Fiat 1996 Brava Service Manual page 334

Engine
Fuel system
10.
1. Air temperature sig- 4. Reference voltage
nal (5V)
2. Battery voltage 5. Air flow rate signal
3. Mass
28
Bravo @ 20v
98 range
AIR FLOW METER (0.281.002.199)
The flow meter is on the air inlet hose and is the
"hot film" type.
The intake air temperature sensor is inside the
flow" meter.
The operation is based on a heated diaphragm
placed 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 above the tempera­
ture of the intake air) by the heating resistance.
The mass of air passing through the measuring
duct tends to remove heat from the diaphragm
therefore, in order to keep the latter at a con­
stant temperature, a current has to pass through
the resistance.
This current, being proportional to the mass of
air flowing to the engine, is measured by a suit­
able Wheatstone bridge and the signal obtained
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 directly measures the
mass of air (and not the volume) there­
by eliminating problems of tempera­
ture, altitude, 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 which passes through the diaphragm re­
moves heat from R3, then the bridge is unbal­
anced.
This situation is detected by the circuit at IC1
which operates the transistor T 1 , in a manner
which is proportional to the bridge imbalance,
which then allows more current to pass through
Rh in order to heat R3 and restore the bridge
balance.
Circuit IC2 measures the current passing
through Rh.
This current, allows the equilibrium of the
bridge to be maintained and is therefore propor­
tional to the mass of air passing through the air
flow meter.
Print no. 506.668/16