'95 Q45 Knock Sensor/Ignition Timing Questions

[jacnc]

Hopefully the more technical among you ("us" now I guess) can answer about KS and timing by the ECU. Can some one tell me what amount of timing the ECU does under normal conditions?

I suspected at least one KS was bad on my brother's '95 Q with 138K. Using a ScanTool, confirmed code 34. While connected, decided to watch what the timing was under various conditions. Observed about 28-29 deg BTDC under cruise, around 0 deg to "-6" deg (6deg ATDC) under heavy throttle... yikes!! Cool... more gas less power. Tested both KS at the connector... found one bad... then found the other bad a few days later (heat. weather?).

OK... I'm not pulling the plenum "today", so decided to use 470k resisters like I had previously in my '93 Maxima. After a little fab... done. Runs like I remember when he bought it a few years back. Hooked up the scan tool to observe... again 28-29 deg and 10-15 deg BTDC under throttle... better! We're running 93 octane.

But, I started thinking (uh-oh). I read a lot of older domestic oriented magazines and see that most of the engines built/dynoed typically like around 34 degrees advance at WOT on pump gas. Some "fast burn" head engines take a little less. And I remember that they all take a bit more when cruising. OK: I figure that the VH engine has a little more efficient head/combustion chamber than an older domestic, and that emissions are a consideration, so maybe 29 deg advance is about right for light throttle cruise. But 10-15 @ WOT? Seems, in my head anyway, like the engine could take a bit more.

So: 1: does anyone know or have you observed what the stock ECU typically does with timing under various throttle and load conditions? Is what I've seen correct?

2: Any chance that the 470k resistora are a little on the low side and the ECU interprets that as a signal?

3: will a 5 deg advance (cheat) at the CAS accomplish something useful or cause a problem?

Many Thanks

[Q45tech]

Unfortunately the emergency 470-550k solution doesn't work like real KS in place just no code.................since the circuitry doesn't hear the engine normal noises full advance is not engaged.

Nissan engineers were smart enough to protect against such tampering If no noise [not knock] is heard half way measures are incorporated so if peak power/economy is required new KS and harness are required.

Observed about 28-29 deg BTDC under cruise, around 0 deg to "-6" deg (6deg ATDC) under heavy throttle

28-29 deg and 10-15 deg BTDC under throttle... better! We're running 93 octane.

Normal steady cruise can be 35-40 degrees and WOT is 22-26 degrees!

19-21 at 4,000 rpm rising to 25-26 at 6,000 rpm.................Nico ecu adds 2-3 degrees .

I noticed this years ago when I had to use resistor to substitute for one defective KS until I could change it out..................Sometimes I would have near normal advance and sometimes not..............never tried both but your test confirmed my worse fears.

One of techs is running my "test harness" and is having a similar condition.

There is no free lunch at radio shack......................never bother to check other Nissan engines since our job is to return things to oem.

All the numbers provided depend on coolant not being above 195-199F and WOT air density normal [not above 70-80F outside and say no higher than 1000' AMSL].............AC OFF.

Coolant temp retard can be as high a 5 degrees.

[jacnc]

Thanks for your response, Q45Tech. Questions that I have like this are born both from my ability to understand and from my lack of knowledge/experience. If I question your response, it's not me saying I think you're wrong but to get more info so that I can understand better.

My understanding of a knock sensor is that it generates a voltage when it detects a vibration (frequency?) that it is tuned for. Is the analogy of a specifically-tuned microphone correct? Your response suggests (more than suggests) that the KS is picking up all kinds of engine noise and sending low voltages to the ECU always, and that ECU is programmed to receive and react to voltages from "nomal noise" and then to specifically react when higher voltages are received indicating noises above a certain threshold. Is that what you are saying? Interesting. Maybe I need to do a quick study on piezo-electrics... or- even tap in and watch voltages from a fresh KS.

Do you know if the KS in the Q/Nissans is ceramic or crystal? (I just read that much). Might explain longevity.

Regardless, your experience/observations indicate that my resistor "fix" may not be providing 100% operation, even if 93 octane fuel would otherwise be more than enough for full ECU programmed advance. Curiosity: any idea if a higher or lower resistance value would produce different results?

I will say this for my observations: it's been hot here in NC for a while (GA too), so temps have been above what you state... even at night; I probably ran with the A/C on; and it's difficult to watch a ScanTool and watch the road when running a Q45 at full throttle. Maybe I need someone else with me, or to figure out how to use the ST's logging feature if it has one.

Thanks again for your responses. I know more today than yesterday... at least enough to go "hmmm", think some, and ask more questions.

J

[Q45tech]

Nissan knock sensors are wide band acoustic microphone [unlike other designs] and inside ecu their are 2 frequency filters per knock sensor one narrow band tuned to knock frequency and another wider to hear engine running.

The wider one sets the threshold average engine noise floor and the knock freq one is peak hold decay.

Nissan KS in ecu is more complex than GM or Furd of the same year it is more like what is found in current year designs.

Same knock sensor can be used on all Nissans only the filter in ecu needs to be changed for bore diameter which set knock frequency........done in software.........http://delphi.com/manufacturers/auto/sensors/et/flat/

Look at these to get the jest of how it works:http://db.s2.chalmers.se/downl...4.pdfh ... an9770.pdf

The ecu has an extra circuit that measures if the KS resistance is ~~correct at start up this is what sets the 90-95 code not the presence or absense of a acoustic signal.

If the basic running noise is not present the ecu ignores the KS system and subsitutes some SAFE value for the added or subtract advance number added in the equation.

Relocating working KS on some Nissan works because the KS hears some engine noise [lower than normal ?] but the ecu then relies on KS values as real just no knock sensitivity.......................this can be very very bad to relocate working KS as it offer ZERO protection.

People seem to ignore that the KS has a max temperature of 150C [272F] and that is easily achievable under the plenum in hot soak summers........eventually like all sensors they fail after 7 years and need replacing and replacing again 7,14,21 etc. depending on summer climate.

Arizona/Texas vs Minnesota/NYC..................my first one failed at 10 years in Georgia but both were cracked................now 7 years later another new one is acting up.

Seems to correlate with the life of rubber under plenum cross over coolant hose.......................heat is heat!

[Q45tech]

The point is the system must create a NORMAL engine noise floor to determine how strong a knock is and to select the appropriate action [how many degrees retard for how long. It keeps stepping down until it accomplishes its goal then how faster does it step up before starting all over.............................we are dealing in millisecond decisions corrections and resets

Since knock can only occur after ignition [spark plug fires] and before peak BMEP [14-16 deg ATDC] a listen window is opened say -10 to +5 deg TDC for each cylinder every 90 degrees of rotation when a pressure spike would drive the piston/rod straight down towards the bearing and cause excessive wear and breakage.

................[ ]..................[ ]................

The normal engine noise floor changes with load and rpm so the KS must constantly listen and adjust what is normal.

The reason there are 2 KS is that this allows a finer resolution some newer systems BMW V8/V10 use 4 KS all about ecu being smart enough to just stop knock without excessive retard.

Once knock is detected it takes more retard to stop it than just the 1-2 degrees it might have needed for it never to have occurred in the first place.

That is the engineers problem: how to guess what won't knock before it begins as controlling it after the fact wastes many revolutions to get it correct again. Hundreds of dyno hours just setting the KS software.

Newer cars measure fuel temperature and density to try to preguess a decent ignition advance and drive by wire stops/delays sudden accelerations under part load in factions of seconds.

The 470-550k resistance is just there to tell the ecu whether a sensor is in place or not....

Code: Select all

............see IC circuits where a capacitor blocks the voltage applied to measure the resistance from entering the real KS circuits only passing the higher freuency noise and knocks.

An extremely complicated process KS probably the most difficult thing in entire ecu.

Above 4,000 rpm there is not enough time to do the job with 90-93 ecu due to low cpu processor speed so another fail safe guess is engaged by KS and ecu.

My test is to data log advance under 1st gear WOT acceleration and always make sure I have 22>28 degrees [moded ecu] from 4,000>6900-7300 summer or winter and adjust the CAS below 15-16 degrees if I cannot hit 28.........after all in a highway pass 4,000>7300 is what counts and a knock detection can leave you splattered on the front of  a semi.

The engineers planned everything except the possibility that owners would redesign the system to avoid maintenance and repair.

Why the CAS has 3 separate system as back up to make sure the car will run they did everything they could to give 6 years and 70k of trouble free driving..................unfortunately US drivers don't rebuild as they should.

[Q45tech]

2004 and later engines actually adjust each cylinders advance retard compared to every other cylinder to not waste retard on those that don't need it. Thanks to faster ecu and IC that now deal in nano-microseconds vs milliseconds.................the 8-10-12 MHz ecu went first to duals and now to 50-150 Mhz; deliver > 100 DMIPS at 32 bit.

Vehicle CAN system are now 624 MHZ ARM Processors

http://www.xilinx.com/bvdocs/whitepapers/wp169.pdf

[jacnc]

Ohhhhh Kaaayyy.

Seriously: I know it took some time to think out and type all of that out. I *really* do appreciate it.

The more I know, the better I understand and the better decisions I make. You have given me (us) info that I probably would have to scour (sp?) a long time for, if i found it all, esp. re: Nissans Further, this is the most in-depth explanation I've ever gotten on any forum.

Hmmm... now to get to work on that artificial "noise" generator. Or... Maybe I'll just replace the KS.

Thanks again.

[Q45tech]

Do the converse, build a variable frequency and depth notch filter to decrease sensitivity to knock. Because the noise spectrum would have to vary with rpm or the KS ecu would catch your cheat.

[jacnc]

I'm guessing that you meant to put a smilie face with you response. :-)

[Q45tech]

$300 [parts] is a small price to pay for the return to increased performance that accurate ignition advance provides.

[Q451990]

I can see myself sometime in the future... sitting in front of my computer...searching for this thread! Any chance this can go to the Articles section?

Heath