Airflow sensor - diesel Bosch common rail

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

How to perform the test

  1. Plug a BNC test lead into Channel A on the PicoScope.
  2. Fit a black clip on the black test plug.
  3. Fit an acupuncture or multimeter probe onto the colored test lead.
  4. Place the black clip on the battery negative terminal.
  5. Probe the air flow sensor's output terminal with the acupuncture or multimeter probe as shown 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.
  6. With the example waveform displayed on the screen, press the space bar to start looking at live readings.
  7. 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 centre the waveform when capturing the output.

Example waveform

Waveform notes

The voltage output from the Air Flow Meter (AFM) should be linear to airflow. This can be measured on an oscilloscope and should look similar to the example shown. The waveform should show about 1.0 volt when the engine is at idle, rising to an initial peak as the engine is accelerated and air volume is increased. 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 volts.

This voltage depends, however, on how hard the engine is accelerated, and a lower voltage is not necessarily a fault in the AFM. On deceleration the voltage drops sharply as 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 2 seconds or more is used, enabling you 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 induction pulses as the engine is running.

Technical information

The Bosch common rail diesel system may be either turbocharged or normally aspirated. In both cases the airflow sensor is located next to the air filter.

The airflow meter monitors the volume of air and supplies the ECM with the relevant data. It uses a conventional method of monitoring the mass air by means of a 'hot film'. The incoming air passes over the hot film, cooling it and altering the output voltage. The voltage at the Air Flow Meter's (AFM) output terminal is directly proportional to air flow, with the voltage increasing as the airflow increases.

The component has a 6-pin electrical connector (using only 5 terminals as number 4 is blank), with all pins terminating at the ECM.


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 (Turbo Diesel)