Oscillations:
Steady, uninterrupted up-and-down swings,
or waveforms.
Oscilloscope:
An oscilloscope is an instrument commonly
used to display and analyze the waveform of electronic sig-
nals. In effect, the device draws a graph of the instanta-
neous signal voltage as a function of time. Also known as a
lab scope.
Oxides of Nitrogen (NOx):
Various combinations of nitro-
gen and oxygen that form when exposed to high cylinder
temperatures. NOx is responsible for the formation of pho-
tochemical smog.
Oxidize:
To combine an element with oxygen, or convert it
into its oxide. For example, when carbon burns, it combines
with oxygen to form carbon dioxide or carbon monoxide.
Parade Pattern:
Oscilloscope display that shows voltage
traces for all cylinders displayed one after another in firing
order from left to right across the screen.
Potentiometer:
A variable resistor with three terminals. Re-
turn signal voltage is taken from a terminal attached to a
movable contact that passes over the resistor.
Powertrain Control Module (PCM):
The PCM is an on-
board computer programmed to control the operation of
the engine and many transmission functions.
Preignition:
A premature ignition of the air-fuel mixture be-
fore the spark plug fires. It is caused by excessive heat or
pressure in the combustion chamber.
Pull-Up Resistor:
A resistor installed in the PCM in series
with the sensor thermistor to protect the electronic compo-
nents from a short to ground.
Pulse Width Modulation (PWM):
A signal with a variable
on-time to off-time ratio. Usually used to control solenoid-
type actuators
.
Raster Pattern:
Oscilloscope display that shows voltage
traces for all cylinders stacked top to bottom or bottom to
top on the screen in firing order sequence.
Residual Pressure:
A constant pressure held in the fuel
system when the pump is not operating.
Resistor:
An electronic component that impedes current
flow. Resistors are used to lower the voltage applied to an-
other device.
Scan Tool:
A test instrument that is used to access power-
train control system trouble codes, freeze frame data, and
bi-directional control of system actuators.
Schrader Valve:
A service valve that uses a spring-loaded
pin and internal pressure to seal a port.
Sensor:
A device that provides an electric signal to a com-
puter to indicate a certain physical condition. In an automo-
tive electronic system, sensors monitor such conditions as
vehicle speed, throttle opening, or steering angle.
Serial Data Stream:
The electronic data transmitted by an
onboard computer to a scan tool frame by frame.
Short Term Memory
: Values stored in the PCM pertaining
to the air-fuel ratio at specific engine RPM and load condi-
tions. Short term memory is constantly updated during en-
gine operation.
Signal Voltage:
The variable input voltage a sensor trans-
mits to the PCM. Signal voltage equals reference voltage
minus the voltage dropped to ground by the sensor.
Solenoid:
An electromagnetic actuator consisting of a
movable iron core with an induction coil surrounding it.
When electrical current is applied to the coil, the core
moves to convert electrical energy to mechanical energy.
Spark Tester:
A special tool with an adjustable spark gap
used to check spark intensity.
Specific Gravity:
The weight of a liquid relative to water,
which has an assigned value of 1.0. In an automotive stor-
age battery, the specific gravity is measured with a hydrom-
eter. When corrected to a temperature of 80°F (27°C), the
specific gravity of a fully charged battery falls in the 1.260
to 1.280 range.
Stepper Motor:
A direct current motor whose armature
moves in small increments, or steps, as different coils are
energized in sequence. As the armature moves, a threaded
pintel moves in and out along with it.
Stoichiometric:
The air-fuel ratio of approximately 14.7:1
that provides the most complete combustion and combina-
tion of oxygen and hydrocarbon molecules.
Superimposed Pattern:
Oscilloscope display that shows
voltage traces for all cylinders one on top of, or superim-
posed upon, another to form a single pattern.
System Pressure:
Fuel injection operating pressure creat-
ed by the fuel pump.
Thermistor:
An electronic component whose resistance to
electric current changes rapidly and predictably as its tem-
perature changes.
Thermostatic Bulb:
A device that automatically responds
to changes in temperature to actuate a damper in the air in-
take.
Thermostatic Vacuum Switch:
A device that automatical-
ly responds to changes in temperature to operate a switch
that opens or closes a vacuum port.
Glossary
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