Fiat 1996 Brava Service Manual page 443

Bravo- Brava 9 **
1998 range
Engine
Fuel system
AAA
1 1
n r-i
1 2
— i
70
3
63
56
10.
KNOCK SENSOR
(KNE-03)
This sensor is the piezoelectric type and is fit­
ted on the cylinder block/crankcase in a posi­
tion which is symmetrical in relation to the
pairs of cylinders 1 -2 and 3-4.
This position is determined by the need to
detect the onset of knock in the same way for
all cylinders.
When there is engine knock, vibrations of a
particular frequency are produced .
The effect generates mechanical repercussions
on a piezoelectric crystal that sends a signal to
a control unit. The control unit then reduces
the ignition advance on the basis of this signal
until the effect disappears. The advance is
then gradually restored to the baseline value.
Principle of operation
The molecules in a quartz crystal are electri­
cally polarised.
Under rest conditions (A), the molecules are
not aligned in any particular direction.
When the crystal is subject to pressure or im­
pact (B), the molecules line up and the degree
to which they align is directly proportional to
the pressure on the crystal. This alignment
creates a voltage at the crystal terminals.
A. Rest position
B. Position under pressure
Checking the resistance
The sensor resistance can be measured by dis­
connecting the connector and connecting an
ohmmeter to the sensor terminals
Resistance 532 - 588 ohm at 2 0 C
Connector wiring
The sensor is connected to the control unit by
twisted cables covered in a shielded non­
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 the electrical connection, undo the
knock sensor retaining bolt and remove.
Copyright Fiat Auto
31