b16 knock sensor infinite resistance

b16 engine, knock sensor seems to have infinite resistance. this
normal? (just bought the knock sensor used on ebay; i know about ebay
sellers, but there is no reason to think the seller is screwing me, he
has long good history, and i haven't seen a lot of complaints anywhere
about a high proportion of knock sensors going bad)

if it is infinite resistance, how does the ecu (obd1) know whether
you have one hooked up, but you're not knocking? cause it gives you an
error code if you don't have it. being solid state, i'm dubious that
it's a superhigh impedance that my meter can't detect, but maybe, FETs
and all....

(for those with burning curiosity [tegger you listening?], presumably
it's in limphome mode when it gives me the no knock sensor error code:
but vtec still switches over, runs great at middle rpms, stumbles a
lot from idle if you floor it from idle, but if you stick with part
throttle until the revs are up a bit it does perfectly fine; might or
might not stumble once or twice when floored at highish rpm (below
vtec level), not sure but that seems to be dependent on temp more than
anything, maybe it plays it safe at full throttle if it sees the
engine is warm but doesn't have to if it sees it's cold? or maybe it's
thinking about something else entirely and the temp is a coincidence.
at any rate, limphome mode is certainly more drivable and more
complicated than it's been described to me as being. certainly,
everybody always says limphome has no vtec, but you can't mistake it
with a b16, it's operative exactly when you expect it. maybe there are
more than one limphome modes, depending on the particular failure? and
please don't tell me my engine is going to knock itself to pieces, i'm
running on premium and i'm pretty sure the thing isn't set up to
default to aviation gas type tuning without the knock sensor to
constantly detune it to run on mere premium. oh, and it's bone stock,
no blower or anything of course)

 

FWIW the knock sensor on my Subaru Impreza is supposed to be between 400k
and 700k Ohms. What resistance range does your service manual say is OK
for the knock sensor? If it's outside that range it needs to be replaced.

I can imagine that some knock sensors might be capacitively coupled and
would appear to be an open circuit measured with a DC ohmmeter. It all
comes down to what the troubleshooting chart in your manual says. If you
don't have a manual and you're trying to troubleshoot by replacing parts,
buying a used part isn't a good strategy (of course diagnosing by part
replacement is more costly and less effective than the correct method).

 

FWIW the knock sensor on my Subaru Impreza is supposed to be between 400k
and 700k Ohms. What resistance range does your service manual say is OK
for the knock sensor? If it's outside that range it needs to be replaced.

I can imagine that some knock sensors might be capacitively coupled and
would appear to be an open circuit measured with a DC ohmmeter. It all
comes down to what the troubleshooting chart in your manual says. If you
don't have a manual and you're trying to troubleshoot by replacing parts,
buying a used part isn't a good strategy (of course diagnosing by part
replacement is more costly and less effective than the correct method).

 

I believe that they are just a piezo element. Shake them at the right
frequency, and a very high voltage (but hardly any current) comes out.
There's no actual electronic stuff in the sensor aside from the element.

You could probably connect one to a high-Z microphone input and tap on it
to see if it was working.
--scott

 

I believe that they are just a piezo element. Shake them at the right
frequency, and a very high voltage (but hardly any current) comes out.
There's no actual electronic stuff in the sensor aside from the element.

You could probably connect one to a high-Z microphone input and tap on it
to see if it was working.
--scott

 

(Scott Dorsey) wrote in

my Haynes manual says that the knock sensor must be checked with an AC
voltmeter on the most sensitive range.
Neg probe to the sensor body and pos probe to the sensor terminal.
tap on the sensor with a hammer or similar device and observe voltage
fluctuations on the meter.

a piezo sensor would be capacitive.

 

Sure, I tested mine by measuring the resistance, then by putting
my voltmeter on the sensor (on AC millivolts) and tapping with a
screwdriver near the sensor and sure enough there was a signal
coming out. Even though it was putting out a signal and the
resistance checked out I replaced it with a new one, figuring the
fault might be an intermittent open that only happens when it's hot.
So far a couple of weeks and a couple thousand miles later no codes
have popped up.

 

Sure, I tested mine by measuring the resistance, then by putting
my voltmeter on the sensor (on AC millivolts) and tapping with a
screwdriver near the sensor and sure enough there was a signal
coming out. Even though it was putting out a signal and the
resistance checked out I replaced it with a new one, figuring the
fault might be an intermittent open that only happens when it's hot.
So far a couple of weeks and a couple thousand miles later no codes
have popped up.

 

Hi,
It is Piezo transducer precision tuned to specific freuency. Just
replace it with new one if it gives trouble.

 

Hi,
It is Piezo transducer precision tuned to specific freuency. Just
replace it with new one if it gives trouble.

 

indeed.

 

indeed.

 

Admittedly my knock sensor experience is with Nissans rather than Hondas,
but it really sounds to me like you're barking at the wrong tree. I've not
heard of a knock sensor that can put a car into limp home mode. In a
Nissan, it doesn't even light the check engine light when it malfunctions.

Usually a knock sensor code comes up in conjunction with other codes when it
"hears" other things related to the real problem clanking around in there.
Your description sounds more like a transmission problem.

 

Admittedly my knock sensor experience is with Nissans rather than Hondas,
but it really sounds to me like you're barking at the wrong tree. I've not
heard of a knock sensor that can put a car into limp home mode. In a
Nissan, it doesn't even light the check engine light when it malfunctions.

Usually a knock sensor code comes up in conjunction with other codes when it
"hears" other things related to the real problem clanking around in there.
Your description sounds more like a transmission problem.

 

a knock sensor just changes(retards) the ignition timing to stop knock.
http://wiki.answers.com/Q/What_does_a_knock_sensor_do

 

to be pedantic, the sensor doesn't do that - it provides info to the
computer which can then be used in ignition timing calculations. with
no sensor input, the ignition is at a default [comparatively retarded]
setting, but with sensor input, ignition can be advanced to optimum.