Car knock sensor: special features and check
A knock sensor is required to ensure proper operation of the internal combustion engine. This is a small device that tracks the load on the engine. To replace it, you do not need to be an experienced car mechanic, you just need to know how to use wrenches. But in order to clearly identify the malfunction of this site, you will need to know the main symptoms that occur in case of a breakdown.
Characteristics of the sensor
This is a small electronic device that gives a signal about the operation of the engine control system (ECU). It works in conjunction with other readers. And their total number in cars VAZ - more than ten. Knock sensor VAZ monitors the entire cycle of the motor, and if detonation occurs, it sends a signal to the control unit. In addition, it determines the cause of the detonation.After the signal has been received by the control unit, the characteristics that affected the engine operation are corrected.
Where is the knock sensor in cars? It is installed directly on the cylinder of the motor. Such a location allows the control system to instantly respond to detonation and get rid of the reasons for which it arose. On VAZ cars, the sensor's installation site is the engine block, between the third and second cylinders. Getting close to it for replacement is not easy, but this procedure is extremely rare, since the reliability of the device is very high. VAZ cars of the latest models are equipped with both broadband and resonant sensors.
Principle of operation
The sensor is based on a piezoelectric element - a plate that, under the influence of vibration, generates electrical impulses. This is called the piezoelectric effect, on its principle and the operation of the device is realized. Moreover, the voltage at the output of the device depends on the oscillation frequency during vibrations of the engine block. In other words, as the vibration increases, the voltage generated by the plate also increases.
When the engine starts to shake, a knock sensor, the signs of a malfunction which will be discussed below, begins to generate a small voltage. This signal immediately goes to the control unit. The latter has an algorithm by which the microprocessor determines how to proceed in a particular case. He analyzes the level of vibration, and then makes adjustments in the right direction: first sets the ignition time, and then changes the quality of the mixture (the ratio of air and gasoline) to the current settings.
What is detonation?
This question also should not be avoided. After all, in order to study the entire process of the sensor, you need to know what it measures. On modern cars with such devices in their scheme, the external signs of detonation can not be noticed. But they are clearly manifested in the early models that used carburetors. Fingers tapped - this is the first sign that is felt. The reason for the appearance of such a sound lies in the fuel system and ignition.
The combustion process is destabilized, and the moment of ignition remains at the same level. This is the root cause.As a result, small explosions occur in the combustion chambers of all engine cylinders. Gasoline mixed with air ignites itself, and a blast wave hits the walls of the combustion chamber with force. From the side you can’t even say that this is a micro-explosion. When the engine is running it looks more like a metal knock, as if it hit the anvil with a hammer.
Why does detonation appear?
Having a little understood what this phenomenon is, you can talk about the reasons for its occurrence in the engine. The most frequent are poor-quality fuel or low octane number. Try to refuel only on proven gas stations. Moreover, gasoline should be the brand that is recommended by the manufacturer of your car. Often the cause of the appearance of metallic knocks is the incorrectly set ignition advance angle. Aim at the service station so that you can set up the moment correctly, on modern equipment.
Too poor mixture is also the cause of the knocking. The temperature of combustion in the chamber of the air-fuel mixture increases dramatically. To make sure that you really need to adjust the quality of the mixture, unscrew the candle from any cylinder. Pay attention to what color the electrode has.If light gray or white, then this is a clear sign of a lean air-fuel mixture. The cause of this phenomenon was the detonation sensor. Symptoms of a malfunction are in the appearance of metallic knocks due to a malfunction of the fuel system and ignition.
In fact, the presence of this device ensures that metal knocks will not be heard. But if they appear, try to repair the node as soon as possible. But do not rush to immediately remove the sensor and run to the store for a new one. It is possible that there is no malfunction of the knock sensor, the reason - in the wiring. Therefore, first of all, inspect all the wires that are suitable for it. If they are torn or oxidized, the sensor will not work. The signal applied to the computer, has an irregular shape or is completely absent.
Therefore, first clean the contacts. You can use a rust converter or a penetrating lubricant. These solutions are good at combating oxide. But if the destruction of contacts is significant, it is better to replace it. The cost of the connector "mother" - no more than one ruble, and you can require a maximum of two, depending on the design of the sensor itself. But if there is no damage to the wiring, then we can conclude that the knock sensor is damaged.
Removing the sensor and checking
There is not a lot of work, you just need to use the key for 20 or 13. It depends on what kind of sensor is installed on your car. Unscrew the mount, after disconnecting the wires from the device. That's all, now you only need to check the serviceability. For this purpose, you need to use a voltmeter. Moreover, the measurement limit should be 0.2 V. If the sensor has one output, then the voltmeter is connected with a plus to it, and a minus to the metal part of the body.
If there are two contacts at the sensor, then you need to connect a measuring device to them. And now you need to remember what function this sensor has. That's right, it measures the level of detonation, which means that you need to apply several sharp blows to its body. The arrow of the voltmeter should deviate (if you apply digital, the voltage value will appear on the display). The stronger the blow, the more the arrow deviates from the zero value. If this does not happen, then you need to purchase a new knock sensor and install it.