callaway95

Following a recent post on knock retard and associated loss of performance, I'm looking to see if anyone has tried either the "telfon - TPE tape wrap" and/or the resistor fix......and if there has been measured improvement (timeslips) from them.

I've reviewed each section of the '95 service manual that discusses the knock sensor/module section.

The manual does state that no additional thread sealant should be added to the threads of the KS as it may prevent it from detecting detonation. This would seem to support the TPE tape theory since it may "soften" the suspected knock - false or true knock. If TPE tape is placed on the threads, can it prevent the circuit from reaching or maintaining a good ground connection off the block - I suspect it is possible, but maybe not enough to eliminate the ground all together.

Secondly, the manual does say that the KS provides a voltage to the Knock Module/PCM, in which case a "ZERO" knock retard equates to a voltage of approximately 1.5 volts.

Now here's where I get a little confused in the manuals terminology as I don't know whether or not a CODE will appear if the PCM's voltage or that which is generated by the KS goes above or below certain values. For example, a CODE will appear if the voltage is < 0.78 volts, greater than 4.1, etc. for 5 seconds..... Is this the voltage generated by the KS?

"TIME OUT HERE..... YAnkees just tied the game in bot 9th!!!! again!!"

I believe the "resistor" fix which has been mentioned is really acting as a voltage-divider circuit, although the ones which have been mentioned have applied to a single KS arrangement. My '95 has (2) KS's, so a little circuit manipulation is required (unless I'm missing something). The manual does state that the nominal resistance range is 3400 to 4400 ohms, with the mean being around 3900 ohms.

Based on this arrangement, the actual resistance seen by the PCM should be 1950 ohms (2 knock sensor "resistors" in parallel), so in order to de-sensitize it, yet maintain the original level of resistance, a 500 ohm resistor should be place in series (between the PCM and just before the splice of the two KS wires) and then a second 10,000 ohm resistor between the PCM and ground. The values are not exact, but resistors are not 100% either - the tolerance is 5 to 10%.

On a 2 KS arrangement, this should result in a desensitized voltage of approximately 30% of it's original value (1.5 volts x 70%). If I am interpreting the manual correctly, you would not want to go below 0.78 volts, otherwise a CODE would appear. Therefore, the most you would want to desensitize the circuit is by 43% (which would give a true 1.5 volt value a PCM "seen" value of 0.80 volts or just above the 0.78 minimum).

I can't see how someone could just replace the KS with a 3900 or a 1950 ohm resistor and not get a CODE since the PCM is looking for some level of voltage generated or pulled-down by the Knock Sensor.

It's late, but does anyone have any thoughts on this?

Dave

GlennS87

Dave,

My tuner is going install an elbow that screws in where the knock sensor is screwed in and then attach the KS to that. It's a trick that is supposedly used on Mercruiser inboard boat racing motors to desensitize the KS. As a matter of fact, he told me he got it from a Mercruiser dealer.

It's supposedly going to be installed on 11/12, so I should have it ready for the last Corvette Challenge. I'll run with my Ease scantool the week prior and then again on the 11/17 to see if it helps.

Half Fast

Instead of going to all that trouble have you tried using the LT4 knock module? It is less sensitive and mounts at the PCM. AC Delco part #16214681. It's a simple fix for false knock on Gen. II LT1's when using roller rockers and high lift cam. :chevy

ASRoff

I second that motion!!! I ganined 2 tenths with the switch to a LT-4 module due to false knock.

silver & red CE

My 96 has a start up routine that calibrates the knock sensors, the PCM leans out the a/f mixture until knock is detected. As I understand it, the tricks that work on older L98s and LT1s won't work (tape, resistor divider). I think the only way to desensitize the knock sensors on 94-96 is to use the LT4 knock module and/or reprogram the PCM to ignore the input.

Eric


[Modified by silver & red CE, 3:14 PM 11/2/2001]

C4 Suke

The Hayne manual says to put some teflon tape crap on the thread, I'm changing mine this weekend... But then again, it is a Haynes manual, and it really sucks... I won't be adding any teflon tape.

callaway95

I had installed the LT4 knock module earlier in the year - it didn't do a thing as far as I can tell - I"ve made enough 1/4 mile runs since it's install with no Et/mph gain.

Eric - that's an interesting note, although from what I've read in the manual, the PCM will only activate spark retard once it has received an input from the knock sensor above a pre-set limit. From what you've said, it appears as though the PCM is adjusting to avoid knock before it happens, whereas the I think it is a corrective measure - it has to sense knock before it will make corrections... :confused:

Glen - I think you have the right idea - seems as though you're simply isolating the knock source further away and some will get lost through the elbow before it reaches the sensor - I like that idea....

I think I'm going to try the resistor arrangement for a run or two to see if anything changes. If not, I'll try to go that way...
Thanks,
Dave

mackeyred96

Dave, I think Julio told me he took care of mine with PCM programing.
I'll find out for sure.

callaway95

John.....

thanks, I'd like to hear how it was corrected or minimized....

I don't know if there's any difference in how the PCM interprets/receives the signal between our cars. They are both LT1 based, although since your's is a '96, you're set-up with OBD II, whereas my '95 is OBD I .

Dave

silver & red CE

Dave, under what circumstances are you seeing the spark being retarded (due to knock being detected)?

callaway95

I'm most affected at top end (heavy load).....I have a scanning program that has shown 8-9 degrees of retard..........also, I have seen 4-5 degrees of retard sitting in my garage & then rev the engine hard (up to about 4500 - 5000 rpm)...

mackeyred96

Dave, just get some pipe fittings and space the sensor away from the block. That will help.

HighHopes85

Generally with PCM tuning, you can control the amount of spark taken out and the rate at which it is taken out. In other words, you can set the retard tables up to a really low value (i.e. 1 degree) and set the knock recovery rate to to a low value.

IOW, once knock is detected, only one degree will be taken out (just hoping it really is false knock), plus, full advance will be reinstated once that degree was removed.

This is how knock is controlled in the 91's and even though you have a PCM instead of an ECM, the retard control should be the same or slightly improved...that is why it is most likely still controlled this way. I will double check the PCM routine if I can find it and post what I can. -Matt-

EDIT-I just checked the parameters at http://www.tunercat.com and the 95 PCM does have the tables I mentioned above. Go to this website, then on the left, Supported ECMs. Yours is at the bottom of the page. Click on the i with a blue circle around it (need Adobe) and got to page 2. It is the 7th-10th options in the tables section.


[Modified by HighHopes85, 1:42 PM 11/3/2001]

DOCTOR J

I agree with what Matt said.

The place to diagnose & fix a knock/spark retard problem is in the ECM - at least on a '91 ECM it has several areas that can be fine-tuned (and I presume a 95 is similar). For example:

1. The "knock attack rate" is adjustable - i.e. how much spark the ECM takes out when it first 'hears' a knock event. The stock GM program takes out several degrees of timing, and will keep taking timing out until it stops 'hearing' the problem, up to a specified maximum number of degrees of spark. The attack rate can be varied with RPM, but GM obviously sets the threshhold low to protect the engine from the 'average' driver.

2. The "knock decay rate" is adjustable - i.e. how soon the ECM puts the spark back in once it stops 'hearing' knock events - the stock program puts spark back to normal very slowly (over several seconds - also varies with RPM). This is obviously a self-protective measure for the engine - in fact, most ECM's have a "low octane" routine that runs in the background to protect against bad fuel - if it hears too much knock over a long period of time, it takes out a bunch of timing and essentially never puts it back in (until you turn the key off). I wouldn't even try to turn off that feature - I've gotten crap fuel from a couple of major-brand gas stations around here.

3. Of course the important thing is to set up the main spark table to match what your engine likes in the first place, to minimize knock in your particular combination. This takes some time, and a lot of Diacom runs, but it's the only way to be sure you have optimized the engine & ECM program.

The GM acoustic knock sensors aren't the greatest system in the world (seems to me I read a while back they are developing an ionization knock sensor at Saab) but it's there to protect the engine. I'd be reluctant to tamper with it too much without knowing exactly what the ECM/engine is doing - teflon tape sounds too much like a band-aid that might mask a more serious problem. In my case, I spent a few weeks last summer chasing a knock problem that turned out to be a bad injector - (that's why I know a little abut the ECM program) - but I had to track down 'real' knock from false knock in order to fix it correctly.

HTH

drj


ps-

Dave -

Re your original question (now that I'm off my soap box), as far as I know the KS is a piezo transducer mechanically and electrically filtered to 'hear' knock events. Seems to me I read a Bosch SAE paper that put the frequency around 4 Khz, but that was a long time ago. If that is in fact the frequency, it obviously falls right in the general 'noise band', so the KS can't really differentiate knock from any other vibration at that freq. Adding tape or resistors to the KS circuit can reduce the amplitude of the 'knock' signal - but that is going to hurt the engine if it's in fact picking up real knock instead of extraneous vibrations. Also, reducing the KS output signal too much is going to cause another problem - the ECM runs a self-diagnostic to look for a KS output, and if it doesn't get one it's going to set a code or go into fail-safe.

All in all, I think playing with the sigal voltage is a band-aid. Either find the parts that are vibrating and tie them down, or adjust the ECM to be more tolerant of KS signals - but be aware of the consequences if it really IS knocking.


[Modified by DOCTOR J, 10:02 PM 11/3/2001]


[Modified by DOCTOR J, 1:41 PM 11/4/2001]