Fiat 1996 Brava Service Manual page 361

Bravo-Brava ty™
98 range
Engine
Fuel system
10.
TIMING SENSOR
This Hall effect type sensor is fitted on the cylinder and is facing the camshaft pulley (exhaust).
There is an opening in the pulley which allows the timing sensor to detect the engine timing position
and signal it to the injection control unit.
The injection control unit uses the timing sensor signal to recognize TDC at the end of the compression
stroke.
Operation
A semiconductor layer, through which a current flows, immersed in a normal magnetic field (lines of
force perpendicular to the direction of the current), produces a difference in power known as Hall volt­
age.
If the intensity of the current remains constant, the voltage produced only depends on the intensity of
the magnetic field; therefore if the intensity of the magnetic field simply varies periodically this produces
a modulated electrical signal, whose frequency is proportional to the speed at which the magnetic field
changes.
To produce this change, a metal ring (inner part of the pulley) with an opening passes through the sen­
sor.
As it moves, the metal part of the ring covers the sensor blocking the magnetic field with a consequent
low output signal; conversely, by the opening where the magnetic field is present, the sensor produces a
high signal.
This signal, together with the signals relating to the rpm and TDC, allow the injection control unit to rec­
ognize the position of the pistons and determine the injection point.
AIR FLOW METER
The flow meter is located on the air inlet hose
and is the hot film type.
The intake air temperature sensor is incorporat­
ed inside the flow meter.
Operation
The operating principle is based on a heated di­
aphragm in a measuring duct through which the
intake air which enters the engine flows.
The hot film diaphragm is kept at a constant
temperature (about 120 °C higher than the tem­
perature of the inlet air) by the heating resis­
tance.
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 is passed through
the resistance.
This current, being proportional to the mass of
air which flows to the engine, is measured using
a 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
12
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