Air Mass Meter (AMM) sensors and have no movable mechanical parts to wear out or fail. The output signal, depending on design, can be an analog DC voltage, or a digital waveform where the duty cycle or the frequency is the information carrier.
The Hitachi MAF commonly used by GM puts out a high frequency with large air flow and a low frequency with a low air flow. Common range is 2900-5000 Hz. The old GM style had a range of 45-72 Hz.

Bosch (top) and Hitachi (GM) (bottom) sensors

The BOSCH HFM5 MAF sensor output is an analog voltage that can vary from approximately 0V up to 5V. Commonly, with no air flow, (key on, engine off) the output voltage is 0.98-1.02V. These sensors are capable of measuring reverse airflow through the intake duct. With reverse air flow, due to, for example, aggressive camshaft profiles, the voltage can be less than 1V.
MAF or AMM sensors failures can be a total lack of signal from the sensor. However, more common failures are under or over reporting the actual air flow to the engine management computer. In some cases the sensor is accurate but "slow". If the sensor is slow, the engine may not start as easy as it should or may not accelerate as it should, especially when cold.
These problems are commonly caused by contamination of the sensing element.
The self diagnostics system incorporated in to the engine management computer may not be able to detect a MAF sensor being out of calibration or slow. In some cases the best and only diagnostic approach is to use an oscilloscope.
The speed of the MAF can be checked using an oscilloscope and performing a snap throttle test.
When performing this test, set the USB Autoscope II to record the waveform. Attach th input lead from the oscilloscope to the signal wire and set your timebase and voltage settings as appropriate.
When the engine is at idle the pressure in the intake manifold is very low (high vacuum). The weight or mass of the air present is negligible. When the throttle is quickly opened to WOT, as in a snap throttle test, air will quickly rush in to fill this low pressure area. Air will continue to rush in until the pressure on the inside of the intake manifold is equal to the pressure on the outside.
However, due to the large velocity of the incoming air, the manifold will actually briefly be "overfilled" as evidenced by the spike in the accompanying waveform. The voltage then settles at the value corresponding to the actual air flow. As the engine RPM increases, the air flow and the MAF voltage increases as well. The pulses or ripple present in the waveform is due to the opening and closing of the intake valves.

The waveform from the BOSCH HFM5 MAF or AMM at snap throttle. A: This is the voltage value at the cursor and corresponds to the maximum voltage from the sensor; Snap throttle the moment in time that the throttle was "snapped".

The maximum value should be close to the maximum value that can be achieved by running the engine at maximum load and RPM. For this particular sensor the voltage should go above 4V.
When diagnosing these sensors, also check the voltage value with the engine stopped. For this sensor the value should be 1Vą0,02V.
The speed of the sensor can be estimated by measuring the time from applying voltage to the sensor and until the sensor is providing a usable signal.

The waveform from a good BOSCH HFM5 MAF at turn-on. A: This is the signal value from the sensor key on engine off and is equal to 0,99V; dT The time between the two cursors. In this case it is the time between applying power to the sensor (turning the ignition key to the run position) and achieving a usable signal from the sensor. In this case it is ~0,5ms.

It has been shown that as the sensing element becomes contaminated, the time interval increases significantly.

The waveform from a bad BOSCH HFM5 MAF at turn-on. A: This is the signal value from the sensor key on engine off and is equal to 0,92V; dT The time between the two cursors. In this case it is the time between applying power to the sensor (turning the ignition key to the run position) and achieving a usable signal from the sensor. In this case it is ~70ms.

Vane air flow meters use a movable flap attached to a return spring and a potentiometer. The flap will move according to the the volume of air flowing in to the engine. As the flap moves, the wiper on the potentiometer will move as well.
The resistance track for the potentiometer is executed on a ceramic substrate. As the wiper or contact arm moves, a varying voltage signal, corresponding to the air flow, is sent to the engine management computer. The signal can be low with high air flow and high with low air flow or the opposite, depending on design. In either case, over time, the constant movement of the wiper against the resistance track can cause the track to wear out, causing intermittent signal drop out and drive ability concerns.
The Vane air flow meter is sensitive to dirt, from, for example, a dirty or missing air filter. If the dirt contamination becomes severe enough it can cause the movable flap to seize. In some cases the dirt accumulation can cause an improper signal value to be sent to the engine controller, also causing drive ability concerns.