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Sensors
Air Flow Meter Vane Type E-mail
The Mass Airflow sensor (Vane Type) is used to measure the amount of air drawn into the engine by means of a hinged Flap. It has the following parts.
  • Hinged Flap. This is connected to a potentiometer. The incoming air moves the flap and this moves the contact along the potentiometer. The different resistance is converted into a signal voltage and is sent back to the Engine Control Unit.

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  • Thermistor. This is used to measure the temperature of the incoming Air. The control unit reports the air temperature as a signal voltage back to the Engine control unit.

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  • Co-Potentiometer. Mainly fitted to non catalyst vehicles. When fitted to vehicles with a catalytic converter it is normally factory sealed. By Adjusting this screw it alters the co signal voltage back to the Engine control unit. The Engine control unit responds by altering the injection duration to either lean or richen the mixture.
This type of Air Flow Sensor usually has 4 or 5 terminals.

 1: Co Adjuster Signal
2: Air Flow Sensor Signal
3: Air Flow Sensor Supply Voltage (5 Volts)
4: Earth Connection (0 Volts)
5: Intake Air Temperature Sensor Signal

The Engine Management light will be triggered if the Air Flow Meter signal voltage moves o0 Ohms to chassis or Ecu pin0utside of normal operating values. It may also have the capability to show open or short circuits in the power supply and earth circuits.

Testing with a Voltmeter: The voltages will be as a above for testing. When testing the AFM signal voltage it will be low at idle and increase rapidly when the throttle is snapped open. It will also drop rapidly when the throttle is closed.
You can manually test this Sensor my moving the flap with your hand. As you move it open  the signal voltage will slowly increase. Faults with this sensor can occur were the potentiometer wears out in one particular part. The potentiometer consists of a carbon track or strip. The flap has an arm connected to it which rubs along the potentiometer strip. The worn part can be seen on a voltmeter. The voltage steadily increases and will then drop off or show an erratic reading when the contacts reach the broken part of the potentiometer.
With the Air flow meter disconnected you can also check its operation using Resistance. The Resistance will increase and decrease when the flap is moved.

Testing with an Oscilloscope:

        Readings taken with a Pico Automotive Oscilloscope.

 

you can see above, the initial rapid rise in voltage as the throttle is opened. It drops slightly after the initial intake of air and then regains a high reading as the engine speed is kept raised. It falls rapidly when the throttle is closed. The voltage reading is hashed because of fluctuating manifold pressure.
If Your Airflow meter readings are correct but the problem persists then double check you readings at the Engine control unit plug. This will show up any wiring faults between the units.