Knock Sensor (KS Sensor)

The Knock Sensor (KS Sensor) identifies or monitors in the engine the moment of the burst in the cylinder chamber and the vibrating sounds produced there. Its purpose is to send signals converted to voltage to the computerized modules.

What is the Knock Sensor?

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It is an electrical vibration device or part of the vehicle's engine. It works in conjunction with the so-called propulsion system control modules (PCM), engine control unit (ECU), or electronic control module (ECM). The KS Sensor delays ignition, preventing engine damage. When there is a vibration that is not normal, the sensor picks it up and signals accordingly.

The KS is a sensor that identifies harmful knocking to the engine, suggesting that the engine timing is too advanced. What the KS accomplishes is to delay the acceleration process until it is balanced, so that the engine runs smoothly without damage.

How does the Knock Sensor work?

The KS works to provide efficient engine operation, which results in fuel economy. Automotive ignition is faster and more powerful. When the detonation sensor identifies an abnormal vibration, which indicates that the car is accelerated, then it delays the time or degrees of spark activation at the spark plug.

Actually, the operation of the Knock Sensor is in the manner of an automotive synchronizer. This is because the KS Sensor keeps the detonation in a frequency range adjusted for the proper functioning of the engine. Inside the Knock sensor, there is an electrical element that produces a voltage when it detects vibration or undue ignition pressure. Thus, it regulates the vibratory frequency to the appropriate levels.

Proper operation of the knock sensor helps to obtain the necessary power and maximum pressure in the chamber for efficient fuel economy. This results in the spark voltage igniting the air-fuel mixture a few seconds before the top dead center (TDC), i.e., the position of the piston that gives the lowest volume in the combustion chamber.

The KS Sensor uses an oscilloscope to measure the signal utilizing a 50 ms horizontal scan and 2V per division.

The function of KS Sensor

The function of the Knock Sensor is to detect knocking or untimely vibrations in the engine. When this happens, the sensor sends signals in the form of voltage to the computer to make the necessary adjustments. That is, it regulates the timing between about 17 and 22 degrees, depending on the manufacturer, through the external module that controls the ignition spark.

The function of the detonation sensor itself is to prevent uncontrolled cyclic combustion, which raises the temperature in the cylinder. The temperature stresses the pistons, cylinder head and valves unnecessarily.

Knock Sensor Types

Among the Knock Sensor types, there is only one, the piezoelectric. It has two terminals:

  • ECU signal output.
  • Ground or earth.

The piezoelectric sensor pursues the highest automotive performance and fuel economy. For this there are two cycles:

  • Otto cycle: the ignition point occurs as close as possible to the correct point.
  • Diesel cycle: it is the point closest to the burst limit.

The piezoelectric element of ceramic or ionic crystals, and a metal mass are the main components of the detonation sensor, in addition to 1 or 2 connection wires, positive and negative.

There are also types of sensors according to their impact: resonators, which emit a signal at a specific frequency for each type of motor, and non-resonant sensors with a wide coverage from 5 to 15 kHz. They can be used in various kinds of motors and their detonation is determined when calibrating the motor. These types of sensors maintain constant sensitivity throughout their reading range, with slight variations.

Among the different models there are also those with or without discharge resistance. In general, detonation sensors work under these rigorous and well proven specifications:

  • 1 to 20 kHz working range.
  • 5 kHz of specific sensitivity in each sensor.
  • -40 ºC to 140 ºC temperature.
  • 800 to 1400 pF capacitance.
  • >25 kHz resonance.

On the other hand, the piezoelectric sensor is composed of 3 main elements: electrical part, metal part and positive and negative wires. In more detail the components that compose it are:

  • Plastic part that covers the sensor.
  • Connecting wires that transmit information.
  • Contact points between the poles.
  • Body or base.
  • Nut to hold the parts together.
  • Washer that communicates the vibrations of the motor.
  • Seismic mass that conducts motor vibrations.
  • Ceramic piezoelectric ring that acts as a transducer.
  • Isolating discs.
  • Screw to attach the sensor to the engine.

Common Knock Sensor Failures

Knock sensor faults are known for:

  • Loss of power.
  • Accelerated explosions.
  • Knocking-in the engine, resulting in mechanical damage.
  • Unsteady running at idle.
  • Increased fuel consumption.
  • Check engine light comes on.

When any of these faults occur, the detonation sensor itself and the connectors can be checked for cracks, deformations, carbon deposits, corrosion, bad anchorage, and dead signal.

How to test if the Knock Sensor is working?

Excessive pressure and heat in the engine create a detonation that is heard as knocking. To prevent damage to the cylinder head plug, there is the KS on most motor vehicles. If in doubt, the sensor can be tested with these steps:

  • Park the vehicle on a level surface, with the emergency brake active.
  • Check the location of the sensor with the help of the manual.
  • Verify that the wires and the KS itself are in their proper place and that there is no sulfation or breakage.
  • Gently tap with a wrench on the intake manifold over the Knock Sensor, and check if the sound is different.
  • Remove the electrical cable and screws holding the sensor.
  • Use an ohmmeter or multimeter and connect the KS.
  • Check if there is continuity between the body and the terminal. If so, the sensor requires replacement.
  • Measure the resistance at the terminals, which should be approximately 120 to 280 Ohms, depending on the vehicle model.

The fault codes that can be generated by problems in the KS Sensor are the following:

How to clean the Knock Sensor?

Cleaning and maintenance of the Knock Sensor is simple, if you take the proper precautions to avoid physical damage:

  • Inspect the plastic housing for cracks.
  • Check that the harness is not split or sulfated. In the latter case, apply an electronic cleaning spray to the terminals.
  • If the wires are broken, it may be better to replace the entire sensor or its wiring if it has connectors.
  • Test the sensor preferably with a sensor tester.
  • Do not tap the sensor as this may damage the internal electronics.
  • Do not use washers or grease on the KS sensor.

The Knock Sensor is vital to keep the engine in good condition. Although it is a small item, it is highly necessary in motor vehicles. It is preferable to buy a new sensor in case of damage than to replace an entire engine.

Knock Sensor Location

The location of the KS can be in three places:

  • Engine block.
  • Valve cover.
  • Intake manifold.

In the engine block, the external location of the Knock Sensor is attached by the direct screw to a strategic area to clearly perceive the vibrations of the cylinders when denotation occurs. You can know the exact location of the Knock Sensor by acquiring the car's electrical diagrams.


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