Switches & Sensors
Air Mass Sensor Troubleshooting
They have been installed in many motor cars for more than a decade: the air mass sensor has become one of the most important components in engine managment. If it fails, frequently nothing else will work.
As the name says, an air mass sensor measures the air mass supplied to the engine. Its signal is used for calculating the injection quantity, and on diesel engines also for controlling the exhaust gas recirculation. It is consequently an essential component in the air supply system and for controlling exhaust gases.
Together with increasing requirements on environmental protection measures, new generations of air mass sensors have become more and more precise – but also more sensitive. A defective or clogged air mass sensor can supply incorrect input signals to the engine control unit which, as a result, controls other components incorrectly.
On turbo-diesel engines in particular, air mass sensors are subjected to extremely high stresses, since both air flow and air speed are very high.
Air Mass Sensor Design
The complete air mass sensor consists of a duct ("tube"), in which the intake air streams past the actual sensor. Depending on the application and vehicle, the air mass sensor is available fully integrated in a plastic tube or the actual sensor as an individual plug-in module. Both versions, whether with tube or separate, are called "air mass sensor".
Older models were equipped with a hot wire sensor element. By briefly heating it after the engine was cut, the hot wire was "burnt free" from impurities. Left: Older type air mass sensor
More recent models work with a film-like heating resistor on a support. The burning-off process is no longer necessary.
All present-day air mass sensors function according to the hot-film principle. The "hot-film sensor" is heated to a constant temperature of about 120 to 180°C above the intake air temperature, depending on the vehicle manufacturer. The air flowing in cools down the hot-film sensor. This cooling-down is compensated by a heating current via the control electronics. The heating current is a measure for the taken-in air mass. This method takes the density of the air stream into account.
On versions with two 2 separate measurement bridges, the detection of pulsations and return flows is also possible. Below: Present-day air mass sensor