can i read a knock sencor with dmm

 

not accurately, the signal is going to be quick and in an analog form. DMMs suck for measuring fine analog signals.

 

does it measure in volts, can i make a led light that flicker at detonation.

 

The knock sensor is virtually useless. It only works at low RPM.

It's a piezo sensor that generates it's own voltage, but will not drive an LED directly. You can use a simple transistor arrangement to make a LED blink when the knock sensor fires, but really it's just academic information. It won't save your engine.

 

low rpm? how low? you r not using 1 since its useless? and about megasquirt how can i tune it without laptor

 

3500 RPM or so. After that, detonation occurs too fast and mechanical noise from the engine is too high.

Why would I use a knock sensor?

You can't tune a Megasquirt without a computer.

 

You can test the knock sensor a couple ways..
one, warm up the engine completely. Connect a timing light (if you have one) and firmly, yet carefully tap the engine mount next to the knock sensor using a plastic hammer. Watch to see if your ignition retards itself...

To test with a DMM (the right way) disconnect the knock sensor and using back probes, like a T Pin you would get from a fabric store etc (you can also use sewing needles but they can be pokey) insert them into the sensor and check for a Resistance Value, remember the sensor must be removed from the circuit to check resistance. This will tell you if your sensor itself is shorted internally or has an open connection internally.

You can also do the tap test (again while back probing the rear of the knock sensor) and watch for a change in voltage on your DMM, as long as you have a quality DMM you should be able to read the Voltage change.

Good Luck...