On the fuel-injected Volkswagen air-cooled engine, the air flow meter can be found attached to the air cleaner housing.
The air flow meter is nothing more than a fancy variable resistor. There is a flap inside that deflects in relation to how much air is entering the engine. This flap is attached to a wiper arm which wipes across a resistive track. The ECU sends a fixed voltage signal to the air flow meter. As the flap opens, the electrical resistance of the air flow meter increases, giving the ECU a measurement of how much air is entering the engine. The air flow meter can be tested electrically by attaching the leads of an ohmmeter to the proper terminals and measuring the resistance offered by the air flow meter as the flap is moved through its travel. The ECU uses the data supplied by the air flow meter to help determine how much fuel to inject.
The air flow meter also contains a temperature sensor for the intake air. This sensor is a semiconductor that varies resistance with temperature in an inverse relationship; that is, the higher the temperature, the lower the resistance. This information also helps the ECU determine how much fuel to inject, as intake air volume (supplied by the flap) and intake air temperature (supplied by the sensor) can be used to calculate intake air mass, whereupon the computer calculates the correct amount of fuel for that given mass of air. The computer sends the temperature sensor a fixed voltage and measures the resistance, which indicates intake air temperature. As you might expect, the temperature sensor can be tested by attaching an ohmmeter to the proper terminals on the air flow meter and measuring the resistance at various temperatures.
The air flow meter also contains a fuel pump safety switch. The system is designed so that the fuel pump only operates when the starter is cranking or the engine is running. If the vehicle were to be in an accident, for example, and the engine stopped, the air flow meter would shut off the fuel pump to prevent a fire. This is accomplished through a set of contacts. When the air flow meter flap is open any amount, the fuel pump runs and when the flap closes completely, the fuel pump shuts off. Naturally, this can be tested also with an ohmmeter or just by observation. With the ignition on, if you deflect the flap by hand, you should hear the fuel pump activate.
And last but not least, the air flow meter provides the provision for idle mixture adjustment. There is a screw in the housing of the air flow meter that is used as a valve for an air bypass channel. The air flowing through this channel is not measured by the air flow meter and so affects the fuel/air mixture. Turning the screw clockwise blocks off the channel and so richens the mixture. Turning the screw counterclockwise opens the passage and so leans the mixture, as the extra air is not measured by the air flow meter and so the computer does not compensate for it.