Don’t Knock The Humble Knock Sensor – Piezoelectric Knock Sensors – Design, Theory And Operation
What They Are
This is a quick technical dissertation will cover the design, function and how to test piezoelectric knock sensors on gasoline engine equipped vehicles.
Often buried deep down in the bowels of the engine, the knock sensor is one of those things that rarely fails, or is even discussed, until a problem occurs.
To put it simply, knock sensors are vibration sensors that are well suited to detect structure-borne acoustic oscillations.
Purpose
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.
Theory Of Operation
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
Testing
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.
Testing and Measurement – Knock Sensor Signal
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.
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.
Scope Pattern 1 – Knock Sensor Signal Without Knock Present
Scope Pattern 2 – Knock Sensor Signal With Knock Present
Still need help diagnosing an engine control input on any vehicle? Get in touch, we will get that vehicle fixed for you.
Value
At Atlantic Motorcar, we’re all about providing our customers not only Great Service but also Value. We understand that “An ounce of prevention is worth a pound of cure.” (my mom would be proud I remembered that.), in other words, preventing, or catching problems like these brake lines early, can save you more than just money.
Finally
Questions, or if we can be of help in any way with service on your Audi, BMW, Mercedes, Mini Cooper or other European (and now Japanese) import, please contact us. Our team of Service Specialists is here to help, for even the newest autos! (207) 882-9969.
Knowing, not just “doing”, that’s the Atlantic Motorcar Center way.
Thanks!
Warmly,
The Atlantic Motorcar Center Service Team
Awww Audi
The only true and permanent repair is to replace the blower motor, control unit, and repair the wiring harness. It’s just not worth the risk, do it right the first time, and it’s good to go.
Let’s Talk About Steering “Pull”
Why Does Conicity Occur?
y, but quick notification is best. If you allow the tires to wear significantly, the wear may be [falsely] blamed for the pull and warranty may be much more difficult. Replacing the tire solves the problem.
The Problem
The only true and permanent repair is to replace the ignition lock cylinder assembly with the housing, matching it to the client’s key sets, so the car does not have two different sets of keys. The challenge arises that the part seems to be nearly obsolete, and were quoted a “maybe” 5 week delivery time from the UK.
Background
The Importance Of Antifreeze/Coolant Service
The engine radiator is used to remove the heat from the engine. Hot coolant from the vehicle’s engine is passed through a winding tube of the core. Fins attached to the core tubes serve to increase surface for heat transfer to surface air that is forced past them, by a fan, thereby taking heat away from off the engine coolant.
Why It Happens
Why AGM Batteries?
Be Sure To Program If Needed
The Problem
What We Found
How It Fails
The Proper Repair
Symptoms of Fuse Failure
What to Do Next
Many owners of late-model Mercedes-Benz cars equipped with 3.0 and 3.5l V6 engines (M272 or M273), as well as some of the later V8s, have or will likely soon experience a near-universal Mercedes Benz intake manifold problem. Generally, we find problems in performance, which include poor idle, loss of power, and a Check Engine Light.
th intake runner manifold can switch between two sets of intake runners with a flap built into the manifold. This way, you can have one set of intake runners optimized for when the car is idling, and picking up low RPM speed. Then if you get on the throttle and request power for sportier or more demanding driving, the flap switches over to the second set of intake runners that is optimized for the higher rpm range. Many of these intakes also have specifically shaped flaps that cause a vortex, adding even more efficiency.