Knock sensors

[Tom Cloud]

there have been several threads with questions about
knock sensors on both the diy-efi list and the eec list.

Yesterday, I researched and bought a knock sensor for
a Ford -- apparently there is one which is pretty standard
for most V-8 applications -- it's a Borg-Warner S-8041.
Don't know the Ford part number, but the Ford dealer quoted me
$36 retail (didn't go there, so don't know what it would have
actually cost me -- usually about 20% less), and PepBoyz quoted
$45 retail with Net of $27.

There is one of these on my '91 F-150 -- though I can't find it.
For the '90 and '91 302 Bronco and F-series speed density (all
they had) fi, there's this knock sensor (could be on other
models -- didn't check).  I was led to believe from discussion on
the lists that the knock sensor was only on MAF vehicles.

 - interesting note ... for those models, the knock sensor is
   only on the 302 (5.0l) and not on the 351W (5.8l) .... why ??

 ** I must recant something I said on the Bronco list re knock
    sensors -- I think I said that knock sensors were only on
    MAF vehicles  --  if so, I was w...w..w-wrong !

I have the little bugger here in my Coca-Cola stained hand
and the mounting threads appear, to my highly calibrated eyeball,
to be 3/8", but mic'ing them shows about .39" and a thread gauge
shows something between 16 and 18 tpi -- and my metric thread
gauge is a round gadget that won't let me measure the short
stud -- so, I can only deduce that it's ISO 10 X 1.5 mm (but it's
so close to 3/8-16 it'd be easy to run a 3/8-16 die over it  ;-)

  -  Question -- does anyone know where this thing plugs onto
     the block?

Checking it out on the bench -- it has two terminals, neither
of which are grounded -- i.e. isolated outputs.  Hooking it to the
'scope shows a bipolar (AC) output of as much as 4 Vpp when I tap
it lightly (very scientific test  ;-)  with the blade of my
electricians knife.  [The Ford Shop Manual says to put a DVM
on it, on the AC scale, and tap it with a small wrench to test
it.]

This is very clearly a piezoelectric (crystal) transducer
(I know, I know  .... that's what you said  8^)

Since I don't have a connector, I figure I'll cut off the
plastic shroud and rubber seal and solder a trailer plug to it
(my poor old Bronco -- it's getting hard to find different
types of trailer plugs so I won't accidently plug the wrong
things together  8-(

Here's what Probst says about the frequencies believed associated
with "knock".

"Depending on the engine, knock may be signalled at frequencies of
about 6 kHz -- 5450, 5700, 6000, 6150, and 6400 Hz.  Some engines
require a different knock sensor because they have a different
resonant frequency -- 9,500 Hz for some 4.9L, 5.0L, 5.0L
Econoline/Broncos."

I don't know if he's saying that those frequencies are the
resonant frequencies of those blocks -- I cirrusly (a Cloud
joke) doubt it  ... I'd think the mass of the 5.0 and 5.8 would
make for a much lower resonance than 9.5k or even 6k.  Does anyone
know?

I don't recommend much from RatShack, but they do sell a couple
of things that are a good deal (IMHO).  First, unrelated, is
their set of jeweler's screwdrivers -- it's a big set for about
$12.  Then, there's a little hand-held audio amp that's powered
by a 9 jolt battery -- great for audio testing.  When I did
communications work, I used it all the time for checking
circuits -- just be sure the input is AC coupled so it's not
damaged hooking it into circuits with DC bias joltages (that means
put a 1 uF cap or so in series with the input). It uses an LM-386
audio amp, for those that care.

I thought I'd get a cheap equalizer (i.e. one that's lying
around -- free) and use the above-mentioned RatShack audio
amp to listen in on the engine whilst tuning out the stuff
I don't want to hear -- until I read Probst's book.

The highest note on a piano is around 7,000 Hz .... the sound
from a typical piezo buzzer (like on your watch alarm or micro-
wave beeper is about 4,000 Hz).  Frequencies up to 9,500 Hz can
get pretty hard to hear -- harder for a crummy hand-held amp like
I wanted to use to reproduce, and even harder to distinguish the
sound from surrounding noise (esp with my new, "quiet" mufflers  ;-)

I'd like to be able to "listen".  There's a trick called
"heterodyning" where you can "mix" (actually multiply)
two frequencies together to get a third frequency in a
different frequency range -- in my case, I'd like to shift
the 9,500 down to about 2,000 or so .... and I might do it
"some day", but now we're getting serious.  I don't want
to drive around with a spectrum analyzer lying on my floor
board -- I can whip up a notch filter and drive an analog
meter (or, for that matter, an AGC circuit that has a lower
frequency into it so that I can *hear* some feedback).

Sorry for the rambling .... has anyone done anything like
this.  Todd Knighton, and some others, keep knocking me (pun
intended) to get the commercially available knock detector
(don't remember who by at the moment  ....  J&S  ?? ) -- but it's
like $400, as I recall .....

  Gonna go get a Coke ....  lemme know what you think

Tom Cloud

They say the mind is the first thing to go ... and I can't remember the second