Hot-film air-mass sensor

The hot-film air-mass sensor records the intake air mass; it is located in the intake port between the air filter and throttle valves.

Design and function

A heated surface of the hot-film air mass sensor in the flow of intake air is controlled to a constant temperature with respect to the intake air. The intake air flowing past this surface cools this heated surface and thus changes its resistance. The heating current is permanently regulated to retain the excess temperature. This heating current is the measured variable for the air mass drawn in by the engine. The DME control unit uses it to calculate the load signal and thus the basic variable for the injection timing.

Important advantages:

• Changes in air pressure (air density) are recorded
  

• Temperature influences are compensated
  

• Low pressure drop in the intake pipe due to low air resistance particularly at high air mass flow rates.
  

• Compensation of pressure pulsation in the intake duct
  

• No moving parts
  

• Large measuring range
  

Self-diagnosis and emergency operation

Self-diagnosis of the hot-film air-mass sensor system includes the following:

• Test of the intake air temperature sensor in the hot-film air-mass sensor
  

• Check for breaks in wiring
  

If a fault is detected during engine operation, a fault is entered in the fault code memory of the engine management system.