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Mass air flow sensor

Mass air flow sensor

The mass air flow sensor continuously monitors the air mass available for combustion while the engine is running. The sensor comprises two main parts, the connection pipe and the sensor section.

The mass air flow sensor is powered with 5V on pin 4 from pin 26, connector A, on the control module. It is grounded through pin 3 to control module pin 11 on connector B.

The control module sensor input, pin 30 connector A, is connected to pin 5 on the mass air flow sensor, the voltage of which varies according to the intake air mass.

Measurement is conducted in the following way. Some of the air flow through the mass air flow sensor passes a measuring plate. The surface of this plate is a thin diaphragm of semi-conductor type on which a heating element and a number of temperature sensors are fitted. The heating element is a resistor, which radiates heat when current passes through it. Together with the resistor, there is a temperature sensor that enables the heat in the heating element to be regulated exactly. The temperature of the heating element is dependent on the temperature of the incoming air.

There are two more temperature sensors on the diaphragm, one upstream and one downstream from the heating element seen from the direction of the airflow. If there is no air flowing past the diaphragm, the temperature will be the same across its entire surface. The two temperature sensors upstream and downstream from the heating element will then detect the same temperature.

When air starts to flow through the mass air flow sensor, the diaphragm upstream from the heating element will cool down, which the temperature sensor on the diaphragm will detect. The temperature of the diaphragm and the temperature sensor downstream from the heating element will be the same as before the airflow started.

The air mass can be calculated by measuring the difference in temperature between the two temperature sensors (upstream and downstream) in the airflow.

The direction of flow can also be detected by determining which of the temperature sensor is the hottest and which is the coldest. All necessary calculations take place in the mass air flow sensor.

The signal available to the engine control module corresponds with the air mass passing the mass air flow sensor that is available for combustion.