Fiat 1996 Brava Service Manual page 401

Engine
Fuel system
Bravo-Brava ig}
TD100
10.
Air flow meter
The air flow meter is the "heated film" type; the operating principle is based on a heated diaphragm
which is placed in a measuring channel through which the intake air which enters the engine flows.
The film diaphragm is kept at a constant temperature ( ~ 120°C higher than the temperature of the air) by
the heating resistance which is in contact with it.
The mass of air which passes through the measuring channel tends to remove heat from the diaphragm
therefore, in order to keep it at a constant temperature, a certain current should flow through the heating
resistance; this current is measured by a suitable Wheatstone bridge.
The current measured is therefore proporational to the mass of air flowing through.
1. Connector
2. Measuring channel
3. Hot film sensor
A= Air intake
P4A22EJ01
NOTE This flow meter directly measures the mas of air (and not the volume) thereby eliminating prob -
/ems of temperature, altitude, pressure, etc.
Description of the operation
The Wheatstone bridge (made up of R3, R2, Rs, Rt+R1) is in equilibrium when Rs is at around 120°C
higher than the temperature of the air.
The air which passes through the diaphragm removes heat from Rs, therefore the bridge is not in equilib­
rium. 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 with the result that more current passes through Rh to heat
up Rs and the equilibrium of the bridge is restored.
Circuit IC2 measures the current which passes through Rh. This current makes it possible to maintain
the equilibrium of the bridge and is consequently proportional to the mass of air which passes through
ill its air fluw me lei.
T1
J *
IC2
Uk
Rsfi"
C4 R5
HI-
R2
RT
IC1
IRS T
y^
O T T c r c r
1 2 3 4
Air flow meter wiring diagram
22
IX-96 - Update
Print no. 506.670/08