| Please VTF. What is a knock sensor or detonation sensor?
Although it may not look like it, the knock sensor is really nothing more than a car microphone that listens to the engine. Similar piezoelectric pick ups are used on guitars, cell phones, etc. They're used both to convert sound to electricity and to convert electricity to sound using the same principals. The knock sensor works by converting pressure or noise pulses into an electrical voltage signal.
More details on exactly how piezoelectric sensors works can be found here on Wikipedia. Who cares?! The typical failure mode of the knock system is through corrossion of the copper terminals in the connectors. Why is this important when the Z community typically relocates or disables the knock system completely?   
The knock sensor is part of the OEM knock system which is significantly more sophisticated and useful than most people realize. It's often disabled by tuners who simply override the system to enable them to tune up the ECU for max power without running into the extra road blocks the knock sensor system creates for them. Doing this puts complete trust in the tuner's equipment and experience to retune the the ECU to avoid knock at all conditions. That's all well and good as long as the car run in conditions similar to those run on that particular dyno. Accidentally using the wrong gas or bad gas, overboosting, slightly different atmospheric conditions, and relevant hardward upgrades can in the worse cases can cause such an engine to knock into oblivien. The knock sensor is your last line of defense. Plus, in a healthy running OEM knock system there exists significant potential to re-optimize the ECU at part power conditions for best throttle response and max torque. For more details on such re-optimizing contact zromtech_. How can you test the knock sensor?
The FSM indicates that you can test the sensor by confirming connectivity between the sensor input terminal A and ground. 
What's causing me to do this write-up is that I was planning on soldering the harness onto the knock sensor and wasn't sure if I'd done everything correctly. Connectivity should exist but what's not mentioned in the FSM is that connectivity will be measured with a huge resistence! Below is my solder job and JB weld cover. 
After I finished this and had put the most of the upper plenum on I decided to double check things but I mistakenly measured the resistence of the knock sensor with an analog ohmmeter on the higher sensitivity settings. As you can see below there was no reading so I figured I had soldered the wrong connection or that the knock sensor I had just installed was bad. I tore everything back apart again!  
Eventually I realized that neither the sensor nor connection was bad but rather my understanding of the knock sensor was bad. Here's a simplified schematic of the sensor itself:
 You can measure the resistence Ri between the two terminals on the knock sensor to be around 600K ohms. -- that's a lot-- therefore the FSM should really clarify that a VERY, VERY SMALL connectivity should exist between the knock sensor terminal A and ground. Otherwise this level of resistence could easily be confused with corrosion resistance. Measuring resistence between terminal B (the right one) and ground will definitely show connectivity. Thats by design so do not connect the white wire to terminal B or you'll short out the knock system completely! Appropriately adjusting the multimeter to a lower sensitivity shows there's connectivity through both terminals but "connectivity" through terminal A (below right) is much less.  
Probably this confusion wouldn't have happened had I used a digital meter from the start but I've also been fooled by the capricious readings off digital meters when measurement connections aren't real good. 
There is only one white wire coming into the sensor at terminal A (the left one). Since the sensor is grounded to the block already the exta terminal B connection is unnecessary. Here's a quick shot showing the resistence measurement between the terminals to prove this. 
There are however two wires going into the knock sensor harness. The second wire provides RF shielding only and doesn't connect to the knock sensor at the far end.
Knock sensor sub-harness:
 The FSM connectivity test is likely not a good indication of knock sensor degradation. To measure sensor degradation requires bench testing the sensor and exciting it over a significant frequency band with the help of an oscilloscope. I'm assuming knock sensors don't degrade much over time but if you know better or know someone who can answer that question for us please chime in! A new knock sensor costs over $150. How sensitive is the knock sensor really?! The knock sensor is extremely sensitive! Yes! A simple tap of a pencil on the sensor will produce a significant voltage spike of around 3V. Link to YouTube video of Pen Tap. YouTube video of a clean O-scope signal produced by a shaver. Nice related write-up. It is possible to test the knock sensor with a digital multimeter but because spikes are so fast it's even hard to tell if you're seeing something real on the digital read out. Taping the end of a socket extension close to the knock sensor while it's on the engine will result in a mild response. YouTube video link using a digital multimeter So now that you see how sensitive the sensor is you probably realize that the game is really about discerning knock from normal engine noise. Again, to understand this level of tuning contact zromtech_. Why is the knock sensor located where it is?
The knock sensor is fixed on the engine block below the lower plenum manifold in the valley between the cylinder heads because that location is as close as possible to all cylinders and therefore gives the most accurate knock measurement for all cylinders. The ECU listens to each cylinder independently and since this happens so quickly relative position is important. 
Overtightening the knock sensor degrades the sensor's sensitivity a little as you're preloading the sensor with unecessary stress. Undertightening to the point where the sensor becomes loose would introduce spurious harmonics into the knock sensor output and confuse the ECU into pulling back timing. Neither of these two scenarios are very likely but take care when installing the sensor and torque it to spec. How accurate is it at predicting knock?
Knock system accuracy and usefulness depends strongly on the position of the sensor and the sophistication of the knock system software... and the OEM ECU has a lot of sophistication! I'm not going to get into the details of the knock system. I'll leave that up to zromtech_. It's important to note that a knock sensor very similar to the Z32s is also used in the 2007+ GTRs. Certainly there have been major advances in the sophistication of the software but the sensor itself hasn't changed much. Bottom line is a properly working OEM knock system can save the day... and with proper precautions it may provide some additional power. (flame suit on now)
OverZealous (CT)
My General posts (click)
My Technical posts (click) |
Knock Sensor / Detonation Sensor: How it Works & How to Test - 
