The computer has to hear the knock...

The Knock-Sensor is in essence a microphone tuned to hear the pinging noise the engine produces whenever there's a problem. The knock sensor is always screwed on to the engine block for better sound reception. However, it blocks out all non-pinging engine sounds and only picks up on the actual ping. A ping is always caused by ignition out of timing or lower octane fuel, depending on the engine design.

Theory of Operation

In order to meet tougher emissions and efficiency standards, todayís engines are burning less fuel than ever before. The governmentís CAFE standards (miles per gallon) or fuel efficiency has risen over the years to higher and higher levels. Because of higher efficiency, modern, leaner running engines also have to cope with the ever present pre-detonation or pinging problem. The leaner the air/fuel mixture the more prone it becomes to premature detonation. This is the actual explosion or combustion of the air/fuel mixture before the piston reaches TDC (top dead center) or the uppermost point of piston travel. Premature detonation causes the familiar pinging sound as if the engineís ignition timing was overly advanced. The actual engine components doing the pinging are the piston rings vibrating against the piston ring grove, although some engineers disagree on this subject. Such pinging can cause severe engine damage if left to itself.

The knock sensor was the answer to this problem. This sensor is actually a microphone screwed to the engine block, with usually only a single lead. The one lead (wire) sensor is grounded at the body. Two lead knock sensors are also used. The second version has the ground provided by the ECM. The knock sensor is responsible for detecting engine pinging. The sensorís microphone material uses the piezoelectric effect to do the actual knock detection. The piezoelectric effect states that when a crystal vibrates it produces an AC signal, as in this case when exposed to sound waves. The knock sensor is made of a crystal, usually quartz, and is tuned to the specific frequency of a pinging engine. In other words, the knock sensor only listens to a pinging engine and blocks all other sounds and noises that the engine can produce. This is true so long as the engine noise is not in the same frequency as what the knock sensor is tuned to.

The knock sensor is specifically designed for each particular engine and is not an interchangeable component. Special consideration has to be taken at the factory when tuning each knock sensor crystal element to the particular engine block, which the sensor is supposed to operate on. Each knock sensor is therefore unique in design. Knock sensors in general are sometimes biased at some particular voltage level, usually 5 volts, although some manufacturers use different bias voltages. This means that the ECM provides a voltage on the signal line. This is done to avoid noise and interference associated with all ground circuits. The sensor itself divides the bias voltage in half because of its internal resistance. The knock signal therefore rides on the 2.5 volts bias voltage (if using a 5.00 V ref.). The knock circuit due to its higher voltage level (bias voltage) will not pick up any ground noise interference. The bias voltage also lets the ECM know when the knock sensor circuit has either open or short circuited.

NOTE: On some newer vehicles, the knock circuit has no particular bias voltage. In suchcases, the ECM usually provides itís own filtered noise free ground to the sensor itself.

The knock sensor has a direct influence on the engineís ignition timing. The ECM uses the knock sensor signal to retard ignition timing and thereby reduce pinging. It is common to see a 2 degree or so ignition timing retardation in a step effect. This means that when the ECM sees a pinging engine it retards timing 2 degrees. If the pinging continues then it adds another 2 degrees of ignition timing retardation. The ECM keeps retarding timing in steps until the pinging stops. It is possible to see the effects of the knock sensor on engine timing by looking at a graphed scan tool reading of the knock PID and/or the ignition timing PID. By graphing these PID readings a relationship can be seen between the two signals. Being able to graph the scannerís PID is of great importance in establishing relationships between two or more signals.

NOTE: PID stands for parameter identification, which means one single parameter or scan reading corresponding to a particular signal, sensor or calculated value.

Conditions that Affect Operation

The knock sensorís sole job is to listen for engine pinging. Any type of interference with this microphone-like sensor will affect the ECMís ability to control timing.

NOTE: Most early European manufacturers connected the knock sensor to the ignition module (the EZL module) and not the ECM itself. This made sense to the Euro makers, since the knock sensor has a direct effect on ignition timing. Later models with more advanced computers (Motronic) integrated the knock sensor operation into the ECM itself.

Oil and dirt on the knock sensorís connector as well as electromagnetic interference from the ignition wires are all detrimental to the knock sensor operation. Even though the knock sensor signal rides on a voltage bias, in extreme cases of deteriorated ignition wires, a spark arc could cause havoc in the knock sensorís signal line. This may cause the vehicle to exhibit lack of power, since the ECM is constantly retarding timing. The same goes for any mechanical problems present that may be causing an engine noise with the same frequency as an engine ping. A good example of this is a broken or cracked flywheel/flex plate. Such condition causes a noise similar in frequency to a pinging engine. The result is a noisy engine with severe lack of power due to the ECM severely retarding ignition timing. Always remember that on knock sensor equipped engines, a noisy mechanical fault can almost surely cause a lack of power symptom.

The sensorís crystal material itself could also get damaged, making the knock sensor literally deaf. Such a condition would render the sensor inoperative and in some cases the bias voltage would not be affected. If this happens, the ECM will not have the ability to detect a pinging engine and no possible ignition retarding would be available. Such an engine would ping severely without any action being taken by the ECM. On the other hand, modern OBD II systems will most likely set a code due to an inoperable knock sensor, even without a bias voltage. This is so because of the functional nature of the OBD II system. OBD II systems will always try to detect a faulty sensor before the fault actually happens. OBD II is sort of a pre-emptive system. The actual sensor does not have to be completely faulty for the system to set a code. It does this by running specific functional tests, called ďmonitorsĒ during a drive cycle.

NOTE: On older (EEC IV) FORD vehicles, at the end of performing a KOER test, you are instructed to perform a brief WOT. This is done in order for the ECM to test the knock sensor. When the WOT is performed, the ECM advances timing and listens for a knock signal which is supposed to hear. A lack of knock signal at this point will set a KOER code for the knock sensor.

Component Testing

Testing the knock sensor is a simple mater. Simply probing on the signal wire with an oscilloscope and tapping on the engine block can generate a signal. Remember that it isnít necessary to tap too hard, since damage can be caused.

Set the scope to DC couple and check for the bias voltage first. If the knock sensor has two leads then it probably will not have a bias voltage. If a bias voltage is seen (usually 2.5 Volts), tap on the engine block with a suitable tool and check for an AC signal riding on the bias voltage line. On knock sensors with two leads (ECM provided ground) the actual AC signal rides on 0 volts and not on a DC bias voltage.

NOTE: Remember not to set the scope on a very high time base. Knock sensor signals fall within the higher audio frequency mark in the frequency spectrum, which is quite low to begin with. If the scope is set too high, nothing will be seen at the screen.

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