Air sensor engine operation-8-

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Mass Air Flow (MAF) sensor / Intake Air Temperature (IAT) sensor

 
1---Housing
2---Housing cover
3---Control electronics
4---Sensor element
5---Sensor measuring cell
6---Heating zone
 

The MAF sensor works according to the hot-wire principle.
The MAF sensor is powered via the Powertrain Control Module relay in the BJB. The MAF sensor and the IAT sensor are connected to earth via the PCM.
The MAF sensor has an extended moulding which projects into the centre of the outlet pipe of the air filter, where it measures the air mass aspirated by the engine and the intake air temperature.
The air mass aspirated by the engine is determined on the basis of the cooling effect of the intake air via a hot-film element in the MAF sensor. The greater the aspirated air mass, the greater the cooling effect and the lower the electrical resistance of the hot-film element. The electronics in the MAF sensor process this resistance value and send a voltage signal to the PCM corresponding to the aspirated air mass.
This analogue voltage signal is between 0.5V and 5V. With a small aspirated air mass the voltage signal is low, with a large aspirated air mass it is correspondingly high.
From this voltage, the PCM calculates the air mass aspirated by the engine. Using this value, the quantity of fuel to be injected and the ignition timing are determined.
The MAF sensor is monitored by the PCM and in the event of malfunctions corresponding fault codes are saved in the error memory of the PCM. If a fault occurs in the MAF sensor, a substitute value is calculated from the engine speed and the throttle flap position.
The MAF is integrated into the IAT sensor. This is realized as an NTC (negative temperature coefficient) resistor, whose resistance diminishes with decreasing temperature. Via the PCM, a 5V reference voltage is applied to the IAT sensor. The voltage at the IAT drops by a varying amount depending on the resistance. From this voltage drop, the PCM determines the intake air temperature.
If the IAT sensor is faulty, a fixed value of 20°C is used for the calculation.

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