Fiat 1998 Marea Weekend, Service Manual

Page: 209 / 846

Marea- Marea Weekend @ i
'98 range
TD
Engine
Fuel system
10.
The principle of operation of the flowmeter may be observed from the following wiring diagram of the
electrical circuit which drives the sensor:
Rk
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Y777. V/77/77/,777777777/777*
R1
Rh
Wiring diagram
Rh = hot film
Rk = temperature compensation sensor
R1, R2, R3 = bridge resistors
R2
R3
Vb
In the diagram, the component Rh represents the hot sensor, which is kept at constant temperature in
relation to the air temperature.
An increase in the quantity of air enveloping the hot element causes it to cool down, and so reduces its
resistance, unbalancing the bridge.
The circuit restores the balance of the bridge by increasing the current supplied to the hot element until
its resistance (and so its temperature) assumes the original balanced value.
The increase in current which passes through the hot element is proportional to the air flow passing
through the flowmeter. This value thus gives the air flow: to measure precisely the current passing
through the hot film, the voltage at the ends of the fixed resistor R3 is measured; this voltage thus pro-
vides the information on the mass of air passing through the flowmeter.
To compensate for the measurement error due to the variation in air temperature, the resistor Rk is in par-
allel to the hot film. Its high value only changes when the air temperature changes, compensating for the
increase in resistance of the hot sensor, so the bridge does not undergo unbalancing due to the variation
in the air temperature.
The hot element consists of platinum. For the same area, film technology offers shorter response times
than hot wire. In addition, having a larger area than wire, it has greater immunity to inorganic contami-
nation.
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