Mass air flow sensor - How It Works

Mass air flow sensor

 General points

The mass air flow sensor is used to determine the intake air flow. It comprises of a pipe-shaped housing with flow rectifier, sensor protection and a sensor module screwed onto the outside. It is installed in the intake pipe between the air filter housing and the intake manifold.

Function

There are two temperature-dependent metal film resistors attached to a glass membrane arranged in the air flow. The first resistor (RT) is a temperature sensor and measures the air temperature. The second resistor (RS) is used to record the air throughput. Depending on the amount of air intake, the resistor RS cools down to a greater or lesser extent. In order to compensate the constant temperature difference between resistors RT and RS again, the flow through the resistor RS has to be controlled dynamically by the electronics. This heat flow serves as a parameter for the respective quantity of air intake by the engine. This measured value is required by the engine management control unit to calculate the amount of fuel required.

Effects of failure

A faulty mass air flow sensor can become noticeable as follows:

■ The engine comes to a standstill or the engine management control unit continues to work in limp-home mode. 
■ Engine warning light comes on 

Reasons for failure of the mass air flow sensor can be: 

■ Contact fault at the electrical connections 
■ Damaged measuring elements 
■ Mechanical damage (vibrations, accident) 
■ Measuring element drift (exceeding the measuring framework)

Troubleshooting
The following test steps should be taken into account during troubleshooting: 
■ Check connector for correct fit and good contact 
■ Check the mass air flow sensor for damage 
■ Check the measuring elements for damage 
■ Check voltage supply with the ignition switched on (circuit diagram for pin assignment is necessary). Ref. value: 7.5 -14 V 
■ Check output voltage with the engine running (circuit diagram for pin assignment is necessary). Ref. value: 0 -5 V 
■ Check the connection cables between the removed control unit connector and sensor connector for transmission (circuit diagram for pin assignment necessary). Ref. value: approx. 0 ohm. 
■ Electronic test of the mass air flow sensor by the engine management control unit. If a fault occurs, a fault code is stored in the control unit and can be read out using a diagnosis unit.


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