Prizm L4-108 1.8L DOHC VIN 8 MFI (1998)
Knock Sensor: Description and Operation
Knock Sensor System
Knock Sensor System Description
Ignition timing is the relationship between the time the spark plug is fired and the time that the piston reaches the end of the piston's upward travel in the
cylinder (TDC). If the spark plug ignites the compressed air/fuel mixture too late, not all of the air/fuel mixture has time to burn while the fuel is highly
compressed. Late ignition causes a decrease in fuel efficiency, decreased power, and increased exhaust emissions. If the spark plug fires too soon, too
much of the air/fuel mixture starts burning before the piston reaches the top of the compression stroke. Early ignition of the air/fuel mixture causes
detonation, commonly referred to as spark knock. Constant spark knock in the motor is undesirable. Excessive spark knock can reduce engine
performance. If severe enough, detonation can cause engine damage. Every engine has an optimum ignition timing value. Optimum ignition timing is
usually the earliest or most advanced firing of the spark plug that is possible without causing detonation. An engine's optimum ignition timing is designed
to be the most advanced ignition timing possible during the most demanding conditions. Optimum ignition timing is affected by all of the following
variables:
^
Engine load
^
Engine temperature
^
Air density (atmospheric pressure)
^
Fuel quality
^
Fuel octane rating
Ignition systems equipped with a knock sensor (KS) can be engineered for optimum ignition timing. A knock sensor (KS) can enable the powertrain
control module (PCM) to adjust ignition timing in order to adapt to any of the variables that affect optimal ignition timing. The KS detects when the
engine is experiencing detonation. The KS then signals the PCM to reduce spark advance until detonation is no longer detected.
Operation
The Knock Sensor (KS) system has two major components.
^
The KS module part of the Powertrain Control Module (PCM).
^
The knock sensor (1).
The KS detects abnormal vibration (detonation) in the engine. The KS is located in the center of the engine block below the intake manifold. The KS
module receives the KS signal and communicates within the PCM. After the correct calculations are made the PCM adjusts the ignition timing in order
to reduce detonation.
When the KS detects detonation, the KS module opens a circuit in the PCM. In response the PCM retards the spark advance in order to reduce
detonation. The amount of timing retard that the PCM applies is based on engine speed and the length of time that engine detonation is detected. Once
the spark timing is retarded, the KS module performs calculations in order to determine whether more or less spark timing advance is required. Normally
the ignition timing advance is increased until zero retard (normal ignition timing) is re-established. If detonation occurs again the whole cycle will repeat
itself. The alteration of ignition timing by the KS often occurs continuously while the engine is running even though no detonation is heard by the
vehicle's operator.
Results of Faulty Knock Sensor (KS) Operation
Loss of the knock sensor (KS) signal or loss of ground at the KS module causes the KS signal to the powertrain control module (PCM) to remain high. A
constantly high input to the PCM from the KS causes the PCM to control the ignition timing as if no detonation were occurring. Failure of the PCM to
retard ignition timing when necessary could cause any of the following concerns:
^
Excessive engine detonation
^
Engine damage during heavy engine loads
^
Higher exhaust emissions
A KS that falsely indicates detonation can cause the PCM to retard ignition timing unnecessarily. Reduced spark advance can cause any of the following
conditions:
^
Poor fuel economy
^
Sluggish engine performance
^
Higher exhaust emissions
