Knock Sensor: Description and Operation

Detonation "Knock" Sensor Circuit:

PURPOSE
The Knock Sensor converts knocking vibrations into electric signals. The signals are used by the Engine Control Module (ECM) to control ignition timing and turbo boost pressure.

Knock Sensor:

CONSTRUCTION
The sensor is a piezo-electric crystal made of three elements: Titanium Oxide, Zirconium Oxide and Lead.

KNOCK CONTROL SYSTEM

KNOCK SENSOR
When engine knocking occurs, the crystal shakes and produces an A/C voltage signal. When engine knocking is detected by the ECM, it retards the timing (maximum of 12° at the crankshaft) and reduces boost pressure (via the wastegate solenoid) to reduce the incidence. Some reduction in engine power will be noticed with the reduction in timing and boost.

WASTEGATE SOLENOID
Normally the wastegate solenoid is fully grounded when the key is on, allowing maximum boost by bleeding of wastegate operating pressure. However if the sensor is shorted (0 Volts) or the circuit is open (5 Volts), the ECM retards the timing and lowers the maximum boost from 12 PSI to 9 PSI, dependent upon the load and RPM information. When this happens it controls the wastegate solenoid operation to eliminate knocking three ways:

1. The ECM can disconnect the ground (14 Volts) from the solenoid circuit, causing the solenoid to open and allow manifold pressure to open the wastegate, thus lowering the boost.

2. The ECM can fully ground the solenoid (0 Volts) until it sees a high load signal (based on input from the Air Flow Sensor). It will then duty cycle the solenoid from 0 to 14 Volts as long as the high load is present.

3. The ECM may fully ground the solenoid circuit (0 Volts) until a light load signal at high RPM (more than 188 Air Flow Hertz at 4500 RPM) is found. It will then duty cycle the ground up to 14 Volts for a second or two. This will only happen in neutral and revving the engine or while shifting gears.