Where is the detonation sensor and what does it do?

Beat gently, but strongly: what is a detonation sensor and how to check it without a scanner?

There is such a sensor in the car – the detonation sensor. Many people know that it exists, some will even say that it somehow miraculously monitors detonation (the purpose of the sensor gives away its name). But what’s next? How does it do it, and what happens if it suddenly stops working? And how do you know if it’s not working? Everything is much simpler than it seems.

What is detonation and why watch for it

Everyone knows that an internal combustion engine requires that very combustion – the ignition of the fuel mixture. For this purpose, a gasoline engine has a spark plug that ignites the mixture at the end of the compression stroke.

The normal speed of the flame front propagation is 30-50 m/s. But sometimes there is a thing called combustion at the shock wave front, which is properly called combustion. In this case, the speed of combustion can increase up to 2000 m/s. A situation arises when the normal propagation of the flame front no longer exists – there is an explosion. And this is detonation.

In terms of physics, it looks quite boring, but if we simplify it, we can say that the order of combustion of the fuel-air mixture is violated. In detonation, the flame front does not even have time to reach the edges of the combustion chamber, and the mixture there ignites itself under the increasing temperature and pressure.

Detonation produces a sound, which, upon hearing it, was commonly referred to as “tapping fingers. Of course, piston pins do not knock during detonation – the clearances there are not the same. It is the walls of the combustion chamber themselves that start tinkling.

Sometimes they confuse with detonation quite another phenomenon when the engine does not want to stop immediately after switching off the ignition, and sometimes it can even crank “in reverse” (of course, first of all old carburetor engines are concerned). Of course, it’s not detonation but glow ignition – a phenomenon in which the fuel-air mixture ignites on its own from too hot parts (for example, from overheated spark plugs with the wrong glow factor). However, if detonation has gone too far and the engine suffers from it with too much regularity, it may well cause glow ignition – detonation leads to engine overheating.

Detonation is a very bad thing. It causes enormous shock loads on the cylinder/circuit components, it may well destroy the piston rings and the pistons themselves. And if you do not pay any attention to it, then the block as well.

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I will not tell you in detail about the causes of detonation – there is a risk to go a long way away from the detonation sensor and sink in the swamp of Newtonianism and Mendeleevism. In short, there are many reasons: from bad or “wrong” gasoline with a low octane number to the curve of the firmware during chip tuning. However, with a very bad chip tuning, the diagnostics can be simply “cut”, and there will be no detonation sensor error. There will be only a sound. It can also be blamed on soot on pistons and in the combustion chamber, poor mixture, engine overheating, or driving at too low rpm under high load.

All modern engines run on the edge of detonation (usually at a very early ignition advance angle). In this case, it is possible to get the maximum efficiency. In the era of trambler engines with automatic ignition advance angle controllers, it was difficult to achieve a very precise angle, so the “fingers knocked” often back then.

Now a very small detonation sensor is responsible for the advance angle, the signal from which allows changing this angle, and, if necessary, the fuel mixture composition.

If the sensor stops working correctly, theoretically there shouldn’t be anything bad: the ignition should become later (such refusal is foreseen in the engines ECU, and in case the ECU loses the signal, the angle correction will be impossible, but the ignition will become too late), the detonation will not happen, but the car will have a noticeably worse ride. Other consequences are also possible: engine overheating, soot on the spark plugs, the same detonation sound, glow ignition, increased gasoline consumption. Much depends on what caused the detonation itself. If the combustion chamber is riddled to death on a direct injection engine, no shift of the angle to a late value will save you. Well, and of course, Check Engine may light up. What to do in this case?

Find and disarm!

Of course, the easiest way is to connect a scanner and read the error. But hardly all motorists have diagnostic scanner somewhere in the storeroom between a drill and a microscope (all garbage from Chinese stores I do not call scanner by principle, although I do not deny the ability of this garbage sometimes show something). Therefore we will try to do without complicated equipment.

First of all we need to find this sensor. It sounds ridiculous, but it is. It is necessary to look for it on the cylinder block. The easiest thing is with inline four’s: the detonation sensor is usually in the middle of the block between the second and third cylinders. That’s where you should look for it, usually just below the intake manifold. Such location of the sensor on the block allows it to “hear” the detonation of all four cylinders, and the location of the engine – longitudinal or transverse – does not affect the position of the sensor.

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The sensors themselves come in two types: resonant and broadband. Their task is the same for all of them: to detect the knock in the engine (i.e. the same detonation), but the operating algorithms are slightly different. The resonant sensor is tuned to a certain detonation frequency, in which it checks the noise. The frequency is calculated by the formula f(kHz)=900/( * r) , where r is the piston radius and is the Pi number (3.1415. ). If the resonance sensor hears a knock at this frequency, it panics and asks the ECU to take appropriate action. It “hears” them with the help of a piezo element. Thus, the sensor is simply an accelerometer that can convert block vibrations into electrical signals.

Broadband sensor also listens to sound, but it is not concentrated on any particular frequency, but simply transmits all knocks to ECU. And the ECU itself thinks whether it is detonation or not and what to do now.

It is easy to distinguish these sensors: one wire goes to the resonant one and two to the broadband one.

If the ECU understands that detonation has started, it starts changing the advance angle, making the ignition later. Swap and listen to the sensor. Any detonation? OK, I’ll move it a little more. It’s gone? Great, that’s the way to go!

Let’s say the sensor was found and even removed from the car. What’s next? There are several simple ways to check it, but I will traditionally tell only about the most basic one. For this you need a multimeter which can measure very small voltages – thousandths of a volt, millivolts (check yours – mine, bought once for 120 rubles, does not have enough threshold). Put the multimeter in voltage measurement mode, apply “minus” to the sensor body, and gently press the plus probe to the connector of the control contact. Now it is necessary to clamp the sensor in the fist and tap a little with the fist on the table. Since the piezo element has no ears, it hears exactly the knocks, and the good sensor reacts to them by changing the voltage. The changes are very small – approximately within 150 mV, and if the knocking is weak, they are 30-40. In this case (if at least this minimum is there) it is necessary to knock with the fist with the sensor a little bit stronger. If the voltage at the moment of knocking at least a little bit jumped up, the sensor is good. If there is no reaction to the bump, the sensor is probably dead. There is no sense in knocking on it with a hammer in an attempt to resuscitate it – there is more chance to kill the very sensitive piezo element than to restore the sensor operability.

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Theoretically, you can still check sensor resistance, but you need to know the exact sensor resistance value from your car. Shocks are somehow easier and more reliable.

What to do next?

Of course, there are expert rebuilders of these sensors or they select a similar sensor of another car, “filed” it in place with additional resistors and capacitors. Probably, sometimes there is no other way out (well, maybe they drive a Bugatti Veyron and cannot find this sensor quickly and cheaply), but still the best way is to put a new one, and calm down, it costs usually not expensive. Unfortunately, there are more complicated situations in life: the sensor works, but it does not show some errors.

This is simple: you need to check the wiring. There can also be “glitches” in it, and the detonation sensor readings must be accurate for normal ECU operation.

One last thing. Sometimes the detonation sensor can go crazy with extraneous noises that the engine should not make. Clanking of hydro-compensators, “dithering”, cracks of phasers, knocking of chain timing – all these extraneous sounds sometimes randomly the sensor can consider detonation. In this case, abnormal ignition advance angles should alert, although the sensor itself will turn out to be serviceable.

As I said before, the detonation sensor is not the part, failure of which will stop the car. No, it will go. But you should not relax, because if you have a detonation, it kills the engine very quickly. Especially, a modern engine with small volume and with supercharging. So, if you have any suspicions, it is better to go to the service immediately.

Checking the detonation sensor, what it is responsible for, how it works and its faults

In a gasoline internal combustion engine, under a certain set of circumstances, there is a metallic knock. “Fingers pounding,” some drivers say. In fact, this phenomenon is called detonation and its occurrence is highly undesirable, because it can lead to engine failure due to the huge speed of the flame front propagation (over 2000 m/s) and high impact loads on the cylinder walls, piston and block head. To control the danger level, a detonation sensor is installed on the cylinder block.

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It is an accelerometer, that is, a device that perceives and converts the energy of the mechanical vibrations of the cylinder block into electrical impulses. The detonation sensor continuously sends signals to the electronic engine control unit, and the electronics respond by changing the quality of the mixture and the ignition advance angle. This device also helps to achieve a more economical operation and develop the maximum power of the engine.

The frequency with which detonation is likely to occur depends on three main factors.

First of all, the probability of occurrence is affected by the chemical composition of gasoline, more specifically, its octane number. The higher it is, the more resistant it is to this phenomenon. The second factor, which affects no less, is the design features of the engine, namely the compression ratio, the shape of the combustion chamber, the location of the spark plugs, the shape of the piston bottom, etc. For example, an engine with a higher compression ratio is more prone to detonation and needs high-octane gasoline. Otherwise, why do manufacturers write the minimum permissible octane number on the gas tank hatch? The third factor is the engine working conditions. The probability of detonation is affected by the composition of the working mixture, the load, the selected gear, the soot. How the detonation sensor works The principle of operation of the sensor is based on the piezo effect. This device is a piezoelectric plate placed in the housing, at the ends of which, in the event of detonation, a voltage appears. With the increase of the amplitude and frequency of the mechanical vibrations of the engine the voltage increases.

detonation sensor construction

There is a certain safety threshold, if the voltage value exceeds it, the electronic control unit will give the command to reduce the ignition advance angle.

Detonation sensor failure When a detonation sensor failure occurs, the control indicator on the dashboard lights up. At the same time, the engine is running, and the car can be quite driven. To the natural question “why do we need this sensor then” the answer is the following.

On old cars, not equipped with an electronic control unit, the ignition advance angle was corrected manually by turning the ignition timing advance distributor cap. This made it possible to adjust the ignition system depending on the octane number of gasoline, which can be very different at different gas stations. The modern engine has a different device of the distributor, its cover is not moving, so the ECU performs this function. Accordingly, if the detonation sensor fails, the ignition advance angle cannot be corrected.

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A faulty detonation sensor affects the dynamics and economy of the engine. The principle of operation of the electronic control unit is such that if the sensor malfunctions, it sets a deliberately late ignition for safety reasons in order to exclude the possibility of engine destruction. As a result, the power unit works, but begins to consume much more fuel, and the dynamics of the machine deteriorate. The second one is especially noticeable at higher loads.

Checking the detonation sensor The main symptoms indicating that this device has failed are:

drop in power; deterioration of acceleration characteristics and a sharp increase in the “appetite” of the engine; smoky exhaust. At the same time, the engine malfunction indicator lights up on the panel. Moreover, it can both burn permanently and light up briefly when the load increases.

how to check the sensor

It is not always possible to have a scanner at hand to read and decode the fault code. It is not always possible to get to the service station. The question arises: how to check the detonation sensor yourself? From the tools you need a digital multimeter.

First of all, it is necessary to find out what resistance should have a functioning sensor on a particular model of car or engine, since all manufacturers have different values. If it differs from normal, you need to replace it.

You can also check the voltage on the electrical contacts of the sensor, for which you need to disconnect the electrical power connector of the sensor and remove it from the engine. After that, the multimeter is put in millivolts voltage measurement mode, its plus probe is connected to the signal contact, and the minus one to the sensor mass (the hole through which the bolt to the engine passes).

Checking the detonation sensor consists in clamping the sensor with the probes attached in the palm of the hand, which must then be lightly tapped on some surface. When tapping, the multimeter should register the appearance of voltage (usually it is about 30-40 mV). The principle is simple: the stronger the shock, the greater the potential difference between the electrodes. Since the voltage is small, not every device will be able to measure it, so you should first make sure that the measuring device you have at hand is designed to measure this. The complete absence of a potential difference indicates that the detonation sensor is defective.

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