Knock sensor voltages

[redhotrod]

Anyone out here know what voltage value the knock sensor puts out

 

[John at J&S]

GM sensors come in three overlapping frequency ranges, to cover bore diameters from 70mm to 120mm. Sensor frequencies are 7.0 kHz, 6.0 kHz, and 5.2 kHz.

Inside the sensor is a piezo electric crystal. The crystal turns vibrations into an analog signal.

Key on, engine not running, sensor connected to the EST module, the sensor voltage will be 2.5 volts.

The sensor has an internal load resistor to ground, and the input of the EST module has a resistor of equal value tied to 5v. This biases the sensor at 2.5v.

The signal level is about 500mv/g. I have seen 40v peak to peak.

 

[redhotrod]

40v p-p, that is of course a.c. voltage, right? wow, thats some serious detonation. So with engine off, key on, thats 2.5v dc, right. In my 84 buick T-type, with 3.8 turbo, what voltage would start to show knock, would it be anything over 2.5v dc. Also, this sensor goes to the est 1st, then to the ecm, right, does the ecm pay attention to the ac waveforms or just the dc signals?

 

[John at J&S]

The ac signal swings above and below 2.5 volts. Normal engine noise can easily be 20v peak to peak.

The ECM never sees the ac signal. The module interprets the signal, and pulses low if it thinks the engine is knocking.