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Drive-by-Wire:

A throttle control system which relies on

electronic signals from the accelerator pedal position sen-
sor to direct the PCM or throttle actuator control module to
operate the throttle blade(s).

Duty Cycle:

The percentage of total time in one complete

on-off cycle during which an output device is energized.

Dwell or Dwell Angle:

Also called cam angle. The meas-

urement in degrees of how far the distributor cam rotates
while the primary circuit is closed.

Dynamometer:

A machine used to measure mechanical

power, such as the power of an engine.

EEPROM:

Electronically Eraseable Programmable Read

Only Memory - memory that does not erase when voltage
is removed. A scan tool is required to erase DTC’s stored in
EEPROM.

Electrolyte:

The chemical solution composed of sulfuric

acid and water, that conducts electricity and reacts with the
plate materials in a battery.

Electromagnet:

A soft iron core wrapped in a coil of a cur-

rent-carrying conductor. Current in the coil induces a mag-
netic field around the core.

Electrostatic Discharge (ESD):

The passage of a static

electrical charge from a component to ground. Generally
associated with the potential for damage to electronic com-
ponents.

Field Circuit:

The charging system circuit that delivers cur-

rent to energize the generator field.

Frequency:

The number of periodic voltage oscillations, or

waves, occurring in a given unit of time, usually expressed
as cycles per second or Hertz.

Graphing Multimeter:

A multimeter displays signals in

conventional digital format or in a value over time relation-
ship.

Gulp Valve:

A valve used in an air injection system to pre-

vent backfire. During deceleration it redirects air from the air
pump to the intake manifold to lean out the air-fuel mixture.

Hall Effect Sensor:

A signal-generating switch that devel-

ops a transverse voltage across a current-carrying semi-
conductor when subjected to a magnetic field.

Hard Code:

A diagnostic trouble code that comes back

within one cycle of the ignition key is a "hard" code. This
means that the defect is there every time the powertrain
control module checks that circuit or function.

Hertz (Hz):

A unit of frequency measurement equal to one

cycle per second.

High-Impedance:

A high level of opposition to current cre-

ated by the combined forces of resistance, capacitance,
and inductance in a circuit.

Hydrocarbon (HC):

A chemical compound made up of hy-

drogen and carbon, which is major pollutant given off by an
internal combustion engine. Gasoline itself is a hydrocar-
bon compound.

Ignition Control Module (ICM):

An electronic module de-

signed to control the primary circuit to the ignition coil.

Induction:

The creation of an electrical current as conduc-

tor moves through a magnetic field or when a magnetic field
moves across a conductor.

Infinite Resistance:

A condition caused by an incomplete

or open circuit that prevents current through the circuit.

Intermittent or Soft Code:

A diagnostic trouble code that

is not there every time the PCM checks the circuit is an “in-
termittent" code.

Magnetic Pickup:

A signal-generating device that uses a

permanent magnet to create a voltage pulse. The trigger
wheel movement creates magnetic flux changes in the
pickup coil to induce a varying-strength voltage.

Malfunction Indicator Lamp (MIL):

A lamp mounted on

the instrument cluster to alert the driver that an emission
system component is operating out of its specified range.

Manifold Absolute Pressure:

Combination of atmospher-

ic and manifold pressure or boost pressure

Manifold Pressure:

Vacuum, or low air pressure, in the in-

take manifold of a running engine, caused by the descend-
ing pistons creating empty space in the cylinders faster
than the entering air can fill it.

Ohmmeter:

A meter designed to measure the resistance of

a circuit or component. DMM’s can be set to an ohmmeter
function.