Air flow meter - air vane

The purpose of this test is to evaluate the voltage output from the internal track of the Air Flow Meter during engine idle, WOT and over-run conditions.

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 the 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 unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Using manufacturer data identify the output signal circuit for the AFM.
  2. Connect PicoScope channel A.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Start the engine and allow it to idle.
  5. Select GO or press the space bar to see live data.
  6. Carry out a WOT test to capture your waveform.

Example waveform

Waveform notes

The waveform example shows the initial idle voltage rise rapidly to almost 4.5 volts, then fall to around 2.5 volts as the throttle is snapped open. This initial peak and drop is due to the rapid influx of air at the instant the throttle opens. The voltage then climbs to around 3.5 volts with the throttle fully open.

You can then see a rapid drop in voltage as the throttle closes. Inertia of the flap movement and an amount of engine vacuum pulls the flap and voltage below idle levels before the vane recovers position and then gradually drops to the normal idle position.

The ‘hash’ on the waveform is due to fluctuations caused by induction pulses from the engine.

Waveform Library

The Waveform Library has 8 examples of moving vane air flow meters.

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select ‘Air flow meter (AFM) moving vane type’.

Further guidance

This type of Air Flow Meter was probably one of the most popular versions used. It has been used on systems such as Bosch L, LE, LE3, Motronic and Ford EEC IV. Several Japanese manufacturers have also based their systems on this type of unit.

The vane type air flow meter works by having the intake air for the engine pass through the metering unit and act on a spring-loaded flap which then moves in proportion to the rate of air entering the engine. The air vane movement in turn slides a contact across a carbon track, the resulting voltage changes are reported back to the Electronic Control Module (ECM). The ECM will then adjust fuelling relative to air flow.

The vane type air flow meter does have a number of technical drawbacks, the vane itself is a restriction to air flow and its movement inertia can lead to electrical inaccuracies. The mechanical carbon track method of voltage adjustment will ware and become inefficient.

For these reasons most manufacturers have moved away from this method of air flow measurement.

The Vane Type AFM has a varying number of electrical connections dependant on vehicle application. The common ones are listed below.

Four-terminal units have, a voltage supply, an earth connection via the ECM, an output from the air temperature sensor and the output from the air vane meter.
Five-terminal units have, as above, but with an additional output from a Carbon Monoxide (CO) potentiometer.
Seven-terminal units have, as the four-terminal unit plus an extra wire to the air temperature sensor and two terminals to the fuel pump contacts. These contacts close and complete the circuit to the fuel pump when the engine is cranked and the incoming air moves the flap by approximately 5°. This unit is fitted to certain Range Rovers.

The AFM also has an internal compensation chamber that helps to stabilise the movement of the flap caused by induction pulses. The CO content adjustment is via an internal air bypass or a potentiometer, depending on the version.

Generic DTC’s MAF Sensor

P0100 - MAF Sensor Circuit Insufficient Activity

P0101 - Mass Air Flow (MAF) Sensor Performance

P0104 - Mass Air Flow Circuit Intermittent


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


  • Back-pinning Probe Set


  • Large Dolphin/Gator Clips


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


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Guided test: Air Vane AFM