Air flow meter (digital) - frequency

The purpose of this test is to check the output from a digital Air Flow Meter (AFM).

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the AFM output signal circuit.
2. Connect PicoScope Channel A and Channel B and join the test leads.
3. Connect the joined leads to the AFM circuit.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Run the engine and allow to idle.
6. Start the scope to see live data.
7. Depress the accelerator pedal fully until the engine speed approaches maximum RPM, then release the pedal.
8. With your waveforms on screen stop the scope.
9. Turn off the engine.
10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

• Channel B shows the digital signal switching between two voltages.
• Low voltage just above 0 V, and a high voltage just below 5 V.
• Channel A shows the digital signal frequency.
• Low air flow at idle indicated by low frequencies, around 2.5 kHz, and high air flow at WOT indicated by high frequencies, around 6.5 kHz.
• When displayed as frequency the digital sensor provides waveform characteristics similar to its analog counterpart.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Mass air flow sensor (MAF) digital / frequency.

Further guidance

Air flow meters measure the quantity of filtered air entering an engine. As such, they are used by the Engine Control Module (ECM) as the primary engine load sensor.

Digital air flow meters operate in a manner similar to hot-wire (or film) air flow meters but have a digital signal output. The digital signal is not affected by noise and interference in the same way as an analog signal, which improves its integrity.

Furthermore, the scaling and corrections required to convert the signal to an accurate representation of air flow are made within the meter and the subsequent output does not require analog-to-digital conversion. Both these factors reduce the complexity and therefore cost of the ECM.

As the digital signal is represented by only two voltage levels, the voltage amplitude does not indicate the air flow. Instead, the air flow is indicated by the varying frequency of the digital signal.

When testing a digital air flow meter, the output signal must be correctly identified: these units often measure both air flow and intake air temperature and give each a digital output. At first glance, the digital air temperature signal is similar to the air flow signal; however, the former is often a pulse-width modulated (PWM) waveform with a fixed cycle rate of around 20 kHz.

The sensor element and air flow meter body form a calibrated unit and are not interchangeable.

Bosch HFM6 digital air flow meters can be reliably tested with still air conditions, ignition on, engine off and exhaust extraction systems removed. 

Under these conditions the signal frequency must be between 1.76 and 1.93 kHz. A measurement outside of this range indicates a faulty sensor.

GT095