Air Flow Meter/Sensor: Description and Operation

GF07.07-P-6000V Hot Film MAF Sensor, Component Description







Location
The hot film mass air flow sensor is located behind the intake manifold between the air filter housing and the throttle valve actuator.







Task
The hot film MAF sensor detects the air mass supplied to the engine and the intake air temperature.

Design
A temperature-controlled resistor detects the air mass and an integral NTC resistor detects the intake air temperature.

The flow to the hot film sensors is even through the grid. The hot film MAF sensor has an oval housing and 4-pin (production start) or 5-pin plug connector.

The electronics and the housing have undergone a number of modifications (please note part number).

Function
The electronic control in hot film mass air flow sensor controls the temperature of the heating resistor [R(H)] by means of a variable voltage, so that its temperature is 160 °C above the intake air temperature detected by the temperature resistor [R(L)].

The temperature of the heating resistor [R(H)] is detected by the sensor resistor [R(S)]. If a temperature change occurs as a result of an increased or reduced air flow, the electronics adapts the voltage at the heating resistor [R(H)] until the temperature difference is again achieved.

This control voltage is used by the engine control unit as a measure for the metered air mass. The air mass is transmitted via a frequency signal (low frequency for low air mass) to the ME control unit. Using STAR DIAGNOSIS, the cycle duration of the frequency signal can be read out in micro seconds.

The intake air temperature is detected by an additional NTC resistor.







Hot film MAF sensors with the number A 273 094 08 48 and higher provide the engine control unit with an additional signal at a frequency ranging between 17 and 21 Hz (44 to 54 ms). With this so called ratio-metric frequency signal, the engine control unit compensates for the temperature and voltage influences at the air mass signal.

The engine control unit also detects the version of the hot film MAF sensor via the duty cycle.