Skylark V6-3100 3.1L VIN M SFI (1996)
Knock Sensor: Description and Operation
DESCRIPTION
Varying octane levels in today's gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion (burn) in the
combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front produced by the spark plug. The
rattling sound normally associated with detonation is the result of two or more opposing pressures (flame fronts) colliding within the combustion
chamber. Though light detonation is sometimes considered normal, heavy detonation could result in engine damage.
To control spark knock, a Knock Sensor (KS) system is used. This system is designed to retard spark timing to reduce spark knock in the engine. This
allows the engine to use maximum spark advance to improve driveability and fuel economy.
COMPONENTS
The KS system has two major components:
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KS module.
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Knock sensor.
The knock sensor is used to detect engine detonation, allowing the PCM to retard Ignition Control (IC) spark timing based on the KS signal being
received. The knock sensor produces an AC signal which rides on a 5 volts DC signal supplied by the PCM. The signal amplitude and frequency is
dependent upon the amount of knock being experienced.
The PCM determines whether knock is occurring by comparing the signal level on the KS circuit with the voltage level on the noise channel. The noise
channel allows the PCM to reject any false knock signal by indicating the amount of normal engine mechanical noise present. Normal engine noise varies
depending on engine speed and load. If the voltage level on the KS noise channel circuit is below the range considered normal, DTC P0327 will set,
indicating a fault in the KS circuit or one of the knock sensors. If the PCM determines that an abnormal minimum or maximum noise level is being
experienced, a DTC P0326 will set.
The PCM contains a replaceable Knock Sensor (KS) module. The KS module contains the circuitry that allows the PCM to utilize the KS signal and
diagnose the KS sensors and circuitry. If the KS module is missing or faulty causing a continuous knock condition to be indicated, the PCM will set DTC
P0325.
The scan tool has four data displays available for diagnosing the KS system. The four displays are described as follows:
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KS Activity is used to monitor the input signal from the knock sensor. This counter will display "Yes" while knock is being detected.
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Knock Retard is the indication of how much the PCM is retarding the spark. The PCM will retard spark in response to the Knock signal or the
Traction Control system Desired Torque signal.
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KS Noise Channel indicates the current voltage level being monitored on the noise channel.
DTCs P0325, P0326, and P0327 are designed to diagnose the KS module, the knock sensors, and related wiring, so problems encountered with the KS
system should set a DTC. However, if no DTC was set but the KS system is suspect because detonation was the customers complaint, refer to
Detonation/Spark Knock in Symptoms. See: Computers and Control Systems/Testing and Inspection
