Fiat 1998 Marea Weekend, Service Manual

Page: 165 / 846

Marea-Marea Weekend^
1998 range
16v
Engine
Fuel system
10.
KNOCK SENSOR
(KNE-03)
This piezo-electric sensor is fitted to the en-
gine block in a symmetrical position in rela-
tion to cylinder pairs 1-2 and 3-4.
This positioning is necessary in order to de-
tect the onset of knock in all cylinders equal-
ly-
When the engine knocks, vibrations of a spe-
cific frequency are set up in the engine block.
This effect has a mechanical effect on a
piezo-electric crystal, which sends a signal to
the control unit. The control unit processes
the signal and reduces ignition advance until
the phenomenon disappears. Advance is
then gradually restored to original values.
Principle of operation
Quartz crystal molecules possess electrical
polarity.
The molecules are not oriented in any partic-
ular direction when at rest (A).
When the crystal is subject to pressure or im-
pact (B), it takes up a different position. The
entity of the change is proportional to the
pressure to which the crystal is subjected.
The positioning induces a voltage at the crys-
tal terminals.
A. Rest position
B. Position when under pressure
Checking resistance
Sensor resistance may be measured by dis-
connecting the connector and connecting an
ohmmeter across the sensor terminals.
Resistance 532 - 588 ohm at 20 C
Wiring connector
The sensor is connected to the control unit by
mean of twisted wires and covered with a
shielded anti-interference sheath.
Pin 1 - Earth
Pin 2 - Signal
Pin 3 - Shielding
The numbers in boxes indicate the corre-
sponding control unit pins.
Removing - refitting
Disconnect electrical connection, unscrew
screw retaining the knock sensor and re-
move.
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