Knock sensor issues

[scott2093]

Awesome! Thanks for all the help everyone...


So, when trying to test knock sensor ac voltage output, it's so cramped in there, I can't get a good whack at anything while holding three things at once but I can see maybe 100mv on the meter when I tap lightly on the ks housing. I know the tek scope can pick up just about anything but I'd be wanting to see the signal do something with the computer ultimately so I'll wait to bring it out I guess.

too bad I can't see knock counts in memory? Maybe I can?idk.... hard being solo. Could drive with scanner but, I need glasses to read things like the scanner but can't drive with them very easily...I could then at least see what that pinging is doing. It's easy enough to duplicate..

I'll get some extra hands and eyes to really try..,... when I was able to see the scanner and whack the intake pretty good, I wasn't looking for voltage. Just looking for knock activity on the scanner. I feel pretty good I can get it to produce some good signal/voltage on the meter when I try looking at that. Hopefully anyway.

Maybe the scanner won't pick up anything until certain conditions are met anyway is what I'm hearing? Or just being able to see the timing retard won't happen until certain conditions are met? I did see something about how the DTC 43 needs certain conditions to be met or something. I'll get a shot of that blurb in manual...edit...added pic

Which is going to possibly leave me with why, after seemingly needlessly replacing the knock sensor, am I getting this new pinging? Knock sensor too tight? Too loose? Coincidence I got some bad gas pretty much the same time? Man I hope it's not something worse. Flexplate or something...

The emissions manual says "engine idling at normal operating temperature. Tap engine near sensor to look for knock signal on scanner"... I tried that..

Unless there's debris in the hole, or the radiator cap is still tight...the coolant should have been GUSHING out of that hole.

Yes it was coming out good...it sucked.....maybe like when you're past halfway through draining the oil.
Has me a little concerned I drowned the new sensor but so far it seems like maybe it's ok. I'll see what kind of voltage I can get out of it with some good whacks... I'll give it a go tonight after work. God willing of course.

thanks again..

 

[scott2093]

I can't get the knock sensor to put out any ac volts.. I left it in the block and ran my meter to the terminal and ground and read the 3.9kohms so I know it was reading what it should. I reached from the sidewall since I don't have a guard and tapped the sensor itself..nothing...I did get some random readings that would just appear for no reason but couldn't duplicate it and hitting the intake good did nothing either....

Tested my old one just tapping it and can get 100mv spikes easy.. Guess I'll try testing it in a vice too because that seems kinda low but I was just tapping it on it's housing... not sure what kind of voltage it should be able to register?

Maybe I bathed the new one in antifreeze and broke it..idk...maybe it's the torque but, it should be more sensitive since it is much past 14lbs..Part came from Summit

Going to pull it and try to figure out what's up...antifreeze isn't exactly free but I don't want to try swapping again without draining it..

 

[Schurkey]

Guessing the new knock sensor was either damaged during installation, or was defective out-of-the-box.

If the old knock sensor still works, put a decent thread sealer on it, (Loctite/Permatex #592 recommended) have the sensor in one hand when you pull the "new" knock sensor out with the other hand. Swap 'em fast enough, you won't lose much coolant.

Especially if you place a plastic bag or Saran Wrap over the radiator filler neck, under the radiator cap. If air can't get in...coolant will be slower in coming out.

 

[scott2093]

I can get 1VAC with a pretty good whack on the vice....easy to get .5v..etc..with my old sensor...
Going to the zone to grab some thread sealant and a jug of antifreeze......
guessing this is the 592...

https://www.autozone.com/sealants-glues-adhesives-and-tape/thread-locker/p/permatex-hi-temp-threadlocker-sealant-0-2oz/526913_0_0?spps.s=6135&cmpid=LIA:US:EN:AD:NL:1000000:GEN:19502231967&&CATARGETID=120054150001286557&CADevice=c&gclid=EAIaIQobChMI97DAs8PigQMV9yvUAR1MawjYEAQYAiABEgInGPD_BwE&gclsrc=aw.ds

 

[scott2093]

Everything working!
New ks was dead. Showed right resistance but no voltage produced.
Installed old one, hooked scanner up , warmed up truck , whacked the intake and got the knock signal and the timing retarded instantly and audibly.

How to I get my timing back? Unplug battery for a while or am I stuck til it relearns something?

Damn.. Well not too expensive of a lesson.
And holy moly. What's up with the plastic bag trick? Literally lost drops of antifreeze....
Thank you!

 

[Road Trip]

Everything working!
New ks was dead. Showed right resistance but no voltage produced.
Installed old one, hooked scanner up , warmed up truck , whacked the intake and got the knock signal and the timing retarded instantly.

How to I get my timing back? Unplug battery for a while or am I stuck til it relearns something?

Damn.. Well not too expensive of a lesson.
And holy moly. What's up with the plastic bag trick? Literally lost drops of antifreeze....
Thank you!

Good news! Thanks for reporting back with your results -- your empirical testing helps the entire GMT400
community. :0)

By the way, the plastic bag trick? I would call that the Schurkey Effect. The man has a *lot* of experience...
and he's willing to share it. His contributions are one of the facets that makes the GMT400 forum glint & shine on like
a crazy diamond out on the interwebs. :0)

Good stuff!

 

[Schurkey]

Everything working!
New ks was dead. Showed right resistance but no voltage produced.
Installed old one, hooked scanner up , warmed up truck , whacked the intake and got the knock signal and the timing retarded instantly and audibly.

Yeah...that's what I'd expect.

How to I get my timing back? Unplug battery for a while or am I stuck til it relearns something?

That's NOT what I'd expect. The knock sensor when activated, should cause the timing to retard for a few seconds. Not "permanently". Your regular timing advance should come back by itself, probably not more than ten seconds. Five, maybe.

Now I'm concerned again.

Whack it again, see if the idle slows, then recovers.

And holy moly. What's up with the plastic bag trick? Literally lost drops of antifreeze....
Thank you!

Good news! Thanks for reporting back with your results -- your empirical testing helps the entire GMT400
community.

Yup.

By the way, the plastic bag trick? I would call that the Schurkey Effect...

Garsh.

[Elvis Voice] Thankyew, thankyew verra much. [/Elvis Voice]

(Don't forget to REMOVE the plastic bag/Saran Wrap from under the rad cap.)

 

[Road Trip]

Just to round out this thread for future reference. There was a question of
what the piezo transducers can put out for a voltage. @scott2093 in post #14
was able to get a 1 volt (ac) signal with the old sensor in a vice.

I just rewatched my old video in Post #10, and I had channel A set to 1V/division,
so with light tapping I was able to get a 6 division peak-to-peak output displayed
from the sensor alone. (Unpowered, not in circuit, just working into the high
impedance of the o-scope. Peak output could well be different in circuit.)

From my notes taken from the '2 Bar' boost project, here's some fun knock sensor facts:

* They are piezoelectric transducers, turning pressure into voltage.
* There are 2 types: Tuned Resonant (high output like the video) and non-resonant. (low output, ~20mV, have shielded leads for improved S/N ratios. {some Bosch})
* The bandwidth of the resonant knock sensors is ~1 Khz. The resonant frequency of the block is determined primarily by the bore diameter. The larger the bores
the lower the frequency. (Blocks tend to ring at a frequency of ~5-7 kilohertz.)
* There is a SAE Technical Paper written by AC/Delco engineers, titled "Combustion Knock Sensing: Sensor Selection & Application Issues". (#900488)
* Given that they are piezoelectric transducers, they can be damaged by excess torque during installation. And you absolutely run the risk of damage if a
impact gun is used during removal and/or subsequent installation. (ie: Treat it more like an audio microphone than a grade 8 bolt. (!)

****

Good news - when I was originally researching this a dozen years ago, I found a (then) old thread discussing how
these knock sensors actually turn a 'rung by detonation' block into a tattletail AC signal. It's a good read, for they not only discuss
the analog output, but they also talk about how software will set up a timed window to listen for when knock occurs (around TDC)
of the compression stroke for each cylinder and ignoring the rest of the rotation so as not to be mis-triggered by everything else
generating noise in the engine.

They also discuss a little about the need for DSP (digital signal processing) in order to further isolate/extract the valuable info out of
conflicting noise sources. (Piston slap, etc.) As well as a 'load qualifier' to further ignore spuria. (ie: spark knocking/detonation
doesn't happen during very light throttle or fuel-off decel, etc.)

Without further ado, here's the link to that old thread over in the 'eng-tips' website - it still exists!: (Knock sensor discussion)
If you can't read the whole thread, then focus on the "JohnatJandS" reply on 28-Jul-03.

Never got to thank the guy for his insight, but it did help us to ensure that we had a functional knock sensor on the engine we
were pushing the limits with. And also where I got the idea to see for myself with an o-scope and a hammer. :0)

Hope this is helpful to others out there...

 

[Schurkey]

Just to round out this thread for future reference...

Thanks for that. I'll read through it later. I've wondered especially about piston slap triggering a knock sensor. My Luminas both have horrible piston slap, they both get about the same fuel economy on the highway...and at least on the one we've owned since near-new, that's LESS than it used to get before the pistons started rattling.

GM says that piston slap is "harmless" but in my experience it costs the vehicle owner in power, economy, and annoyance. Probably affects emissions based on insufficient timing advance as well.

GM says it's "harmless" because they don't want to recall and fix millions of vehicles they under-engineered.

 

[Road Trip]

Thanks for that. I'll read through it later. I've wondered especially about piston slap triggering a knock sensor. My Luminas both have horrible piston slap, they both get about the same fuel economy on the highway...and at least on the one we've owned since near-new, that's LESS than it used to get before the pistons started rattling.

GM says that piston slap is "harmless" but in my experience it costs the vehicle owner in power, economy, and annoyance. Probably affects emissions based on insufficient timing advance as well.

GM says it's "harmless" because they don't want to recall and fix millions of vehicles they under-engineered.

You know, we're working on & driving older vehicles. Of course with anything mechanical the design is frozen in time,
and from today's perspective some of what we are fighting is because back then the engineers were still learning
as they went with this new-fangled technology. I have no doubt that the software algorithms sifting actionable info
from a noisy background have improved substantially since our prior-century setups?

I have often wondered when enthusiasts were messing around with closed-loop feedback EFI cars, just how often
were they were layering in added noise (via forged pistons & their increased cylinder clearances or aggressive cam lobes stimulating
valvetrain harmonics) ...which was causing the knock sensor subsystem (tuned for stock production engines) to signal
the ECU to pull timing for the wrong reasons in addition to the right ones? And in frustration (because they were working blind
by NOT looking at the data streams like @L31MaxExpress and @PlayingWithTBI have shared with us), they get frustrated,
plunk a carb & HEI distributor on it, and miss out on all that is really available for the fine-tuning/optimization process?

****

One last thing. Putting a scope on the KS signal was a quick & dirty cross-check/instructional moment for the apprentice.
(The young airman just happened to be into making videos & putting them up on YouTube.) To answer your question about
piston slap vs. spark knock, what we really want to use to figure out if different engine sound sources are conflicting with
each other is a good quality spectrum analyzer.

Once you observed a few engines (especially a known-good fresh vs. high-mileage SBC) or stock cast piston vs forged piston motors, I think
we'd get a much better idea of what we were dealing with. Especially if we were also monitoring how the black box
(or 0411) was pulling timing in response to the stimulus. I have a feeling that some of your suspicions re: piston slap would be confirmed.

EDIT: Now that I no longer have to organize my life around supporting 24x365 computer rooms or generating Code 1 aircraft, it's
my sincere hope to eventually pick up from where these old videos left off. A spectrum analyzer hooked up to the knock sensor circuit
while simultaneously looking at the live black box/0411 data streams would make for a great motorhead troubleshooter's video rabbit hole.

So much more to learn than there is time to do so... :0)