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How to Test a Knock Sensor

Nov 07, 2023      View: 1664


Knock sensor is an important sensor used to monitor and control internal combustion engines. Its main function is to detect knocking or knocking noise in the engine, and is also called a knock suppression sensor. When knocking occurs, the knock sensor senses engine vibration or vibration and then transmits this information to the engine control unit (ECU). Knock sensors play an important role in internal combustion engines, ensuring proper engine operation and efficient performance. Therefore, when this sensor fails, timely repair and replacement is crucial.


What is a Knock Sensor

The knock sensor is a key component installed on the engine block or intake manifold. Its main task is to monitor vibration and abnormal sounds in the combustion chamber. During a normal combustion process, combustion should be smooth and produce no unusual noise or vibration. However, once the sensor detects an abnormality, it transmits the collected information to the vehicle's electronic control unit (ECU). The ECU will then take measures to combat this irregular combustion, usually including adjusting the timing of combustion and delaying ignition to ensure a smooth combustion process. This helps maintain engine performance and reliability and prevent abnormal combustion phenomena that could cause damage or reduced efficiency. Therefore, knock sensors play a vital role in maintaining proper vehicle operation and performance.


Working Principles of Knock Sensor

Based on their structure, automotive knock sensors can be categorized into piezoelectric non-resonant knock sensors, piezoelectric resonant knock sensors, and piezoelectric spark plug seat metal gasket-type knock sensors. They can also be classified into two main types: piezoelectric knock sensors and inductive knock sensors.

The fundamental principle behind a knock sensor's operation is that when the engine experiences vibration or knocking, a piezoelectric ceramic generates a voltage peak. The larger the knock or vibration, the greater the peak generated. The engine control unit (ECU) processes the signal received from the knock sensor. If it detects knocking, the ECU will delay the ignition timing to prevent further knocking. When the ECU doesn't receive signals from the knock sensor, it advances the ignition timing to ensure optimal engine performance. Therefore, the use of a knock sensor is aimed at enhancing engine power performance while preventing knocking from occurring.


Symptoms of Knock Sensor Malfunction

Due to the critical role of the knock sensor in the internal workings of an engine, its failure can significantly impact the engine's performance and operation.

  • Unusual Engine Noise

Engine knocking is a telltale sign, even if you might not know what it is. It derives its name from the audible knocking or pinging noise associated with this phenomenon. If you've never heard this noise before, it can be challenging to describe, but it becomes quite evident when it occurs.

It's most easily recognizable during engine load or non-idling conditions. To diagnose if the vehicle is experiencing engine knocking due to a faulty sensor, you can open the vehicle's hood and gently rev the engine in neutral to make the diagnosis easier. The higher the engine RPM, the more pronounced the noise typically becomes.

  • Decreased Acceleration

Engines and all their moving parts are designed to work in harmony and with precise timing. Any disruption in this balance can lead to poor engine performance. When the knock sensor is faulty, it fails to provide information about knocking occurrences, and thus, the ignition timing remains unchanged.

On the road, wind noise can be a distracting factor, making it difficult to hear engine knocking while driving at high speeds. Therefore, it's advisable to conduct a check when experiencing issues with acceleration and excessive engine vibration. The test is more manageable when the surrounding environment is relatively quiet.

  • Poor Fuel Economy

Poor fuel economy can also result from incorrect combustion timing or additional combustion occurring within the engine. While it's rarely the sole indicator of a faulty knock sensor, as it can be associated with various issues in different vehicle systems, it's more of a supplementary symptom of engine knocking rather than a primary one.

  • Check Engine Light Illuminates

The simplest way to identify a malfunctioning knock sensor is to check for the illumination of the check engine light. When the information received from the sensor begins to slow down or doesn't reach the computer at all, the vehicle's ECU easily communicates a fault signal.

Besides physically hearing the knocking associated with the sensor, an OBD scanner is the most straightforward way to diagnose sensor issues, as it displays fault codes specifically related to this problem.

  • Piston Damage

Piston damage typically occurs when vehicle owners fail to address existing issues. Before severe engine damage takes place, several other methods can determine whether the knock sensor has failed. Therefore, it's crucial to investigate these issues promptly when they arise to mitigate more severe damage to major components in the future.


What Causes a Knock Sensor to Go Bad


Engine knocking typically occurs when the air-fuel mixture in the combustion chamber ignites prematurely before the flame front has fully developed. Unburned fuel and air situated in remote areas of the chamber auto-ignite due to elevated temperatures and pressures. The flames resulting from this auto-ignition intersect with those from the regular combustion process, generating substantial pressure and giving rise to abnormal knocking within the engine. The primary causes of engine knocking are as follows:

  • Excessive Ignition Advance

Ignition timing usually takes place prior to the piston reaching top dead center during the compression stroke. If ignition occurs too early, a significant portion of the gasoline combusts during the compression stroke, while the remaining unburned fuel experiences pressure, leading to spontaneous knocking.

  • Carbon Deposits on Pistons

Over prolonged use, a thick layer of carbon deposits can accumulate on the piston crown. This reduces the available space within the combustion chamber, resulting in higher compression ratios and contributing to knocking.

  • Engine Overheating

Prolonged engine operation can lead to the buildup of excessive heat, especially when the cooling system is inefficient. This can result in engine overheating and subsequent knocking.

  • Low-Octane Fuel

High-compression engines require high-octane gasoline, while lower-compression engines can use lower-octane options. Using gasoline with an octane rating that is too low for the engine can be a factor in engine knocking.

  • Abnormal Air-Fuel Ratios

When the air-fuel ratio exceeds 14.7, the mixture becomes lean, causing increased combustion temperatures and, consequently, elevated engine temperatures, which can trigger knocking.


How to Test a Knock Sensor




In the context of the Volkswagen Passat NMS, as illustrated in the Figure, the procedure for testing the knock sensor can be broken down as follows:

Testing the Knock Sensor Circuit

1. Begin by stripping the outer insulation of the wiring harness to expose the two shielded wires, usually in black. Using a multimeter set to the 200-ohm range, measure the wire resistance between T3c/3 and ground, as well as between T3a/3 and ground. A normal resistance reading should be approximately 0.1 ohms. Any significant deviation from this value suggests a wiring harness fault.

2. Proceed to disconnect the connector linking the knock sensor to the Engine Control Unit (ECU). Measure the wire resistance separately for the following pairs: T3c/1 to T121/107, T3c/2 to T121/99, T3a/1 to T121/106, and T3a/2 to T121/99. In the case of normal functioning, the resistance values should hover around 0.1 ohms. If you obtain notably different readings, it indicates a wiring harness issue.

Checking the Operational Performance of the Knock Sensor

1. To assess the operational performance of the knock sensor G61, connect an oscilloscope tool to it. Start the engine and let it idle, then utilize the oscilloscope to record the signal waveform produced by the knock sensor. If the waveform fails to display a knock signal, it signifies a malfunctioning knock sensor.

2. Apply the same approach to measure the performance of the knock sensor G66 for a comprehensive evaluation of its functionality.


Detection of Knock Sensor in Passat New Lingyu


Tips for Preventing Engine Knocking

While the knock sensor can detect engine knock, relay knock signals to the ECU, and help mitigate knocking by adjusting ignition timing, it's important to recognize its limitations. This adjustment is constrained by the engine's operational range. Excessively delaying ignition timing not only leads to reduced power and increased fuel consumption but also results in incomplete combustion, potentially raising engine temperatures and causing damage to components like the catalytic converter and cylinder head gasket.


To effectively eliminate knocking, consider the following steps:

  • Use gasoline with the compression ratio recommended for your engine. Opt for 97-octane gasoline, and avoid adding 93-octane gasoline under any circumstances.

  • Steer clear of driving in high gears at low speeds, as this can increase engine load and advance ignition timing. Lower RPMs in higher gears can easily induce knocking.

  • Routinely clean carbon deposits on the pistons. Avoid prolonged low-speed and idle operation and occasionally rev up the engine to higher speeds.

  • Stay vigilant about engine coolant temperature to prevent overheating. Regularly inspect the coolant level, monitor oil levels, and stick to a routine oil change schedule.

  • It's worth noting that maintaining the engine in a state of mild knocking can actually improve engine power and fuel efficiency.

Final Words

Car knocking often causes serious damage to the engine. Knocking refers to abnormal self-ignition before the spark plug in the combustion process of the mixture in a car engine. It is usually caused by excessive compression ratio, inappropriate ignition timing, poor fuel quality or poor engine working conditions. . Detonation will produce high-temperature and high-pressure shock waves, causing damage to internal engine components and performance, including pistons, valves, cylinder walls, etc. Therefore, it is very important to maintain your car regularly to avoid abnormal combustion and knocking. This helps extend the life of your car, improves fuel economy, and provides better performance.


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How to Test a Knock Sensor

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  • Can a car run with a bad knock sensor?
  • Yes, you can drive a car with a faulty knock sensor, but it's not recommended. The knock sensor plays a crucial role in detecting engine knock or detonation, which can lead to engine damage if not addressed.
  • Is a knock sensor expensive to fix?
  • The cost of replacing a knock sensor can vary based on several factors, including the make and model of your vehicle, the labor rates in your area, and the type of parts used. Generally, the price range for replacing a knock sensor is between $200 to $800.
  • What is another name for a knock sensor?
  • The knock sensor is also commonly referred to as a detonation sensor.
  • How long does a knock sensor last in a car?
  • In general, knock sensors are designed to be durable and reliable, and they can often last for over 150,000 miles if the engine is well-maintained and operates under typical conditions.
  • Where is the knock sensor located?
  • The location of the knock sensor can vary depending on the make and model of the vehicle, but it is typically found on the engine block. It is commonly located on the engine's cylinder head, engine block, or intake manifold. The knock sensor is designed to detect engine knock or detonation, so its placement allows it to sense the vibrations and noises produced by the engine.
  • How many knock sensors does a car have?
  • The number of knock sensors in a car can vary depending on the engine's configuration. Most small engine cars typically have one knock sensor. However, in vehicles with larger or more complex engines, such as V6 or V8 engines, it is common to have two knock sensors. Some straight-six cylinder engines may also use a dual-knock sensor system.
  • Does a knock sensor produce AC or DC?
  • A knock sensor typically produces an alternating current (AC) signal. It is a piezoelectric sensor that generates an AC voltage in response to the vibrations and shock waves caused by engine knock or detonation. The piezoelectric crystal inside the sensor generates an AC signal as it is deformed by the mechanical vibrations, and this signal is then used by the engine control unit (ECU) to monitor and adjust the engine's operation to prevent knocking.