Knock sensors

All about knock sensors

What is it ?
 
The knock sensor in an engine is used to send signals to the engine-control computer if this action is detected, thus preventing a detonation from happening. In a sense, the knock sensor is a microphone placed in the engine to listen to any unusual noises that the engine makes.

A knock sensor is often located bolted on the center of the wall of the cylinder block. There are also some vehicles where the knock sensor is installed on the cylinder head itself. The location of the knock sensor depends entirely on how effective it will be in catching the noise an engine makes.

Most cars that are manufactured today have knock sensors installed in the engine. However, there are a few vehicles that may not have the knock sensor. It is advisable to check if the knock sensor is in place to prevent any potential damage to the engine.

The knock sensor may be mistaken for a spark plug because it looks just like one only smaller. This part has a threaded part on one end and a small pole-like or pin projection on the other end. Most of the parts of the knock sensor are located at its center. The threaded part is used to bolt the device on the engine, although some do not have this part installed. However, a knock sensor without the threaded part will have a threaded bolt hole instead.
How does a know sensor work ?

It detects the slightest noise in the engine and picks up on the "knock" of pre-ignition and sends the information to the ECM (Electronic Control Module). This 'ping' or 'knock' is caused when the mixture of air and gas does not burn smoothly or when it burns too soon. When the timing is off, this can also cause the knocking of the engine. The knock sensor is put in place to regulate these issues. This sensor is usually mounted on the block by a threaded edge that is screwed directly into the block of the engine and is connected to the ECM by wires. When the knocking or pinging is detected, the sensor sends a signal to the ECM, and this in turn retards the engine spark timing at two degree intervals until it has corrected the issue. The sensor's microphone is so sensitive it picks up the knocking when the human ear cannot detect it. It will hear the slightest ping even when the engine is at its top speed. Most vehicles are equipped with a knock sensor, although there are a few that aren't. All production turbo charged high performance vehicles come equipped with this knock sensor, because these engines are prone to pre-ignition issues.
What are the kinds of symptoms ?

The symptoms of a bad knock sensor usually show up at highway speeds or under a load. Be clear that this sensor affects the ignition timing of an engine. The vehicle will have a loss in fuel mileage and acceleration especially at highway speeds. The customer may complain that the vehicle's engine doesn't feel right. Some COP coil on plug systems use strategies that cancel cylinders, see the manufacturer's manual for special instructions. Some of these are located on the cylinder heads under the intake manifold. The vehicle's computer will detect a bad knock sensor, set off a code (P0325-P0332), and enter fail safe mode.

 

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How to test a knock sensor ?

1. Find the electric harness above the thermostat (usually located near the area where the radiator hoses meet the engine). It is usually a gray/black connector with two wires and a red cap.

2. Unplug this connector and fix a voltmeter that can measure more than 10 M ohms to the male connector.

3. Check the foremost terminal and ground the negative point out.

4. Check the voltmeter for continuity after turning the vehicle on. If there is a problem with this, you will need to replace the knock sensor.

5. Also, if your vehicle has run over 100,000 miles without replacing the knock sensor, it is best to get it changed.

6. You will also need to check for frayed wires to confirm a faulty sensor.

7. Another way to test the knock sensor is by striking the intake manifold above the knock sensor with a wrench, while the engine is running. Be careful not to strike the sensor. Hitting the intake manifold should result in a small change in the sound of the engine. If this does not occur your knock sensor is probably dead.

While replacing the knock sensor, you will need to open the intake manifolds, which will give you an opportunity to service other parts such as valve cover gaskets, spark plugs, coolant hoses, coolant passage plates, etc. Watching out for the above mentioned symptoms will keep your vehicle running smoothly for many years to come.


 
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