Ignition timing determines the relationship between the time the spark plug is fired and the time that the piston reaches top dead center (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. The 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. The optimum ignition timing is affected by all of the following variables:

Ignition systems that are 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 the ignition timing in order to adapt to any of the variables that affect the optimal ignition timing. The KS detects when the engine is experiencing detonation. The KS then signals the PCM to reduce the spark advance until detonation is no longer detected.

The knock sensor system has 2 major components.

The KS detects 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 the 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 the detonation. The amount of timing retard that the PCM applies is based on the engine speed and the length of time that the 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, or normal ignition timing, is re-established. If detonation occurs again the whole cycle will repeat. The alteration of the ignition timing by the KS often occurs continuously while the engine is running even though no detonation is heard by the vehicle's operator.

Loss of the KS signal or loss of ground at the KS module causes the KS signal to the 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 the ignition timing when necessary could cause any of the following concerns:

A KS that falsely indicates detonation can cause the PCM to retard the ignition timing unnecessarily. Reduced spark advance can cause any of the following conditions: