Air flow meter (digital) - frequency

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

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

This test utilises both Channel A and Channel B on your PicoScope.

Both channels are connected to the same sensor output.

  1. Join the leads from Channel A and Channel B together.
  2. Connect the joined leads into the signal circuit of the AFM unit.
  3. Join the channel earth leads and connect onto the vehicle earth.
  4. Run the engine and allow to idle.
  5. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  6. Select Go or press the space bar to see live data.
  7. Carry out a Wide Open Throttle (WOT) test to capture your waveform.
  8. Turn off the engine.
  9. Use the Waveform Buffer and Zoom 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.

Example mass air flow meter.

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.

Diagnostic trouble codes

Selection of component related Diagnostic Trouble Codes (DTCs):

P00BC Mass or Volume Air Flow "A" Circuit Range/Performance - Air Flow Too Low

P00BD Mass or Volume Air Flow "A" Circuit Range/Performance - Air Flow Too High

P00BE Mass or Volume Air Flow "B" Circuit Range/Performance - Air Flow Too Low

P00BF Mass or Volume Air Flow "B" Circuit Range/Performance - Air Flow Too High

P0100 Mass or Volume Air Flow "A" Circuit Malfunction

P0101 Mass or Volume Air Flow "A" Circuit Range/Performance Problem

P0102 Mass or Volume Air Flow "A" Circuit Low Input

P0103 Mass or Volume Air Flow "A" Circuit High Input

P0104 Mass or Volume Air Flow "A" Circuit Intermittent

P010A Mass or Volume Air Flow "B" Circuit

P010B Mass or Volume Air Flow "B" Circuit Range/Performance

P010C Mass or Volume Air Flow "B" Circuit Low

P010D Mass or Volume Air Flow "B" Circuit High

P010E Mass or Volume Air Flow "B" Circuit Intermittent/Erratic

GT095

Disclaimer
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

  • Premium 6-way breakout lead set

    £269.00

  • Back-pinning Probe Set

    £40.00

  • Flexible Back-pinning Probe

    £3.00

  • PicoScope Battery Clip

    £2.75

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Guided test: Air flow meter (digital) - frequency