900 SE Convertible V6-2489cc 2.5L DOHC EFI (1997)
Mass Air Flow Meter: Description and Operation
The mass air flow sensor contains a ceramic plate which is position in the air flow and heated electrically to 180°C (356°F). When the air flow increases,
the heating element requires a higher voltage to maintain the temperature at 180°C (356°F).
The voltage level is used by the control module as a measure of the inflowing air mass and constitutes the principal measured quantity for fuel injection.
When the ignition switch is in the ON position, the main relay supplies battery positive voltage (B+) through the transistor and PTC resistor RS to
grounding point G37.
Since RS is cold, the transistor's base current will be high. The transistor will then conduct a heavy current through heating element RH which is baked
together thermally with RS in the ceramic plate.
The ceramic plate heats up and PTC resistor RS reduces the base current through the transistor and so also the current through the heating element. The
temperature stabilizes at 180°C (356°F).
The ceramic plate cools down when air flows past it. The resistance of the PTC resistor drops and the transistor increases the voltage across RH so that
the temperature returns to 180°C (356°F).
The voltage across RH goes to pin 17 of the control module.
To measure the voltage with a high degree of accuracy, pin 45 of the control module is connected directly to the mass air flow sensor. The voltage across
pins 17 and 45 is a measure of the inflowing air mass.
In the event of a fault in the mass air flow sensor or a break in the circuit (no continuity), the control module uses the input from the throttle position
sensor as a substitute value.
