mass airflow sensor
The mass airflow sensors converts the air drawn into the engine into a voltage signal. The amount if air taken in is to e known by the ECM for calculating the engine load. Amount of fuel injected, ignition time of the cylinder, timing of shifting the transmission are all decided by this . The airflow sensor is located in the air intake stream, between the throttle body and the air cleaner, so that the amount of incoming air can be sensed.

OPERATION
a hot wire sensing element is utilised by the sensor to measure the amount of air entering the engine.when the incoming air cools the hot wire, a voltage signal is generated by the sensor and is sent to the PCM. The signal is used by the PCM to calculate the injector pulse width which controls the air/fuel ratio in the engine.a thin platinum wire wound on a ceramic bobbin and coated with glass is used as the sensing element in the hot wire type mass airflow meter. This hot wire is maintained at 392°F (200°C) above the ambient temperature as measured by a constant "cold wire''.

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more details are also available at:
http://freeautomechanicmaf-mass-air-flow-sensor.html
http://samarinsglossary/airflow_sensor.html