by Keith Hamburger

Created on: January 14, 2009

The mass airflow sensor on your vehicle is part of a closed loop feedback system controlling your engine operation. By measuring the amount of air flowing into the engine, the mass airflow sensor lets the car's computer, the electronic control module, know how much fuel to use to get optimal performance and efficiency.

Mass airflow sensors can be electronic or mechanical. Electronic sensors usually operate by temperature changes in a heated wire or film. Mechanical sensors use a vane in the intake airflow for measurement.

Vane type sensors are not true mass airflow sensors as they don't measure the actual mass of the air entering the engine, but the volume. The mass of the air will change with temperature so these sensors usually use a temperature sensor to adjust the output and approximate the measurement of air mass.

With a "flap" or door that opens as air flows through the intake, vane type sensors actually measure the movement of the vane to get a signal that is sent to the computer. As the air flows faster, providing a greater volume of air to the engine, the vane moves more changing the output signal.

Since gravity pulls the vane downward, affecting the position of the vane, a vane type sensor must be properly oriented. Also, acceleration can affect the position of the vane affecting the output. Other than problems with the position of the vane due to external forces the most common issue with these types of sensors is due to contamination or wear on the electrical contacts inside the sensor.

The other common type of mass airflow sensor measures the actual mass of the air entering the engine. By measuring the temperature change due to the airflow past a heated wire, the hot wire sensor and the hot film sensor both give an accurate input to the car's computer system.

The wire in a hot wire sensor increases in resistance as it heats up. When it reaches a resistance to where the voltage and resistance are balanced the current is limited to a fixed amount. However, when air flows past the wire, cooling it, the resistance drops and additional current flows through the wire. This change in resistance is measured and converted into a voltage signal that is sent to the electronic control unit.

Increased airflow cools the wire causing a change in the current. Greater air density flowing past the wire also changes the current. Since both airflow and density affect the temperature of the hot wire mass can actually be measured. Since the mass of the air flowing into the engine is a measure of the amount of oxygen available to be used in the combustion process this is a more accurate measurement for controlling performance and efficiency.

Hot wire mass airflow sensors can be affected by contaminants on the wire. Other than this problem, short of a complete failure due to a damaged wire or failed electronics, there is little that can impair the performance of this type of system.

A membrane or film sensor works similarly to a hot wire sensor. A small, thin electronic circuit is placed on either side, upstream and downstream, of a film or membrane. These circuits are heated and the different electronic characteristics of each side are balanced to determine the airflow.

Since the mass airflow sensor is just one component of an entire closed-loop feedback control system it may or may not be the cause of performance, emissions or efficiency issues. Your service manual will give you a method of testing the sensor to determine if it is operating outside of normal specifications.

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