Knock Sensor FAQ: Everything you need to know about the KS

**jlpicard6** · 7-29-07, 7:15

I wish to compile all the information we need to know about the Knock Sensor. After messing around with this little 'pig-tailed' sweety, I found out there was a lot of mis-information on this forum, and no single source for good info, so I thunk', why not try and 'gen-up- a FAQ about it. If you have any pertinent information you think would be valuable to the 1 Gen crowd please add below and I will edit what is in the quoted area. Thanks.

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The single lead 'pigtail' wire that I have read about here, is actually a tiny coax cable coming off the knock sensor. I clipped mine today, and pulled the oil cooler to get at it. A deep well 1 1/8" socket fit it nice and snug. I was changing my oil, so while the filter was off, I pulled off the oil cooler, easily done.

The coax wire surprised me, I didn't expect it, but after looking hard at the schematic in the FSM I can see it is shielded. The wire routes around the alternator, so I guess it is subjected to some EMI from it, and the shielding helps to keep the knock signal clean. The FSM, and the posts I read here call the output from the KS a 'signal'. Not a DC voltage, but a signal that corresponds to the amount of knock or lack of knock I am assuming. I am wondering what a 'non knocking' output from the KS would look like, has anyone ever 'scoped' one of these to look at it's waveform?

The KS on our cars is matched to the engine. There MAY be some other Knock Sensors on other vehicles that COULD fit, but the KS on the 1st Gen is calibrated to that specific engine, and it's output to the Knock Sensor Module must fall within those Calibrated limits.

The knock sensor protects the engine from knock. Can it be bypassed? I suppose it can, if you developed the Calibrated signal output from the OEM Knock Sensor and routed that to the Knock Sensor Module, or figured out what the Knock Sensor Module output was and routed that to the ECM, but the Knock Sensor system is a safety mechanism designed to keep the engine from frying under a 'knocking' condition. So keeping it intact and functioning is pretty important.

Found a good thread that explains a lot about the Knock Sensor: http://www.wellsmfgcorp.com/pdf/counterp_v4_i4_2000.pdf

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Actually, it's not as complicated as you'd think.

The stock knock sensor is nothing more than a piezoelectric element (ie a cheap microphone), tuned to be sensitive near the frequency that knock would be found in our engine. When the knock sensor vibrates, the motion generates a 0-5v signal. The louder the noise, the higher the voltage.

I can't say for sure how the stock ECU does it, but I can definitely tell you how the AEM EMS handles knock. A normally running engine will make a certain amount of noise (more noise at higher RPMs), so the ECU can't just react of ALL voltage from the knock sensor. The EMS has a table of knock voltage threshold vs RPMs. The higher the RPMs, the higher the knock threshold. Any knock sensor voltage over this threshold is treated as knock, anything underneath is ignored as normal engine noise.

I think it is very likely that the stock ECU handles knock in a very similar fashion. Our ECU's aren't exactly the pinnacle of processing power, ya know.

As far as a warning light goes, any effective warning light would have to have some kind of a RPM-based filtering, and therefor would have to be have a way to read RPM. Normal engine noise at 5000 RPMs would look like pretty significant knock at 2500 RPMs. I suppose you could make a warning light without RPM feedback, and just tune it to warn of knock in the upper RPM range, since that's where knock is most likely. Just to give you an idea, here's my knock voltage table:

RPM : Voltage
0 : 0.12
800 : 0.31
1600 : 0.43
2400 : 0.57
3200 : 0.74
4000 : 0.90
4800 : 1.04
5600 : 1.21
6400 : 1.35
7200 : 1.52

It's been my experience that when I see significant knock, it's usually in the 3-5 volt range, regardless of RPMs. However, any significant knock is usually preceded by some light knock of spikes .5-1v over the threshold, which you wouldn't be able to pick up without an RPM threshold.

If you're just looking for a cheap way to monitor knock, one thing you can do is take a cheap Radio Shack noise amplifier gizmo, and splice it in to the knock sensor voltage. Remember, the knock sensor IS a microphone, so you can listen to it.

The key is to make sure that the pickup you splice into the knock sensor line is very high impedance, so that you don't lower the voltage that the ECU sees. This way you'll be able to hear knock, real time. I was actually going to make something like that before I bought my EMS. I'm pretty sure Autospeed had an article on a headphone knock monitoring device like this. I know for sure I've read a website or two on the subject.

The problem with any knock sensing device, of course, is that it can give a false sense of security. What if you didn't put it together right? What if a connection got loose? If you play around with knock sensing devices, just make sure you do it in conjuction with O2 monitoring (wideband, preferably), reading spark plugs, and a heavy doses of tuning study and common sense. Knock monitoring should be a tool in your toolbox, not your only trick.

Ok the plot thickens! I just now went to my parts car, and probed the threaded hole for the Knock Sensor. It goes no where, not into the water-jacket, nor into the crankcase. It is a dead-ended threaded hole. So I got to thinking, why does the KS HAVE to be at that miserable location? I then got to looking around the block, and found a threaded hole just behind the AC assy. and to the left of the exhaust manifold shields, and guess what.... the KS threads into the hole perfectly. I may burn up my KS, it is probably hotter at that location then back by the oil filter, so I wrapped some metalized-fiberglass header tape around it, and wired it up. Works perfectly, for now. Here is a photo of the location:

After a week of driving with my Knock Sensor in the new location, I find I am operating normally. More information to follow with details for this relocation.