Ok guys, I'm going to try and explain this as easily as possible, and hopefully it will make sense.
Today I bought myself a digital multimeter, because I thought it would be a really good investment. SO I bought one, and so far I am extremely pleased.
When I got home, I decided that the first thing I would do is check the reading on my Knock Sensor. So I went to my car, and started the test. For those who have worked on their KS, or have researched on them, you should know that the KS is in a really uncomfortable place. You should also know that there is a harness that is on the KS itself, and there is a single wire comming from it. If you follow that wire, you will eventually lead to another sub-harness. This harness is way easier to get to, and you can kind of imagine where it is in this pic:

http://forums.maxima.org/showthread....5&pagenumber=2

Its the second pic, the one on the bottom. This sub-harness made it a lot easier to test the KS. Some of you already know that I have been having problems with my KS for quite sometime. I bought a brand new KS from teh dealer a few months ago, and have still been getting the KS code (03 04). What I did was make a "bypass" for the KS, so like this the code would leave, and my car would run normally again, because apparently the KS advanced my timing tremendously. What I did was get a few resistors from RadioShack and wire them in series, connecting one end to a ground point, and the other to the KS harness, The reason for doing this si so that the resistors give off the same amount of resistance that the KS would normally give out, thus sending a signal to teh ECU that the car had a normal functioning KS. Overall this "bypass" worked, but I still felt that the car was not as it should be.
What I did was go to that sub-harness, and disconnected it. One side leads to the "bypass" wire (which would normally lead straight to the KS) and the other side of the harness supposedly heads towards the ECU. I got my multimeter, and tested the first side of the harness, which leads to the bypass/KS. I got a resistance of 540k ohms (more or less), which is what I expected, meaning that I did ti right. Then I decided to check the other side of the harness which leads to teh ECU, and it read a resistance fo about 555k ohms. I honestly dont understand why it gives me a resistance, unless the ECU has a resistance of its own. So far, i'm thinking, nothign seems to be wrong. For the heck of it, and sheer curiosity, I decide to check the actual KS, to see if mine had gone out or something. When I test it, I get a resistance of 550k ohms, which was a surprise because this meant that my kS was functioning properly, but made me wonder why I still would get the KS code on my ECU.
I decided to leave the "bypass" until I can further find out whats wrong with the whole system. So what I do is re-cnnect that sub-harness, but before I go, I decide to test it while the harness is connected. When the harness is connected, I get a resistance of 270k ohms! This is about half of what It gave me when it was disconnected. Can someone explain to me why it does that? I assume that when the car is on I shouldnt be able to get any readings, but why did resistance read 270k ohms? Could I have a problem with my sub-harness? Or is there a problem elsewhere, or is this just normal? Please let me know, I appreciate it, sorry for the long thread.

Devon

The KS is supposed to read 10MEG Ohms or higher from it's lead to ground while it's bolted to the car. (Haynes Manual)

Your bypass does nothing because the KS output is a voltage, not a resistance. Basically your sending more of the KS output to ground with your "bypass" which will probably trip the KS code.

The KS code is set when a voltage output from the KS is outside of the expected range.

If your refering to this test:

Check Input signal Circuit-2
Check resistance between terminal 1 and engine ground.

Reistance:

Approximately 500 - 630 kohm

That resistance value is looking INTO the ECU, not at the KS. It's input signal resistance, not the actualy KS resistance.

EDIT:
Just found this further down that post:
Knock Sensor



Disconnet knock sensor harness connector.

Check resistance between terminal 2 and ground


Approximately 500 - 630 kohm [at 25 degrees Celcius / 77 degrees Far]

- It is necessary to use an ohmmeter which can measure more than 10 Mohm


Something doesn't seem right here. Those values can't be the same looking at the wiring in two direction. I believe a type-o might be to blame here.

According to the Haynes manual, the KS resistance should be very high, greater then 10Meg. And the output the ECU is looking for is a variable voltage signal, not a variable resistance.


Taken from that post:

The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectrical element. A knocking vibration from the cylinder block is sense asa a vibrational pressure.

This pressure is converted into voltage signal and sent to the ECM.

The Knock Sensor has one trip detection logic.

Diagonostic Trouble Code No. = P0325, 0304
Malfunction is detected when = An excessively low or high voltage from the knock sensor is entered to the ECM

Well this bypass is not tripping the KS code, in fact it took it away, I have no codes. My multimeter can read up to 20M ohms, so what is the correct way to do it? I want to follow what you say because in most of the posts I have read, you were the one with the most knowledge on this subject. When I tested the KS itself, I tested it not bolted down, but having the metal peice touching a ground point. And I tested the left terminal of the KS to get the reading. Let me know, thanks!

Ok when I check the KS itself, I checked the #2 pin (which i belive is the one on the left). It wasnt bolted down, but it still gave me that reading which you described as between 500k-630k ohms. I dont know if by bolting it down would make a difference. Also, could you explain to me the reason why the resistance dropped when I connected tat sub-harness? Last but not least, should I check teh KS the same way I did but in volts and not ohms? If you want to go on AIM to talk about this it would help

Interesting.... Your test method seems right. If I had my Haynes book with me I'd look up the resistance test again. I can sware it's supposed to be very high, not low like your coming up with. Can somebody verify that in the Haynes manual for us????

If your bypass method stopped the light, that tells me your KS is indeed working and detecting excessive ping and or knock. The voltage or signal from it is to high, your bypass is sending some of it to ground, thus lowering the signal and making the ECU "think" all is OK.

Ping may be present on your car. When was the last time you have a major service done? Ping can be caused by excessive carbon build up in your combustion chamber. You might try to find a place that can preform a fuel system, intake and combustion chamber clean up. There are a few systems on the market that will clean things up nicely. I think 3M or Champion has a system out, B&G is another one. Check with Nissan too, they might have a service they perform too. Expect to pay up around 100 bucks for something like that. It might be worth it and could cure your KS problem.

I might be interested in that cleanup you were talking about. When I first encountered my KS problem (whch was Halloween night of 2002) I tired everythign to fix it. I have changed my plugs to NGK, changed one coil pack, change fuel filter, air filter, and oil. I just recently cleaned my IACV, and plan on doing the same to the EGR valve. Since I found out that the KS is actually working fine, then that means something is making my engine knock/ping, hence causing the KS to give off a code and retard my timing. I use 93 octane gass all the time.
What bothered me the most out of this, was the fact that when I connected that sub-harness abck together, the resistance dropped by half. This sticks out in my head, and I have no idea why this would happen. Thanks for your help.

If I understand your method, it looks to me that with the harness plugged in you are measuring the 550 Kohm KS in parallel with the 550 KOhm reading you got going through the ECU => which would be about 275 Kohms. Sounds like a useless measurement and nothing to get worked up about.

Yeah, basically when the harness is disconnected, I have one side with 540k ohms and the other side with 550k ohms, then when harness is connected, i get 270k ohms total. This is normal?

If this is the case, shouldn't he be able to hear knocking/pinging?

I don't have a book right now but I think the 550 Kohm value is correct, which measures the KS from the terminal through the grounding point. My guess for a reason a 10 Mohm DMM is suggested is that meters are less accurate near the limits of their range.

Also, a piezzoelement is sort of like a microphone in this application. The more it vibrates due to knock, the higher the voltage it kicks out, which the ECU uses to adjust timing. The resistance measurement is just a value we can use to check its condition when it's "off".

So then what do i do? LoL

Geez I'm confused. I should really get around under there and PLAY with the KS before I continue (disclaimer for being stupid), but let me ask...

Did you wire the bypass up to the KS harness such that the harness was not plugged into the KS anymore? njmax had me in the mindset that the bypass was hooked up AND the KS was still functioning. However, I thought the whole point of bypassing w/resistors was to fake the ECU into thinking the KS was present and functioning properly, without the KS actually hooked up.

Coincidentally I had it in mind that the ECU wasn't always monitoring the voltage the KS spat out, and would freak if it was too high OR too low. I had it more in mind it was constantly monitoring to make sure it was under a specific threshold. If it is more variable than a simple "Yes, it's above, no it's not", then why would a bypass work at all for any piezzo KS? Does the simple resistor setup people use to bypass also send a constant voltage to the ECU such that the ECU confirms that a) the KS is working, and b) no knocking is going on?

I need to have my brother the EE explain to me how resistance and voltage are related and the affects they'd have in the sitch of a KS

The way it works, is that teh KS itself is totally disconnected, it may be still teh car and bolted to teh engine, but teh harness isnt connected to it. Instead, teh resistors act as a temporary knock sensor with teh same resistance, so the ECU thinks that the KS is functioning properly. Now even if something is wrong with the KS or thwere is something wrong in the engine which is causing the KS to advance thwe timing, you wont know because you bypassed the whole thing, which is why this is only tremporary, because if somethign does happen to mess up your engine, you would want to know about it.
Im having problems because supposedly my KS is working fine according to the tests from my multimeter, but something is going on where its setting off the cel, which is then making my timing all messed up. The worst part is that there is no other code with my KS code, just the 03 04.
Usually if the KS code is tripped, uits because somethign else caused it to go off, such as an o2 sensor or what not, or if teh KS code is by itself its most likely because the KS is bad and needs to be replaced. My KS is new, and like said before, Im not getting any other code other than my KS, which sux, and now I dont know where else to go because I dont wanna keep my real KS bypassed.
if there is a way to check the KS if its working while the car is on (idling) and all the connections are normal, then I would try that, but is there a way?

Could be. Probably. Maybe.
Measuring that circuit is not a documented test point that I know of, so it doesn't mean much in determining the condition of anything.

That's correct, no KS is needed if the resistors simulate a good working KS. But I'd STRONGLY suggest using a KS to protect your engine!
That's what appears to be happening to me.