Knock Sensor Diagnostics

Knock Sensor Diagnostics

The knock sensor is not the only sensor responsible for detecting engine knock.

A knock sensor is a piezoelectric accelerometer that acts as a microphone on the engine. It converts vibrations into an alternating current (AC) signal. When the crystal inside the sensor is vibrated, it produces an AC voltage. The greater the vibration or shock, the higher the voltage and frequency.

The ECM can look at the output from the knock sensor and determine if something is going wrong in the combustion chamber and take action, like changing the ignition timing or adding fuel.

The knock sensor is not the only sensor responsible for detecting engine knock. The ECM looks at the crank angle sensor data to determine at which cylinder the event is taking place and whether the crankshaft had a sudden change in speed. That way, the ECM can determine if the driver just hit a pothole, or if a knock occurred. It can also identify if the event happened before or after top dead center.

Testing

If you’re using a meter, set it to AC volts. If you are using a scope, make sure the AC filtering is turned off. If you are testing the sensor on the vehicle, you need to access the connector. Knock sensors have only two wires.

Knock sensors are mounted on the block near the base of the cylinder head. On an inline engine, the sensor is easy to access. On V6 and V8 engines, the knock sensor might be in the valley of the engine under the intake manifold.

There are two schools of thought for testing. If the sensor is out in the open, you can use a blunt chisel and hammer and strike the area around the knock sensor.

If the sensor is inaccessible, you can run the engine and pull the fuse for the fuel pump and let the engine stall. The lean conditions might produce an engine knock. But, this type of testing can be hard on an engine.

Knock sensors rarely degrade; they typically fail and will not produce a signal no matter how hard you hit them. When you are looking at a meter, you will see a change of 0.06V when striking around the sensor. Using the min/max feature, you might see higher voltages. You can also check the amount of resistance across the sensor and verify the value with the service information. A scope can provide you with a better picture of what the sensor is doing, even while the engine is running.

Some engine management systems will send out a brief bias voltage signal to check the condition of the circuit when the vehicle is started. If the circuit has higher-than-normal resistance due to corrosion or a bad connection, the voltage will drop, and a code will be set for low voltage. The ECM also knows what types of voltage the knock sensor should produce during startup and idle, and it will set a code if it does not see the correct value.

If a knock sensor-related code is set, the ECM may use a more conservative fuel map or even limit engine loads and RPM. This is to protect the engine and safeguard against possible damage to the combustion chamber.

You May Also Like

Understanding Differences Between Aluminum And Cast Iron

Sheer material differences aside, what are the features and benefits of aluminum and iron engine components?

It doesn’t take a rocket scientist to realize that the rising cost of raw materials and supply chain disruptions over the last few years would transform our industry. However, we never envisioned the sudden demand for billet blocks to surpass that of cast iron or aluminum. The cost of cast aluminum blocks are creeping ever closer to that of their billet counterpart, therefore the additional expense is now a reasonable and practical upgrade for our racers. 

Battery Charging and Programming Diagnostics

Temperature changes how the battery discharges and the amount of current that can be delivered.

Adapting Sensors For Changing Inputs

Let’s focus our attention today on one small facet of the technology inside modern vehicles: Adaptations.

Ignition System Do’s and Don’ts

Why do ignition systems give technicians problems when diagnosing ignition-related misfires? The answer is that some technicians use tests that might give inconclusive results or do damage to the coil or drivers inside a module.

Belt Service For Hybrid and Stop/Start Systems

Knowing how to “force start” a hybrid vehicle can be helpful if you are trying to diagnose a noise problem that involves the accessory belt drive system.

Other Posts

Mercedes Ignition Coil Pack Diagnostics

Replacing a single coil might not be the best approach to curing an engine of a chronic misfire.

Ball Joint Inspection

It’s important to remember not to miss a worn joint. If a ball joint fails, the driver loses control of the vehicle.

Fuel Pumps and Cranking

Diagnosing the problem comes down to understanding what causes a loss of fuel pressure.

BMW 4-Series Service

The 4-series does not stray far from the typical BMW engineering and operation.