Description

Detection of the air drawn into the intake is performed by a hot film mass air flow sensor (1). It is fitted between the air cleaner housing (2) and the throttle body (3).




The air mass is required for the following DME control unit functions:
- Injection
- Ignition
- Oxygen sensor control
- Venting of the carbon cannister
- Knock regulation
- Plausibility test




Advantages of the Hot Film Mass Air Flow Sensor:

- Measures the passing air mass (gas mass) per time unit independent of its density and temperature.
- Large measurement range
- High sensitivity, particularly at small air flow rates
- No moving parts, therefore no wear
- Insensitive to dirt
- No free burning necessary




Design of the Hot Film Mass Air Flow Sensor

A hot film sensor is installed in a measurement channel. It consists of a ceramic substrate that carries the following thick film resistors:
- Heating resistor (RH)
- Sensor resistor (Rs)
- Air temperature resistor (R)
- Compensation resistor (R1)

A hybrid circuit and a power electronic component are installed behind the measurement channel in the electronics housing. A heat sink is mounted on the outside of the electronic housing.

Construction
The electronic housing (the complete measuring element) and the measuring channel are fitted into the housing of the mass air flow sensor with two screws. An additional seal is used to seal the measuring element against the air mass sensor housing.

On the inlet and outlet sides of the air mass sensor a protective screen is fitted to smooth out the turbulences in the air and therefore to ensure an even air flow around the hot film. Next to the connecting plug, an arrow is imprinted in the housing of the mass air flow to indicate the direction of air flow.

Note: The measuring element in the air mass sensor housing must never be removed since the components are calibrated during manufacture of the unit.