How does the [DSM] knock sensor work?

No, it will not work.

First of all, the knock sensor for a DSM engine is nothing more than a microphone. It picks up engine noise - all of it. There is no signal processing or filtering. The output signal is difficult to interpret, to say the least. It is not possible to hook up an LED, DMM, digital display or any other form of indicator directly to this signal.

The ECU is responsible for processing this signal and getting useful information from it. The results do not exist outside the ECU memory. So, to measure knock as the ECU does it would be necessary to duplicate the ECU's signal processing algorithms.

To make things more interesting, the ECU only 'listens' for knock in certain portions of the engine cycle. So timing would have to be known as well.

It is far easier for most people to simply purchase a datalogger or Pocketlogger. These instruments will read the correct knock sums directly from the ECUs memory.

The Last Word: Ages ago, there was a way to hook up an LED to the knock sensor "microphone" using a threshold detector. The LED would light up - sort of - when the signal from the microphone got "loud" enough, which supposedly indicated knock. Cute, but not necessarily a good idea.