The knock sensor is an auxiliary sensor used to detect the
onset of detonation. The sensor has no affect on fuel or
emissions and is actually part of the electronic spark control
circuit. Consequently, it affects ignition timing only.
When the knock sensor detects the characteristic pinging or
knocking vibrations produced by detonation, it signals the engine
control module or electronic spark control module to momentarily
retard timing. The computer then backs off the timing a fixed
number of degrees or in increments depending on how it is
programmed until the detonation stops. Then timing returns to
The sensor, which is mounted on the intake manifold or
engine, generates a voltage signal when engine vibrations between
6-8 kHz are detected. The sensor works on a "wall vibration"
principle wherein a vibrating plate inside the sensor presses
against a piezoelectric quartz crystal to generate an AC voltage
signal. The strength of the signal increases with the severity
of the knocking. When the plate oscillates at the right
frequency, the knock sensor signals the computer or control
module to retard timing. On newer GM applications, the signal
goes to the knock sensor or "KS" module located on the powertrain
control module (PCM).
The location of the sensor on the engine is critical because
it must be positioned so it can detect vibrations from the most
detonation prone cylinders. On some in-line five or six cylinder
engines two sensors are needed to pick-up detonation at both ends
of the engine.
KNOCK DRIVEABILITY SYMPTOMS
If the knock sensor circuit fails, the computer won't retard
timing to prevent detonation. The result will be an audible
pinging or knocking from the engine during acceleration or under
load. Light detonation usually causes no harm but heavy
detonation over time can crack pistons and rings, flatten rod
bearings and cause head gaskets to fail.
Knock sensors can sometimes be fooled by other sounds in the
engine, causing the timing to retard unnecessarily. A bad rod
bearing or piston slap in a high mileage engine, for example, may
trigger the sensor. So too can a worn timing chain or mechanical
fuel pump. A drop in fuel economy or performance would result
from retarded timing.
KNOCK SENSOR CHECKS
Just because an engine detonates doesn't mean the knock
sensor is defective. The causes of detonation include:
* Defective EGR valve (stuck shut or inoperative)
* Too much compression due to accumulated carbon in cylinders
* Overadvanced timing
* Lean fuel mixture (or a vacuum leak)
* Overheated engine
* Low octane fuel.
The knock sensor can be ruled out on most applications by
running the engine at about 2000 rpm and rapping on the intake
manifold near the sensor with a wrench (never on the sensor
itself!). Observe ignition timing with a timing light, or if
that isn't possible listen for a change in engine speed caused by
a momentary retard of ignition timing. The sound of the wrench
should simulate the vibrations produced by detonation, causing
the knock sensor to signal the computer to back off the timing.
You should see a corresponding decrease in timing advance of
usually 6 to 8 degrees and/or a decrease of a few hundred rpm in
engine speed. If nothing happens, the sensor, wiring circuit or
computer may be faulty.
You can also use a scan tool to read the knock sensor status
directly. Some systems give a yes/no or on/off indication while
other show the actual number of degrees of spark retard. If the
sensor gives an indication of knock retard when you rap on the
engine, the sensor and its wiring harness are okay. But if the
timing fails to retard, there's a problem in the computer spark
A knock indication that fails to change when you rap on the
engine or one that shows constant retard at idle would indicate a
faulty sensor or wiring circuit (or a false retard caused by a
mechanical problem in the engine as noisy lifters, loose
Trouble codes that indicate a problem with the knock sensor
* General Motors: Code 43
* Ford: Code 25
* Chrysler: Code 17
If you're observing the knock sensor signal on an
oscilloscope, you'll see a straight line constant voltage signal
when the sensor isn't picking up any vibrations. But when you
tap near the sensor (or when the engine knocks), you should see a
momentary blip or oscillation in the trace.
The knock sensor is a sealed unit, so if defective it must be
replaced. No adjustment is possible