Mass Air Flow Sensor (MAF)

 A hot wire mass airflow sensor determines the mass of air flowing into the engine’s air intake system. The theory of operation of the hot wire mass airflow sensor is similar to that of the hot wire anemometer (which determines air velocity). The General Motors division (GM) was the first car company to use the hot wire sensor. This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, like a toaster wire. The wire's electrical resistance increases as the wire’s temperature increases, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s temperature is directly proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement of current into a voltage signal which is sent to the ECU.

If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meter's paddle sensing element, the hot wire responds directly to air density. This sensor's capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume.

Some of the benefits of a hot-wire MAF compared to the older style vane meter are:

1.) Responds very quickly to changes in air flow

2.) Low airflow restriction

3.) Smaller overall package

4.) Less sensitive to mounting location and orientation

5.) No moving parts improve its durability

6.) Less expensive

7.) Separate temperature and pressure sensors are not required (to determine air mass)

There are some drawbacks:

1.) Dirt and oil can contaminate the hot-wire deteriorating its accuracy

2.) Installation requires a laminar flow across the hot-wire

     There are a few reasons for concern if a mass air flow sensor gets very far out of calibration.

1.) This sensor is directly responsible for the amount of fuel calculated to be injected into the cylinder during the combustion cycle. So if this sensor is incorrect about the amount of air being introduced, then the amount of fuel being injected will also be wrong.

2.) As stated above, If this sensor has contaminated it acts as an insulator to the hot wire. If the hot wire is insulated, then the air that is passing by it will be unable to cool the wire and therefor is not being measured properly. This air is known as "Unmetered air". Unmetered air is air that does not have the fuel to match it in the combustion chamber. So if you have more air than fuel, you have a lean condition.

3.) There are O2 sensors to adjust for this error but only up to a certain percentage. If the unmetered air goes beyond what the oxygen sensors can make up for then you will be running the engine lean and causing damage.