The Mass Airflow sensor is used to measure the amount of air drawn into the engine. It consists of 3 main parts a Thermistor, a platinum wire and a control unit.
Platinum hot wire. This is kept at a constant temperature by the control unit. The hot wire is cooled by the incoming air and the control unit increases the current to maintain it's temperature. The change in current is used as a means to measure the volume of air being drawn in by the engine. the measurement is converted in to a signal voltage and this is sent to the Engine control unit.
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. The Thermistor reading is also used to help regulate the current to the hot wire.
Control Unit. This is built in to the body of the Airflow Meter and is responsible for sending signal voltages back to the Engine Control Unit and for the correct operation of the Air Flow Meter itself
This type of Air Flow Sensor usually has 4 or 5 terminals.
1: Power supply. This will usually be battery voltage
2: Earth Connection. This will connect to both chassis ground and and ECU ground (The Ecu ground is monitored for diagnostic purposes.
3: Air Mass Sensor Signal Voltage
4: Volt sensor Supply Voltage (Used for Air Temperature Thermistor)
5: 2nd Earth Connection or Burn of Function (Bosch Type Sensors)
The Engine Management light will be triggered if the Air Flow Meter signal voltage moves outside 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 abovefor 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.
Testing with an Oscilloscope:
Measurement 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 maintains 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.
