Air flow meter (hot wire - gasoline)

The purpose of this test is to evaluate the voltage output and response time of the Air Flow Meter (AFM) during engine idle, Wide Open Throttle (WOT) and over-run conditions. 

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the AFM sensor signal circuit.
2. Connect PicoScope Channel A to the sensor signal circuit.
3. Start the engine.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. Carry out a Wide Open Throttle (WOT) test.
7. With your waveform on screen stop the scope.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• With the engine idling, the initial voltage is around 0.5 V.
• When the accelerator pedal is rapidly and fully depressed, the voltage rises rapidly to almost 4.0 V, then falls to around 2.5 V. This initial voltage peak is due to the inrush of filtered air to the intake manifold.
• When the intake manifold air pressure has equalised with the ambient air pressure, the subsequent, airflow is dependent on engine speed. Therefore, as the engine speed increases to its peak, the voltage rises to around 4.0 V.
• When the accelerator pedal is released and the throttle closes, the air intake and signal voltage drop rapidly.
• As the engine overruns and its speed drops, the sensor voltage gradually reduces as the engine returns to its idle state. Some engine control modules will maintain, or increase, airflow in this phase as an anti-stall feature.
• The waveform hash is due to fluctuations caused by engine induction pulses.

Waveform Library

Go to the drop-down menu bar in the lower left corner of the Waveform Library window and select Mass air flow sensor (MAF) hot wire.

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.

Hot wire air flow meters have a heated wire element located within the intake air flow. The voltage, and hence current, through the hot-wire circuit is varied to keep it at a fixed, hot, temperature. As the air flow increases, its cooling effect increases, and the greater the voltage required to keep the wire temperature constant. Therefore, the hot-wire circuit voltage indicates the air flow to the ECM. The sensor element and air flow meter body form a calibrated unit and are not interchangeable.

Due to their position within the air intake tract, air flow meters can be prone to contamination, e.g. if a vehicle is poorly serviced with ineffectual air filters, or if it has other intake, crankcase breather, or exhaust recirculation faults. In such cases, the meter’s readings can be erroneous, causing the ECM to incorrectly calculate the air-fuel mixture, possibly leading to the following symptoms:

• poor performance
• incomplete combustion events
• fuel trim or mixture related faults

Air flow meter circuits (including the power, ground and signal circuits) are also susceptible to common electrical circuit faults, such as:

• open- or short-circuits
• high resistances

Some manufacturers use hot film air flow meters. The testing procedure and operating characteristics are the same as the hot wire type.

GT007-EN