Air flow meter - hot wire

The purpose of this test is to evaluate the voltage output and response time of the AFM during engine idle, WOT and over-run conditions. 

How to perform the test

  • Plug a BNC test lead into Channel A on the PicoScope.
  • Fit a large black clip on the black (negative) plug on the test lead and an acupuncture or multimeter probe on the coloured (positive) plug.
  • Place the black clip to the battery negative terminal and probe the air flow sensor's output terminal with the Back-pinning Probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available.

With the example waveform displayed on the screen, you can now hit the space bar to start looking at live readings. Snap the accelerator quickly from idle to full throttle and observe the waveform. When testing the air flow meter, it may take several attempts to capture the output with the waveform centred on the screen.

Example waveform

Waveform notes

The voltage output from the Air Flow Meter (AFM) should be proportional to airflow. This can be measured on an oscilloscope and should look similar to the example shown. The waveform should show approximately 1.0 volt when the engine is at idle, but this voltage will rise as the engine is accelerated and air volume is increased, producing an initial peak. This peak is due to the initial influx of air and drops momentarily before the voltage rises again to another peak of about 4.0 to 4.5 volts. This voltage will, however, depend on how hard the engine is accelerated, and a lower voltage is not necessarily a fault within the AFM.

On deceleration the voltage drops sharply as the throttle butterfly closes, reducing the airflow, and the engine returns to idle. The final voltage drops gradually on an engine fitted with an idle speed control valve, as this slowly returns the engine to base idle as an anti-stall characteristic. This function normally only affects the engine speed from around 1200 rpm back to the idle setting.

A timebase of about 10 seconds is used, which enables the operator to view the AFM's output voltage on one screen, from idle, through acceleration and back to idle again. The 'hash' on the waveform is due to the vacuum change from the induction pulses as the engine is running.

Technical information

This form of air flow meter is, in many ways, better than the conventional air vane meter as it offers very little resistance to the flow of incoming air. The mass air flow is measured by the cooling effect on a heated wire that is suspended in the air passage, and it is the air flow's cooling effect on the wire that signals to the Electronic Control Module (ECM) the quantity of incoming air.

The AFM is once again located between the air filter and the throttle butterfly. Inside the component are two wires, one of which is used to convey the temperature of the incoming air and the other heated to a high temperature (approximately 120°C) by passing a small current through it. As the air flows across the heated wire, it has a cooling effect on it causing a temperature change; a small circuit inside the component increases the current passing through the wire to maintain the temperature, and it is the recognition of this current that signals to the ECM the mass air flow.

The current supplied to the heated wire alters proportionally to the air flow. Any wire that is constantly heated forms an oxide coating; so to clean the wire after each journey, a current is passed through the wire heating it to approximately 1000°C, burning off any build-up and ensuring a clean wire for the next time the vehicle is started.

More resources on the web

Air Mass Meter Evaluation

This video shows how PicoScope can be used to evaluate an air mass meter. First a good air mass meter on a Ford Mondeo 2.0 is tested, then an air leak is simulated to show the effect on the air mass meter and the performance of the engine.



This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Multimeter Probes


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Hot Wire (Petrol)