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PLEX Knock Monitor v3

Practical Standalone Tuning

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Discussion and questions related to the course Practical Standalone Tuning

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Not sure where to put this but it seems a standalone may benefit from using the knock monitor the most so I put it here, feel free to move it if needed.

Just got the new Plex Knock Monitor v3HD. Really excited to start using it.

On engines that already have knock sensors, whether they be really advanced stand alone ECU or an OEM ECU, how are people tapping into these signals? Is there a best practice?

I can think of back probing into either the sensor connector or the ECU to feed the knock and trigger signals into the knock monitor but I can also think up of a way to make a connecting harness that sits inline with the knock sensor, cam and crank signals for triggers.

Any input or idea would be greatly appreciated.


It is generally not recommended to share a knock sensor signal. With the two devices connected, usually powered from different sources you can easily create a ground loop and some audio devices are quite sensitive to impedance too. Most tuners attach their own knock sensor in an alternative location for tuning, leaving the ecu one alone. I have sometimes in the past used a longer bolt and stacked my sensor on top of the factory one when I couldn't find a more suitable spot and it seemed to work ok.

I've done the longer bolt and stacking sensors before when I was validating the response of a new version of the Bosch Knock sensors, I actually had three stacked up, with the order being from the bottom of the stack as known good sensor, new sensor, known good sensor. I sandwiched the test sensor between two known good sensors so that i could see if there was any attenuation of the signal between the top and bottom sensor, I also inverted the order mid way through the testing for further validation. All three of the sensors read the same values during this testing.

Amazing device including the logging ability and replay of audio

I have an email into Plex about this as their Help manual says

"It is also possible to connect the Knock inputs to the OEM sensors in parallel to the ecu. For this mode the Knock Monitor must be powered from the vehicle 12V or if powered through USB a ground must be connected to the engine."

I was concerned about ground loops as well but it sounds like they think any ground loops will be taken care of with a separate ground or powered and grounded through the vehicle.

I have a solution for the triggers as I can make a BNC quick connect for two different trigger sets ups, either an inductive pickup for a plug wire or a smaller amp clamp for an injector/coil wire trigger and swap out depending on the vehicle vs back probing at a connector.

I know I have a few cars I tune that have inline connectors for the knock sensors that I can make a harness to plug into inline and take a signal from the existing harness.

When I get a response back I will post it here, and I will wire up a car like Plex suggested and see if I have any issues with ground loops or any abnormal readings.

We usually reply within 12hrs (often sooner)

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