Essential Guide to Knock Sensors for Your Vehicle

As an Amazon associate, we earn from qualifying purchases.

Purpose

The Knock Sensor is an auxiliary sensor that is part of the electronic spark control circuit. The knock sensor detects engine vibrations caused by “knocking” (cylinder detonation), and converts the vibrations into an AC signal that is sent to the vehicle’s computer system. The computer then momentarily retards spark timing to eliminate the knock condition. Once the knocking stops, spark timing returns to normal.

Location

The knock sensor is installed in the engine block in a location that lets it detect vibrations from the most detonation-prone cylinders. A vehicle may be equipped with one or two knock sensors, depending on engine type (“V”, “in-line”) and number of cylinders.

Operation

The knock sensor consists of a piezoelectric quartz crystal, a fixed base plate, and a vibrating plate enclosed in a metal housing. The sensor tip is threaded into the engine block, intake manifold or exhaust manifold.

When detonation occurs, the resulting vibration is transferred from the sensor tip to the vibrating plate, causing the plate to oscillate (vibrate up and down). The vibrating plate presses against the piezoelectric quartz crystal, causing it to generate an AC voltage signal proportional to the amount of vibration detected (the greater the vibration, the stronger the AC signal).

When the vibrating plate oscillates at the correct frequency (roughly 6 to 8 kHz), the knock sensor signals the vehicle’s computer system to retard spark timing.

Drivability Symptoms

The following symptoms may indicate a damaged or defective knock sensor, or trouble in a related system:

• Audible pinging or knocking while accelerating or driving under load. This condition occurs when the knock sensor fails and the vehicle’s computer system can’t retard timing to prevent detonation. Mild knocking is usually harmless; however, heavy knocking can damage pistons, rings, rod bearing, and result in head gasket failure.
  
  NOTE: Pinging or knocking is not always the result of a defective knock sensor. Detonation can also be caused by poor quality fuel or a lean fuel mixture, engine overheating, misadjusted (over-advanced) timing, excessive compression caused by “dirty” cylinders, and EGR valve problems.
  
• Poor performance or decreased fuel economy without audible pining or knocking. These conditions can occur when the knock sensor is triggered by other engine sounds caused by wear or mechanical component failure, such as a bad rod bearing or worn timing chain. Performance and fuel economy problems result from unnecessarily retarded spark timing.
  

Testing

The knock sensor can be tested while installed on the vehicle using the following procedure:

• Start and run the engine at about 2000 RPM.
• Place one end of a metal rod against the engine block, intake manifold or exhaust manifold near the knock sensor. Do not place the rod on the knock sensor.
• Tap the rod LIGHTLY with a soft-face mallet to simulate the vibrations caused by detonation. You can observe knock sensor operation using any of the following means:
  

• Audible/Visual. You should hear/see a 200-300 RPM decrease in engine speed when the metal rod is tapped.
  
• Timing Light. You should see a 6-8 degree decrease in timing advance when the metal rod is tapped.
  
• Scan Tool. Refer to the manufacturer’s instructions to determine the correct indication. Some scan tools provide an “on/off” indications (such as a blinking light) each time the metal rod is tapped; other tools may show the actual number of degrees of spark retard.
  

Troubleshooting

• If the knock sensor shows an indication of spark retard when the metal rod is tapped, the sensor and associated wiring are in good condition.
  
• If the sensor and wiring are in good condition, but spark timing does not retard, there may be a problem in the electronic spark control circuit.
  
• If the knock sensor does not show an indication of spark retard when the metal rod is tapped, or shows steady retard at idle, the sensor and/or associated wiring may be faulty, or noises caused by mechanical problems elsewhere in the engine may be triggering the sensor.
  

Repair/Replacement

The knock sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

Back to Blog Academy