[jansvobodajr]

I am doing my knock sensor and i need to know the torque specs for my '96 Dodge Grand Caravan 3,8L AWD. please and thanks guys!!!

 

[ImperialCrown]

You may need a crowfoot wrench or a deep socket for this job. The proper torque is important for this sensor.
Tighten knock sensor to 10 N·m (7 ft. lbs.) torque.
Over or under tightening effects knock sensor performance, possibly causing improper spark advance control.

 

[jansvobodajr]

Thanks a lot.

 

[Rick Anderson]

Is the direction the sensor installed make a difference? I'm NOT sure, but have heard that, if the knock sensor is an accellerameter, direction would make a difference, but arguably it is NOT an accellerameter, or if it is, only in one plane that the rotation direction won't make a difference.

Regardless, baring any kind of index mark or guidance in a manual, install the new in the same relative rotation direction as the originals can't hurt. You're going to have to have the pigtail face the same direction anyway for wire routing, just mark it and do it more accurately, "just in case".

 

[ImperialCrown]

The knock sensor is a piezo-electric device.
Mechanical stress, like vibrations from cylinder combustion 'detonation' or 'pre-ignition' causes a quartz crystal to produce an electrical voltage. When the PCM sees this voltage, it will retard ignition timing. The knock sensor will 'hear' ping that the human ear can't.
NOx levels soar and engine damage can occur from excessive pre-ignition.

 

[Rick Anderson]

Hear in the sense detonation/pre-ignition will create a vibration through the block, which creates the sound the human ear will hear, that the piezo-electric effect will pick up the vibration and turn into a electrical signal on a wire to the PCM. So it would pick up the vibrations before they got loud enough for a human ear to hear. Can't say its NOT hearing it, cause the closest analogy probably would be how people put a broom handle on different parts of the engine or accessories and then their ear up against the broom handle end, to hear the sound created by vibrations from the problems in just that one area, over the rest of the noise of the engine. Like people can tell if a train is near by putting their ear on the train track.

Its a chicken or the egg argument, regardless IC your explanation makes it perfectly clear how the knock sensor works to tell the PCM there is a detonation/pre-ignition condition in the engine.

 

[valiant67]

The factory service manual (at least for the earlier, probably cruder) knock sensors advised tapping the engine block with a hammer and then to observe the change in timing if the sensor was working properly.

 

[Rick Anderson]

Hmmm, I've read a few things about retrofitting EFI from one engine to another, the knock sensors being one that you just can't swap over and expect it to work. That there is a lot of test and development work to determine the exact signal characteristics of detonation/pre-ignition in that particular engine, and programming the ECU/PCM to recognize that exact signal. That signal would be different on a different engine because of the differences in the size/structure/composition of the block and parts attached, thus the ECU/PCM won't recognize detonation cause the signal is different for a different engine.

So I wouldn't be surprised if more modern knock-sensors that test wouldn't work, they have become much more selective on the freq and amplitude of the detonation, that a random hammer strike might NOT create the conditions its looking for. Or, arguably a hard enough strike of a hammer is going to appear like detonation to any knock-sensor system, all that development work went into picking up detonation even earlier to counter it even earlier and avoid it.