Fiat 1998 Marea Service Manual page 353

Engine
Fuel system
10.
1. Air temperature
signal
2. Battery voltage
3. Earth
28
4A28U02
4. Reference voltage
(5V)
5. Air flow rate signal
Marea- Marea Weekend I P >
AIR FLOW
(0.281.002.199)
The flow meter is located on the air inlet hose
and is the "heated film" type.
The intake air temperature sensor is inside the
flow meter.
The operation is based on a heated diaphragm
located in a measuring duct through which the
intake air entering the engine flows.
The hot film diaphragm is kept at a constant
temperature (about 120 °C higher than the tem-
perature of the entering air) by the heating re-
sistance.
The mass of air which passes through the meas-
uring duct tends to remove heat from the dia-
phragm therefore, in order to keep the latter at a
constant temperature, a certain current must flow
through the resistance.
This current, being proportional to the mass of
air flowing to the engine, is measured by a
Wheatstone bridge and the signal is sent to the
injection control unit.
1. Covers
2. Electronic card
3. Sensor
4. Support plate
5. Support
6. Seal (O-Ring)
7. Temperature sensor
NOTE This flow meter measures the mass of air
(and not the volume) directly, thereby
eliminating problems of temperature, al-
titude, pressure, etc.
Description of the operation
The Wheatstone bridge (made up of resistors
R1,R2,R3,R4) is balanced when R3 is about 120
°C higher than the temperature of the air.
The air passing through the diaphragm removes
heat from R3, therefore the bridge is unbalanced.
This situation is detected by the circuit at IC1
which operates transistor T1, in a manner which
is proportional to the imbalance of the bridge,
and as a result more current is directed through
Rh to heat up R3 and restore the equilibrium of
the bridge.
The circuit IC2 measures the current passing
through Rh.
This current makes it possible to keep the bridge
balanced and is consequently proportional to the
mass of air passing through the air flow meter.
99 range
METER
Print n° 506.763/14