This technical article will cover the design, function and how to test piezoelectric knock sensors on gasoline engine equipped vehicles.
To put it simply, knock sensors are vibration sensors that are well suited to detect structure-borne acoustic oscillations. This can occur as knock in a gasoline engine when pre-ignition or detonation occur (unintended combustion). The knock sensor converts the engine knock into an electrical signal, monitored by the engine control module. The engine control module uses this signal to counteract engine knock via ignition timing adjustments.
Inside the sensor is a toroidal piezoceramic element with an attached mass. The mass reacts to the vibration caused by engine knock, in turn causing movement in the piezoceramic element which generates an electrical signal. This is the signal used by the engine control module as described earlier.
An engine will utilize just enough knock sensors to reliably monitor engine knock. On four-cylinder engines one knock sensor can be used, as it can easily monitor all of the cylinders. As engine cylinder count increases, more sensors are needed. Most times knock sensors are split into banks. For example on a six-cylinder engine, bank 1 may be cylinders 1 -3 and bank 2 would be cylinder 4 – 6.
A Volvo engine equipped with one sensor.
A Mercedes-Benz engine equipped with two sensors.
Sensor are mounted to the engine crankcase and secured with a bolt tightened to s specified torque. This tightening torque is important, under or over torqued fasteners could reduce sensor reliability. Then sensor is mounted directly to crankcase with no washers to a machined and clean surface.
Knock Sensor Crankcase Mounting Location
When sensors fail they can cause a few different faults. First of course if the sensor electrical circuit fails, you will receive a fault related to the circuit, such as a P0325. This code refers to the sensor circuit, so it will likely be an electrical failure. P0326 may set, this fault code refers to sensor range / performance. This can set due to signal out of range. The sensor can also fail, setting no fault codes but causing ignition timing to retard therefore reducing engine power. This can be checked by viewing live data stream and confirming engine timing is being retarded,
When testing turn the Key On with the engine OFF, backprobe the sensor connector to check the bias voltage (this voltage is present for circuit integrity). On most vehicles one wire will be battery negative (ground), the other will be reference voltage, 2.5 volts with connector plugged in and 5 volts with the connector unplugged.
Once you have tested circuit integrity and confirmed DC voltage is correct, you now have to test the sensor signal, which is an AC signal riding along the bias DC volts. When knock occurs, the sensor will produce about spikes from 0 – 4.5 volts (depending on severity of knock). You can lightly tap the area surrounding the sensor to manually test it. The sensor will produce a waveform as shown below.
Scope Pattern 1 - Knock Sensor Signal Without Knock Present
Scope Pattern 2 - Knock Sensor Signal With Knock Present
Scope Pattern 3 - Knock Sensor Signal With Knock Present
You can also test the sensor using a DVOM in AC volt mode. Expect to see voltage generated in the same range as shown in the scope pattern.
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