Testing digital MAF sensors (with frequency outputs)

The purpose of this test is to check the output from a digital MAF sensor.

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 MAF sensor.
  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

Two waveforms are presented by the PicoScope:

Channel B shows the voltage measured at the sensor output. It is a digital signal switching between two voltages; a low voltage at just above 0 Volts, and a high voltage at just below 5 Volts.

As the digital signal is represented by only two voltage levels, the voltage level does not indicate the air mass flowing through the sensor. Instead the sensor indicates the mass air flow by varying the rate (frequency) at which the digital signal switches between the high and low voltages.

Channel A shows how the switching rate (frequency) of the digital signal varies as the air mass flowing through the sensor changes during a WOT test: high frequencies indicate high mass air flow (e.g. around 6.5 kHz at WOT, as in this example) and low frequencies indicate low mass air flow (e.g. around 3 kHz at idle, as in this example).

When represented in terms of frequency, digital mass air flow sensor waveforms display similar characteristics to their analog, voltage, equivalents.

Waveform Library

The Waveform Library has 76 examples of MAF digital/frequency.

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’.

Further guidance

Digital air mass meters have been introduced on to vehicles to reduce the signal conversion and processing demands that their analog counterparts place on an engine control module.

Firstly, the scaling and corrections that transform the internal sensing element’s output to an accurate representation of mass air flow are made within the sensor, removing the need for extra processing in the engine control module.

Secondly, the air mass information is transmitted as a digital signal, which is not affected by noise and interference in the same way as an analog signal, improving information integrity.

Thirdly, the engine control module does not require the analog-to-digital conversion capabilities needed with an analog sensor, which reduces their cost.

When testing digital mass air flow sensors, ensure you have identified the correct output signal. These sensors often measure both mass air flow and intake air temperature, with each having its own digital signal output from the sensor. At first glance, the digital air temperature signal is similar to the mass air flow signal; however, the former is often a pulse-width modulated waveform with a fixed cycle rate of around 20 kHz.

Digital Bosch HFM6 mass air flow sensors can be reliably tested with still air conditions (ignition on, engine off and exhaust extraction systems removed): under these circumstances the signal frequency must be between 1.76 and 1.93 kHz. A measurement outside of this range indicates a faulty sensor.

GT095-4

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

    £48.00

  • Flexible Back-pinning Probe

    £4.25

  • PicoScope Battery Clip

    £2.75

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Guided test: Digital with frequency output