Purpose
Varying octane levels in the gasoline can cause a detonation in performance engines. Detonation is sometimes called a spark knock. A spark knock causes the pistons and the rings to vibrate and rattle, sometimes producing a knocking sound.
In order to control a spark knock, a knock sensor (KS) is used. This system is designed to retard the spark timing up to 15 degrees in order to reduce a spark knock in the engine. This allows the engine to use a maximum spark advance in order to improve driveability and fuel economy.
The PCM uses information from the MAP and coolant sensor in addition to the RPM in order to calculate the spark advance as follows:
| • | Low MAP output voltage equals more spark advance |
| • | Cold engine equals more spark advance |
| • | High MAP output voltage equals less spark advance |
| • | Hot engine equals less spark advance |
A low MAP output or high resistance in the engine coolant temperature (ECT) sensor circuit could cause detonation. A high MAP output or low resistance in the coolant sensor circuit could cause poor performance.
Operation
The knock sensor (KS) system uses a knock sensor that is mounted into the engine block near the cylinders. The knock sensor produces an AC voltage which increases with the severity of the knock. This signal voltage inputs to the powertrain control module (PCM). The PCM then adjusts the ignition control (IC) timing to reduce spark knocking.
