Impreza Coupe FWD F4-1820cc 1.8L SOHC (1995)
Mass Air Flow Sensor: Description and Operation
PURPOSE/OPERATION
The mass air flow sensor measures all air entering the engine through the intake system. This information is vital to the ECM to determine proper
fuel mixture and ignition control configurations.
A hot-film sensing head is used to measure air flow. Air flow is measured by determining the cooling effect on the sensing head. Whenever the
ignition is ON, the sensor produces an output signal to the ECM.
The hot-film sensing head acts as a temperature sensitive resistor. The ECM maintains the sensing head at a preset constant temperature. As air
passes over the hot-film, it has a tendency to cool. Since the sensor head changes resistance based on temperature, a varying current is required to
maintain constant temperature. The internal circuitry of the sensor monitors this changing current demand and sends a varying voltage output
signal to the ECM, tracking current required.
