Theory of Operation
In order to meet tougher emissions and efficiency standards,
today’s engines are burning less fuel than ever before. The government’s
CAFE standards (miles per gallon) or fuel efficiency has risen over the
years to higher and higher levels. Because of higher efficiency, modern,
leaner running engines also have to cope with the ever present
pre-detonation or pinging problem. The leaner the air/fuel mixture the more
prone it becomes to premature detonation. This is the actual explosion or
combustion of the air/fuel mixture before the piston reaches TDC (top dead
center) or the uppermost point of piston travel. Premature detonation causes
the familiar pinging sound as if the engine’s ignition timing was overly
advanced. The actual engine components doing the pinging are the piston
rings vibrating against the piston ring grove, although some engineers
disagree on this subject. Such pinging can cause severe engine damage if
left to itself.
The knock sensor was the answer to this problem. This sensor
is actually a microphone screwed to the engine block, with usually only a
single lead. The one lead (wire) sensor is grounded at the body. Two lead
knock sensors are also used. The second version has the ground provided by
the ECM. The knock sensor is responsible for detecting engine pinging. The
sensor’s microphone material uses the piezoelectric effect to do the
actual knock detection. The piezoelectric effect states that when a crystal
vibrates it produces an AC signal, as in this case when exposed to sound
waves. The knock sensor is made of a crystal, usually quartz, and is tuned to
the specific frequency of a pinging engine. In other words, the knock sensor
only listens to a pinging engine and blocks all other sounds and noises that
the engine can produce. This is true so long as the engine noise is not in
the same frequency as what the knock sensor is tuned to.
The knock sensor is specifically designed for each
particular engine and is not an interchangeable component. Special
consideration has to be taken at the factory when tuning each knock sensor
crystal element to the particular engine block, which the sensor is supposed
to operate on. Each knock sensor is therefore unique in design. Knock
sensors in general are sometimes biased at some particular voltage level,
usually 5 volts, although some manufacturers use different bias voltages.
This means that the ECM provides a voltage on the signal line. This is done
to avoid noise and interference associated with all ground circuits. The
sensor itself divides the bias voltage in half because of its internal
resistance. The knock signal therefore rides on the 2.5 volts bias voltage
(if using a 5.00 V ref.). The knock circuit due to its higher voltage level
(bias voltage) will not pick up any ground noise interference. The bias
voltage also lets the ECM know when the knock sensor circuit has either open
or short circuited.
NOTE:
On some newer vehicles,
the knock circuit has no particular bias voltage. In suchcases, the
ECM usually provides it’s own filtered noise free ground to the
sensor itself.
The knock sensor has a direct influence on the engine’s
ignition timing. The ECM uses the knock sensor signal to retard ignition
timing and thereby reduce pinging. It is common to see a 2 degree or so
ignition timing retardation in a step effect. This means that when the ECM
sees a pinging engine it retards timing 2 degrees. If the pinging continues
then it adds another 2 degrees of ignition timing retardation. The ECM keeps
retarding timing in steps until the pinging stops. It is possible to see the
effects of the knock sensor on engine timing by looking at a graphed scan tool reading of the
knock PID and/or the
ignition timing PID. By graphing these PID readings a relationship can
be seen between the two signals. Being able to graph the scanner’s
PID is of great importance in establishing relationships between two or more
signals.
NOTE:
PID stands for parameter
identification, which means one single parameter or scan reading
corresponding to a particular signal, sensor or calculated value.
Conditions that Affect Operation
The knock sensor’s sole job is to listen for engine pinging.
Any type of interference with this microphone-like sensor will affect the
ECM’s ability to control timing.
NOTE:
Most early European
manufacturers connected the knock sensor to the ignition module (the
EZL module) and not the ECM itself. This made sense to the Euro
makers, since the knock sensor has a direct effect on ignition
timing. Later models with more advanced computers (Motronic)
integrated the knock sensor operation into the ECM itself.
Oil and dirt on the knock sensor’s connector as well as
electromagnetic interference from the ignition wires are all detrimental to
the knock sensor operation. Even though the knock sensor signal rides on a
voltage bias, in extreme cases of deteriorated ignition wires, a spark arc
could cause havoc in the knock sensor’s signal line. This may cause the
vehicle to exhibit lack of power, since the ECM is constantly retarding
timing. The same goes for any mechanical problems present that may be
causing an engine noise with the same frequency as an engine ping. A good
example of this is a broken or cracked flywheel/flex plate. Such condition
causes a noise similar in frequency to a pinging engine. The result is a
noisy engine with severe lack of power due to the ECM severely retarding
ignition timing. Always remember that on knock sensor equipped engines, a
noisy mechanical fault can almost surely cause a lack of power symptom.
The sensor’s crystal material itself could also get damaged, making the knock sensor
literally deaf. Such a condition would render the sensor inoperative and
in some cases the bias voltage would not be affected. If this happens, the
ECM will not have the ability to detect a pinging engine and no possible
ignition retarding would be available. Such an engine would ping severely
without any action being taken by the ECM. On the other hand, modern OBD II
systems will most likely set a code due to an inoperable knock sensor, even
without a bias voltage. This is so because of the functional nature of the OBD II
system. OBD II systems will always try to detect a faulty sensor before
the fault actually happens. OBD II is sort of a pre-emptive system. The
actual sensor does not have to be completely faulty for the system to set a
code. It does this by running specific functional tests, called “monitors” during a drive cycle.
NOTE: On older (EEC IV)
FORD vehicles, at the end of performing a KOER test, you are
instructed to perform a brief WOT. This is done in order for the ECM
to test the knock sensor. When the WOT is performed, the ECM
advances timing and listens for a knock signal which is supposed to
hear. A lack of knock signal at this point will set a KOER code for
the knock sensor.
Component Testing
Testing the knock sensor is a simple mater. Simply probing on the signal
wire with an oscilloscope and tapping on the engine block can generate a
signal. Remember that it isn’t necessary to tap too hard, since damage can
be caused.
Set the scope to DC couple and check for the bias voltage
first. If the knock
sensor has two leads then it probably will not have a bias voltage.
If a bias voltage is seen (usually 2.5 Volts), tap on the engine block with
a suitable tool and check for an AC signal riding on the bias voltage line.
On knock sensors with two leads (ECM provided ground) the actual AC signal
rides on 0 volts and not on a DC bias voltage.
NOTE: Remember not to
set the scope on a very high time
base. Knock sensor signals fall within the higher audio frequency
mark in the frequency spectrum, which is quite low to begin with. If
the scope is set too high, nothing will be seen at the screen.
