Topic of the Week! Knock Sensor related!

This was very popular on another forum but the moderator there did not want it?

So I thought I would try it here and see if anyone is into having a specail Topic to discuss each week. We can do anything EFI related.

This week I picked Knock Sensor! Discussion should be revolved around Knock Sensor and complete circut and anything it does.

I am looking over a BCC ASDU.bin with $42 mask and 1227747 ECM. These came with knock sensor and ESC module then adjust timing through ECM via the EST or sometimes called ICM which is located in the distributor to reduce spark when sensor detects knock.

There are many options noted in TunerPro when using the $42 mask.

False knock has always been an issue. Even the GM manual gives a procedure to check knock sensor is "Hit block with hammer and note if knock is detected" So many things can cause false knock. Things I have read are
Exhaust pipe hitting frame.
Forget a gear drive as they make to much noise.
Off roaders have reported knock readings in data when knock could not have happened, but they remember hitting rocks.
Open headers, to much noise to close to engine.

I have learned to use the knock sesor as a tool in my own vehicle and develope a tune that is safe and bumps against knock, then back out 2 degrees across the board. If I get no knock readings I disable knock sensor.

So some things I would like to know and compare is how much spark does your ECM (engine size or info as well) take out when knock is detected? Mine says
MAP 60 takes out 5.98
Map 70 to 100 takes out 8.09

How long does it take out spark?

How is recovery rate figured?

How long does it take away spark?

Here's the path taken in GM EFI systems that have a ESC module along with a knock sensor. Newer systems have the ESC removed and knock module placed in the ECM in the memcal chip

[attachment=0:37kqrdjw]gm-ign-module-schm.jpg[/attachment:37kqrdjw]

nearly all of the P4 ECMs use a knock filter integrated into the MEMCAL, the only one i can think of that doesn't is the 1227165, that uses the setup posted above.

then, when you get to the P66 modules, only the Northstar still had them integrated into the MEMCAL since it was the only unit that wasn't flash based. the rest used a seperate, removable knock filter.

the first set of OBD2 PCMs used the knock filter as well, but around ~1997/1998, the filter was removed entirely..... not sure how those PCMs account for knock frequency.

got some more info since i was searching specifically for it:

older knock sensors that use an external knock filter module = 100K ohm sensors. the knock filter is also a 4 pole brandpass filter and supposedly doesn't pick up false knock as easily.

the later models that used a knock filter integrated into the MEMCAL = 3.8K ohm sensors. the P4 models boast a 1/250,000th of a second reaction time to knock being sensed. two-pole bandpass knock filters.

all OBD2 applications i can find used 100K ohm sensors, the earliest models used a hardware based bandpass filter very similar to the P4/P6/P66 units, then after that, they used a DSP(digital signal processor) to determine if knock was happening or not and how severe it is. they assume knock is NOT happening at idle(and other low load situations, IIRC), and take multiple measurements at idle, making the highest and lowest voltage it sees as a "window" and whenever the KS voltage gets outside of this window beyond a certain percentage(or maybe raw voltage), that's when it's considered knock and will start pulling timing.

Good info! Thanks Robert!

So old knock sensors are good... have to start collecting a few. But I like the way the newer E/PCMs figure a pattern to use.

I had a freind who was a engineer in Detroit and he told me stories of Ping, Knock and Detonation. Engines were run on an engine stand type dyno room and they added load then spark till they could here ping and the motors lasted forever. Motors that were ran with knock did last longer then expected but I forget the hours. Engines that were run on detonation could be easliy damaged, destroyed or grenaded with in seconds

old sensors aren't necessarily better, it's the superior design of the 4 pole bandpass filter that doesn't let as much false knock through as the 2 pole design that replaced it.

you could use the OBD2 sensors if you wanted, with the older filter, since they're both 100K designs.

Where's the filter?

depends on the ECM. if it's a 1227165 or i think any of the C3 ECMs, then the filter is external and would be between the sensor and the ECM. usually mounted on the firewall.

I never knew of one? Wonder if it's in the ESC on 1227747 type systems?