Knock Sensor: Description and Operation

Varying octane levels in gasoline can cause detonation in engines. Detonation is called spark knock.
To control spark knock, a knock sensor system has been added. This system is designed to retard spark timing up to 20% to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

The knock sensor system has two major components:
- Knock sensor module
- Knock sensor

The sensor is mounted in the engine block near the cylinders, or in the intake manifold at the rear of the engine. When the knock sensor detects abnormal vibration (spark knocking) in the engine, it produces a voltage that is received by the knock sensor module. As long as the knock sensor module sees no voltage from the knock sensor (knock not present), it sends a signal voltage (8 to 10 volts) to the Engine Control Module (ECM) and the ECM provides normal spark advance- When the module detects voltage from the knock sensor (knock present), it turns "OFF" the signal to the ECM and the voltage at terminal "B7" goes to 0 volts. The ECM then retards Ignition Control (IC) to reduce spark knock.

Loss of the knock sensor signal or loss of ground at knock sensor module would cause the signal to the ECM to remain high. This condition would cause the ECM to control IC, as if no spark knocking were happening. No retard would occur, and spark knocking could become severe under heavy engine load conditions.
Loss of the knock sensor signal to the ECM would cause the ECM to constantly retard IC. This could result in sluggish performance and cause a Diagnostic Trouble Code (DTC) 43 to set.
A "Scan" tool will read knock signal in A/D counts. When detonation is detected, knock signal counts will increment, as long as knock is present. "Scan" tools will indicate knock being present either by showing AID counts, or displaying "Yes" (knock present), or "No" (knock not present). If DTC 43 is present, use that chart to diagnose system. If no code is present and knock sensor system is suspected, use the knock sensor system check.