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, Wide Open Throttle (WOT) and over-run conditions.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the AFM sensor signal circuit.
2. Connect PicoScope Channel A to the sensor signal circuit.
3. Start the engine.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. Carry out a Wide Open Throttle (WOT) test.
7. With your waveform on screen stop the scope.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• With the engine at idle the sensor output is around 0.75 V.
• When the throttle is snapped open, the initial air inrush causes the sensor output to rise rapidly to almost 4.5 V. The air flow through the sensor quickly reduces once it has caught up with the initial engine demand and the sensor output falls to around 2.5 V; however, whilst the throttle is held open, the engine speed builds rapidly and the air flow through the sensor increases again, causing the sensor output to rise to around 3.5 V.
• At the point that the accelerator pedal is released the throttle chokes the air supply and sensor output drops rapidly. Inertia of the flap movement and the return of intake manifold vacuum forces sensor output voltage momentarily below the initial idle conditions.
• Once the engine settles at idle the vane recovers its idle position and output voltage returns to that measured at the start of the test.
• The hash on the waveform is due to the effect of induction pulses as the engine is running.

Waveform Library

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.

Further guidance

Vane-type air flow meters indicate the quantity of air flowing into an engine. They consist of a spring-loaded vane flap, which is deflected by the air flowing past it. The air vane is mechanically linked to an electrical contact, which slides across a carbon track as the vane moves. The effect is that of a variable resistor providing a varying voltage output relative to vane position. The Engine Control Module (ECM) uses the sensor output voltage to adjust the fuelling in proportion to the indicated air flow.

Vane-type air flow meters have a number of technical drawbacks.

• The vane has a restrictive effect on air flow.
• The vane inertia affects both the air flowing through the unit and the vane response to it, making it complicated for engine designers to predict its behaviour.
• The mechanical action of the contact against the carbon track causes wear and other inaccuracies.

For these reasons vehicle manufacturers have moved on to alternative methods of air flow measurement.

Depending on the vehicle application, Vane- type units were commonly found with any one of the following different types of electrical connector:

Four-terminal units can have-

• supply voltage.
• earth path via the ECM.
• air temperature output.
• air flow meter output.

Five-terminal units; as the four-terminal unit with the addition of-

• Carbon Monoxide (CO) potentiometer output.

Seven-terminal units; as the four-terminal unit with the addition of-

• air temperature sensor.
• two terminals to fuel pump contacts.

The fuel pump contacts make and break the circuit depending on the air flow through the meter. The circuit is made only when the incoming air has moved the flap approximately 5° from its rest, engine off position. This type of unit was fitted to certain Range Rover vehicles.

Vane-type air flow meters also have 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.

This type of air flow meter has been used on systems such as Bosch L, LE, LE3, Motronic and Ford EEC IV and several Japanese manufacturers.

GT008-EN