Air flow meter - hot film - turbo diesel

The purpose of this test is to evaluate a hot film Air Flow Meter’s (AFM) voltage output with a turbo diesel engine during engine idle, acceleration, and over-run conditions.

How to perform the test

View connection guidance notes.

1. Use manufacturer's data to identify the Air Flow Meter (AFM) output terminal.
2. Connect PicoScope Channel A.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Start the engine.
6. Depress the accelerator pedal fully until the engine reaches maximum RPM then release the pedal.
7. With your waveforms on screen stop the scope.
8. Turn off the engine.
9. 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 at idle, the waveform voltage varies between 1.5 and 2.0 V.
• After the accelerator pedal is rapidly and fully depressed, the engine speed increases rapidly and the voltage rises to over 4 V.
• After the accelerator pedal is released, the engine speed decreases as it overruns, causing the air-mass to reduce and the waveform voltage to rapidly decline.
• With the engine approaching idle speed around 1200 rpm, an anti-stall function is activated, at around 3.5 s in the waveform, causing the air mass to decrease more gradually.
• The waveform hash is caused by induction pulses as the engine runs.

Waveform Library

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

Further guidance

Air flow meters measure the quantity of filtered air entering an engine. Typically, Engine Control Modules (ECM) use these meters as the primary engine load sensor.

Hot film, or wire, air flow meters have a heated element located within the intake air flow. The voltage, and hence current, through the heated element’s 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 element’s temperature constant. Therefore, the heating 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.

The testing procedure and operating characteristics of hot wire and hot film air flow meters are the same.

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 smoke limitation value, leading to poor performance and/or incomplete combustion with excessive black smoke from the exhaust.

GT009-EN