Mass Air Flow Sensor
|Air pressure influences the actuators|
The hot film air mass flow sensor is one of the most important sensors for petrol engines and, also for diesel engines. Through this device the control unit (engine management) has data at its disposal of the
consumed air volume, its temperature and pressure. Accordingly the just sucked in air, it can adjust the injection/ignition, the canister-purge valve and, e.g., the exhaust gas recirculation.
|No free burning and the possibility for measurements in the bypass|
The hot film mass air flow sensor is only seemingly not as sensitive to deposits as the hot-wire mass air flow sensor. The hot-wire mass air flow sensor still had to be burned free until the end with 1000°C and needed
the full suction current. Free burning is unnecessary and you can also measure in the bypass, having a lower flow resistance.
|Bridge connection through thin film|
As for the hot-wire mass air flow sensor the heating resistance consists of platinum. It still forms a bridge connection resistance, using a temperature resistance, a sensor resistance and an adjustable resistance. The
difference is that all these resistances are placed as a thin film on a ceramic layer. The bridge connection is balanced such that the difference in temperature between the heating resistance and the temperature
resistance is 120°C - 180°C. The sucked in air does not cool down the temperature resistance, but the heating resistance. The supplied filament current is the measure for the flowing through aerial masses.
|No problems with pulsation any more|
It is not in all cases possible to replace the mass air flow sensor with the volume air flow sensor. Same engines have very pulsating suction air pillars,
as for example the BMW four cylinders. In this case it was possible that the same air was erroneously measured several times. In the meantime, there are temperature sensors in the bypass channel or double bridge
connections, recognizing returning air from the suction tube.
|Failure, larger effects and error reading|
In particular since the mass air flow sensor is also used in charged diesel engines, its failure rates increased. The sometimes far-reaching effects of these failures stress the importance of the sensor. Besides the
recommendations of the error reader, it makes sense to evaluate measurements of the engine control unit or also own data.
|Measuring with multimeters (older method) or the oscilloscope (newer)|
Older devices carry a continuous tension or current on the signal cable. It changes, e.g., when the load requirement changes, which can easily be adjusted. This can still be checked with the multimeter. Newer mass air
flow sensors make use of digital signals and consequently just adjust the frequency. An oscilloscope has to be used for checking.
|Spoiled or dirty surfaces|
It is important to check the cause or causes of a defect involving the sensor, because replacing the sensor is costly in same cases. VW on the other hand offers favorable conditions for example for the TDI. Rarely the
sensor will show defects because of the electronics, most often, there are external effects causing the mass air flow sensor to malfunction. Faultily filtered suction air operates like sanding paper; oil drops might scatter
through the vent pipe of the crankcase. Additionally, there is the humidity in the sucked in air, or, especially for the diesel engine, the vibrations of the engine.
|Easy to replace, hard to repair|
The exchange of this sensor is relatively easy. It can be traced either as a complete pipe piece in the suction part or just as a plug-in unit in the connecting pipe between air filter and throttle valve. Sometimes it is even
located in the air filter. Maybe it is important to stress that there are very few chances of successful repairing this precision part.