Hatch S.N. Computerized engine controls

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632 Glossary

the technician in diagnosing faults in the con-

trolled system. For example, a functional test

may allow the technician to energize certain

actuators as desired.

Gallium arsenate crystal—A semiconductor

material that changes its conductivity when

exposed to a magnetic field.

Generic Electronic Module (GEM)—A Ford

control module that controls body functions

such as interval wipers, the driver’s door one-

touch-down power window, the illuminated

entry feature, and other features. Similar to

the BCM (Body Control Module) on General

Motors and Chrysler vehicles except that it

does not control the climate control system.

On newer Ford products, the GEM was sepa-

rated into two control modules: the FEM and

the REM.

Generic OBD II Mode—A scan tool diagnostic

mode designed to allow an aftermarket ge-

neric scan tool access to nine generic diag-

nostic modes out of fifteen set aside by the

EPA under OBD II standards. Also known

as Global OBD II Mode by some scan tool

manufacturers.

Global OBD II Mode—The name that some scan

tool manufacturers give to Generic OBD II

Mode.

Grade Logic Control—A control module strat-

egy used by Honda, designed to reduce the

driver’s need to constantly compensate at

the throttle pedal for transmission upshifts

and downshifts when driving in mountainous

terrain.

Grams per mile (GPM)—A measurement of

exhaust gasses using a constant volume

sampling (CVS) method in conjunction with a

dynamometer.

Grid—A fine wire mesh that is placed between

the anode and cathode segments of a vac-

uum fluorescent display (VFD).

H-gate—Two pairs of transistors used to control

current flow direction.

Hall effect switch—A magnetic switching device

often used to signal crankshaft position and

speed to the computer.

Hard fault—A fault that currently exists.

Hard wiring—Wiring designed to carry only

one type of signal 100 percent of the time,

unlike a data bus that can transmit multiple

messages. For example, a throttle position

sensor (TPS) signal wire is traditionally used

to communicate information about throttle

position to the PCM 100 percent of the time.

Hexadecimal—A system of assigning single dig-

its to represent the total decimal value of four

bits of binary code. The values 0, 1, 2, 3, 4,

5, 6, 7, 8, 9, A, B, C, D, E, and F are used,

thus creating a “base-16” numbering system.

Hexadecimal values are often represented

using a dollar sign ($) ahead of the value.

Also referred to as a “hex” value.

High-pressure cutout switch—A pressure-

operated switch in the high-pressure side of the

air-conditioning system that opens when pres-

sure climbs too high. When the switch opens, it

disables the A/C compressor clutch circuit.

High-voltage (HV) battery—An onboard battery

made up of many cells to create a voltage

potential of several hundred volts. Used on

gasoline/electric hybrid-drive vehicles.

Home energy station (HES)—A device that uses

tap water with electricity from a wall outlet to

create the hydrogen needed to refill a fuel cell

vehicle’s hydrogen tanks.

Hot-film air mass sensor—A type of air mass

sensor using changes in the temperature of

the heated element to determine the mass of

the air passing into the engine.

Hot-wire air flow sensor—An air flow sensor

that keeps a special wire at a fixed number

of degrees above ambient temperature in the

intake airflow. The amount of current required

to maintain the heat is converted into a volt-

age or frequency signal that conveys infor-

mation about the mass of the intake air.

Hydrocarbons (HC)—A vehicle emission con-

sisting of unburned gasoline or diesel fuel.

The engine may produce excessive HCs due

to a total misfire or rich air/fuel conditions.

HCs may also be released into the atmo-

sphere because of fuel evaporation.

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Hz (Hertz)—Cycles per second; a measure of

frequency.

Idle tracking switch—An on/off switch indicat-

ing to the computer that it is in charge of the

engine for idle speed.

Impedance—The total circuit resistance, includ-

ing resistance and reactance.

Inductive pickup/Hall effect pickup—Two

types of rotation sensors are used on many

vehicles: the inductive pickup and the Hall ef-

fect. Inductive pickups consist of a wire coil

wound around a permanent magnet. When

a metal tooth passes the tip of the magnet, a

small, polarity-reversing alternating current is

generated (this current can grow much larger

with higher speed). The Hall effect pickup re-

ceives a reference voltage from the computer

and converts it into an on/off signal. Hall ef-

fect sensors do not vary the signal voltage

output with speed.

Inertia switch—A Ford trademark safety feature

and an occasional no-start puzzle. To protect

against a gasoline fire following an accident,

the inertia switch disables the fuel pump cir-

cuit upon impact.

Initialization mode—The beginning state of an

engine control computer when the ignition

switch has been turned to on but the engine

has not yet started. The computer performs a

variety of internal tests and sensor/actuator

checks.

Intake Manifold Runner Control (IMRC)—An

intake air plenum design used on Ford and

Mazda engines that can vary the effective

length of the intake. When the engine is op-

erating at low speed and under light load, it

provides a longer effective length to keep the

velocity of the air flow high so as to provide

crisp throttle response. When engine speed

and load are increased, it provides a shorter

effective length to allow for additional air flow

to meet the greater demand.

Integrated circuit (IC)—A circuit, typically placed

on a semiconductor chip, that is minuscule in

size and consists of semiconductor compo-

nents such as diodes and transistors.

Integrator—A term that refers to the General

Motors name for short-term fuel trim (STFT)

prior to OBD II standardization, referring to the

PCM’s immediate reaction to the last voltage

signal communicated from the oxygen sensor.

Intelligent Architecture—A system that includes

a combination voice recognition and naviga-

tional system, as well as the ability to person-

alize driver settings or download information

or music. The system also has several other

features, including the ability to communicate

with electronic devices using Bluetooth

wireless technology. Also included is real-

time navigation, a feature that can alert the

driver to an accident ahead or even remind

him/her of a vehicle service that is due.

Another feature, called “MyHome,” gives the

driver the ability to communicate with Web-

based equipment at home or elsewhere.

Interface—Circuitry that converts both input and

output information to analog or digital signals

as needed and filters external circuit voltage

to protect computer circuits.

Intrusive test—A self-test initiated by the PCM

to test a non-continuous monitor. During this

test the PCM energizes output actuators, not

because engine/vehicle operating conditions

warrant it, but only for the purpose of per-

forming the test.

KAM—Keep-alive memory, an electronic memory

in the computer that must have a constant

source of voltage to continue to exist.

Kilobyte—1024 bytes of information in binary

code.

Lambda—A Greek letter (

) used to indicate how

far the actual air/fuel mixture deviates from the

ideal air/fuel mixture. Lambda equals actual

inducted air quantity divided by the theoretical

air required (14.7). The actual ideal mixture

depends on the characteristics of the fuel and

air drawn in through the intake system.

Lift pump—An electric fuel pump in or near the

fuel tank that pressurizes the fuel slightly

(about 15 psi) and delivers it to the main fuel

pump. Found on both gasoline and diesel en-

gines. Sometimes called a primer pump.

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634 Glossary

Light-emitting diode (LED)—A semiconductor

device that generates a small amount of light

when forward biased (when voltage is ap-

plied with the polarity such that current flows

across the PN junction).

Limp-in mode—A state of a computer engine

management system in which one or more

major component circuits have failed and a

substitute value is used to retain driveability

until the vehicle can be repaired.

Linear air/fuel (LAF) sensor—A wideband oxy-

gen sensor designed to accurately measure

leaner air/fuel ratios than can be measured

with a standard oxygen sensor. The use of an

LAF sensor allows the PCM to run the engine

quite lean under certain conditions.

Logic gates—Combinations of transistors within

the computer that determine the computer’s

output reaction to a specific combination of

inputs values.

Logic module and power module—Early to

mid-1980s Chrysler products separated the

computer functions into two components: the

power module, which carried the higher cur-

rent and worked the actuators, and the logic

module, usually in a more protected place,

usually more electrically protected. The logic

module assesses sensor inputs and instructs

the power module when to inject fuel, fire

plugs, and work actuators.

Logic probe—A computer-safe test light using

LEDs to indicate if a signal is present. Logic

probes are typically used to provide quick checks

of primary ignition and fuel injector circuits.

Long-term fuel trim (LTFT)—A long-term

adjustment made within the PCM so that

the performance of the short-term fuel trim

(STFT) will cause the oxygen sensor signal

to average between 400 and 500 mV. LTFT is

designed to modify the base fuel calculation

to compensate for wear and aging of the

engine and other components that affect the

air/fuel ratio. Originally known on General

Motors’ products as “block learn.”

Longitudinal—Parallel to the length of the car

(opposed to transverse).

Loop configuration—A configuration of a multi-

plexing network that connects the data bus to

all nodes on the bus in a series circuit.

Malfunction indicator light (MIL)—A lamp on

the instrument panel used to alert the driver

when the PCM identifies a problem with the

engine control system. Known commonly as

a check engine light prior to OBD II standard-

ization.

Manifold absolute pressure (MAP)/intake

manifold vacuum—Manifold absolute pres-

sure (MAP) is the actual pressure in the in-

take manifold, regardless of the difference

between it and ambient air pressure. Intake

manifold vacuum, the traditional concept,

refers to the relative difference in pressure

between the two. Intake manifold vacuum is

always a relative measure. MAP measure-

ments are not relative.

Master—The computer that controls a data bus

network.

Megabyte—1,048,576 (1024 squared) bytes of

information in binary code.

Memory—A computer’s information storage ca-

pability.

Memory DTC—A diagnostic trouble code (DTC)

that has been set in a computer’s memory at

some point in the past. This type of DTC may

represent either a hard fault or a soft fault.

Micron—A unit of measurement that describes

the size of a particle one-millionth of an inch

in diameter.

Microprocessor—A processor contained on an

integrated circuit; a processor (central pro-

cessing unit) that makes the arithmetic and

logic decisions in a microcomputer.

Misfire detection—Part of the required OBD II

program, misfire detection closely monitors

engine RPM for the slight variations when

combustion misfire occurs. In most cases,

the computer disables the injector for a misfir-

ing cylinder (to protect the catalytic converter

from overheating) and sets an appropriate

DTC.

Modulate—To find a position between two ex-

tremes; to alternate between on and off.

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Modulated displacement—A system in which

a series of PCM-controlled electromechani-

cal devices can selectively disable the rocker

arms of particular cylinders, preventing the

valves from opening. The engine then oper-

ates on eight, six, or four cylinders, depend-

ing on power requirements.

Molecule—A small chemical building block that

consists of two or more atoms. It may be an

element or a compound.

Monitor—An onboard diagnostic test that an

OBD II PCM performs to determine if a com-

ponent or system is within design specifica-

tions.

Motor/generator (MG)—A component that can

turn electrical energy into mechanical energy

(as a motor does) and can also turn mechani-

cal energy into electrical energy (as a gen-

erator does). Also sometimes referred to as

an electric motor/generator (EMG).

Motor start-up in-rush current—The initial high

current that an electrical motor draws when

its circuit is first energized.

Motronic—The latest of the Bosch engine man-

agement systems, Motronic is Bosch’s first

full engine control system, regulating fuel in-

jection and ignition timing and dwell in one

unit.

Multiplexing (MUX)—One of the most significant

extensions of computer control technology in

vehicles, multiplexing connects many or all

the control units in a vehicle over a single-

wire network: the data bus. Information from

all components is sent over the data bus and

received by all.

Multiplexor—An electronic chip that, when built

into a node, gives the node the ability to send

messages on a data bus.

Negative ion—An atom that has an extra elec-

tron, thus having more electrons than it has

protons. It therefore has a negative electrical

charge.

Nitrogen (N

)—A gas that makes up approximately

78 percent of Earth’s atmosphere. Ideally, the

nitrogen that enters the engine’s combustion

chambers will go through unchanged.

Node—A computer that is connected to a data

bus and has the ability to send and/or receive

messages on the bus.

Noise-vibration-harshness (NVH)—A special

acoustic diagnosis tool that can identify the

frequencies of various noises and vibrations

to compare with known frequencies. Useful

to determine whether a noise or vibration

is engine-speed-relative, drivetrain-speed-

relative, wheel-speed-relative, or random.

Non-continuous monitor—An OBD II monitor

that is not continually monitored by the PCM,

but rather is tested by the PCM performing an

intrusive test of the component or system. A

non-continuous monitor is only run when spe-

cific preprogrammed enable criteria are met.

Non-powered test light—A tool that can be

used to quickly check for a voltage potential

(differential). Voltage potential should not be

judged based on the brightness of the light.

Unfortunately, due to its low internal resis-

tance, a test light can act as a jumper wire

and unexpectedly damage a computer or de-

ploy an air bag.

Normally aspirated—An engine that uses am-

bient atmospheric pressure to force air into

the engine (as opposed to turbocharged and

supercharged).

OBD II—A standardized computer control self-

diagnostic program, mandated for most

vehicles as of the 1996 model year. Nomencla-

ture, trouble codes, and even the size, shape,

and many circuits of the diagnostic connector

are standardized across the industry.

Ohm—A unit of measurement used to measure

electrical resistance.

Ohm’s law—An electrical law that states that it

takes 1 V of electrical pressure to push one

amp of current flow through 1 ⍀ of resis-

tance.

On-demand DTC—A diagnostic trouble code

(DTC) that results from a self-test (perfor-

mance test), identifying any hard faults that

were found within the computer system.

Open loop—Any operational mode in which a

computer controls an output circuit but does

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636 Glossary

not use a feedback signal from a sensor or

circuit to monitor the results of its own control.

However, the computer may monitor other

input signals/sensors during open-loop con-

trol that do not pertain to a feedback signal.

In the PCM’s open-loop control of the air/fuel

ratio, the air/fuel mixture is calculated based

on coolant temperature, engine speed, and

engine load without benefit of the oxygen

sensor. See also Closed loop.

Oxides of nitrogen (NO

)—A vehicle emission

that is a combination of nitrogen (N

) and

oxygen (O

), which are combined within

the engine’s combustion chamber or in the

exhaust system due to temperatures that

exceed 2500°F.

Oxygen (O

)—A gas that makes up approxi-

mately 21 percent of Earth’s atmosphere.

Most of the oxygen that enters the engine’s

combustion chambers will be used up during

combustion.

Palladium (Pd)—A silver-white, ductile, metallic

element; number 46 on the periodic table.

Parallel hybrid—A vehicle that can be acceler-

ated using either an electric motor/generator

or a gasoline engine or both.

Parameter ID (PID)—A specific data stream

parameter relating to a sensor value, an

actuator command, or some other piece of

information that a PCM or other control mod-

ule may communicate to a scan tool through

the data stream. The scan tool will show the

PID as well as the related information.

Passive voltage—A voltage that is at rest, not

being pulled either high or low by a computer

circuit.

Permeability—A material that strongly demon-

strates the characteristics associated with

magnetism, such as iron, is said to have high

permeability.

Pickup coil—A small coil of wire in which the sig-

nal of a pulse magnetic generator is produced,

such as in many electronic distributors.

Piezoelectric—A crystal that produces a voltage

signal when subjected to physical stress such

as pressure or vibration.

Piezoresistive—A semiconductor material

whose electrical resistance changes in re-

sponse to changes in physical stress.

Pinpoint test—A test that checks a computer

circuit for the exact fault, whether it be a sen-

sor/actuator, an open or short in the circuitry,

a poor or open electrical connection, or the

computer itself. Pinpoint tests may be per-

formed either by following a flowchart or by

following an electrical schematic. Following

a pinpoint test series, the technician should

know exactly what repair needs to be made

to restore the system to its “new” condition.

PIP signal—The profile ignition pickup signal

(PIP) comes from a crankshaft- or distributor-

mounted sensor indicating cylinder position

to the computer.

Platinum (Pt)—A heavy, grayish white, ductile,

metallic element; number 78 on the periodic

table.

Polymer Electrolyte Membrane—A type of fuel

cell that uses a solid polymer membrane (a

thin plastic film) as an electrolyte. PEM fuel

cells combine hydrogen (H

) with oxygen (O

)

from atmospheric air to create electrical cur-

rent flow. The only emissions resulting from

fuel cell operation are water (H

O) and heat.

A PEM fuel cell is also commonly referred to

as a Proton Exchange Membrane fuel cell.

Positive ion—An atom that has a deficiency of

one electron, thus having fewer electrons

than it has protons. It therefore has a positive

electrical charge.

Potentiometer—A three-terminal variable resis-

tor that acts as a voltage divider. Frequently

used as a position sensor for throttle angle

and so on. The potentiometer can be linear

or rotary.

Potentiometer sweep test—The testing of a po-

tentiometer throughout its range of operation

to determine whether any electrical opens

exist.

Powertrain control module (PCM)—The term

and acronym now used (since OBD II) for the

primary computer in charge of engine and

transmission performance.

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Pressure cycling switch—A pressure- operated

switch in the low-pressure side of the air-

conditioning system that opens as Freon

pressure drops below a specific pressure.

When the switch opens, the A/C compressor

clutch is disabled. The purpose of this switch

is to protect the compressor against opera-

tion when the system has leaked empty.

Primary and secondary oxygen sensors—The

oxygen sensors are normal types, heated for

rapid warm-up and closed-loop acquisition.

What is new is the use of tandem sensors

(also called primary and secondary oxygen

sensors), one before and one after the cata-

lytic converter. This combination serves as a

test of the converter’s effectiveness. So long

as the downstream, secondary sensor shows

a relatively flat signal, whatever its output

voltage, the catalyst is working. When the

secondary oxygen sensor starts the charac-

teristic mixture dithering like the primary one

(still used for fuel mixture trim), the catalyst

has become inefficient.

PROM—Programmable read-only memory.

Propagation—As used in automotive technol-

ogy, the spread of the flame front across the

combustion chamber of an engine.

Protocol—A standardized binary code that con-

stitutes the language with which computers

are able to communicate with each other

over a data bus.

Proton Exchange Membrane (PEM)—A type of

fuel cell that uses a solid polymer membrane

(a thin plastic film) as an electrolyte. PEM

fuel cells combine hydrogen (H

) with oxygen

) from atmospheric air to create electrical

current flow. The only emissions resulting from

fuel cell operation are water (H

O) and heat.

The technical name for this type of fuel cell is

a Polymer Electrolyte Membrane fuel cell.

Pulse width and duty cycle—Both pulse width

and duty cycle are measurements of a sole-

noid’s (or signal’s) on-time—pulse width as a

real-time measurement and duty cycle as a

percentage of the total cycle. If the frequency

of the cycle is known (as measured with a

Hertz meter), dividing the frequency into

1000 ms (1 second) determines the length

of one cycle. If this amount is then multiplied

by the percentage of duty cycle on-time, the

result is the pulse width as a real-time mea-

surement in milliseconds.

Pulse-width modulated (PWM)—A signal

that is controlled according to a real-time

measurement, usually in milliseconds or

microseconds. This type of signal may be

used to control fuel injectors, cooling fans, and

blower motors. When a serial data bus uses a

fixed pulse width, typical with two-wire buses,

the binary code is said to be a PWM signal.

Pulsed injection—One of the two kinds of fuel

injector fuel-spray techniques, the other being

continuous injection. In a pulsed injection

system, the injectors are pulsed, in some way,

according to the tach reference pulse of the

ignition system. Therefore, as engine RPM

increases, the frequency of the injector pulse

also increases. TBI injectors may be pulsed

with every tach reference pulse. PFI systems

may pulse their injectors simultaneously (all

at the same time), in groups (half at a time),

in pairs, or sequentially in the engine’s firing

order just before the intake valve opens.

Pumping losses—The load on the engine caused

by the work needed to pump air through the

system. On gasoline engines, because of

the vacuum developed behind the throttle,

pumping losses are greatest at the most

closed-throttle positions: idle and deceleration.

The aerodynamic friction of air through the

intake system also contributes to a lesser

extent to the pumping losses, though that

becomes the principal source at high speeds

and open throttle. Diesel engines, because

they are unthrottled, have very low pumping

losses compared to gasoline engines.

Quad driver/output driver—A power transistor

in the PCM, capable of working either four or

seven different actuators (such as injectors

or ignition coils). Each quad driver or output

driver works when the PCM uses it to ground

a component’s circuit, completing it.

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638 Glossary

Quick test—The whole set of procedures that

make up the Ford diagnostic procedure.

RAM—Random-access memory.

Rationality testing—The diagnostic testing per-

formed by a computer using its ability to com-

pare input values to each other, or to operate

an actuator while watching the change in a

related input circuit. Also known as efficiency

monitoring.

Rear Electronic Module (REM)—A Ford control

module that controls the exterior lighting at

the rear of the vehicle, as well as the deck lid

release solenoid.

Reference voltage regulator—The device within

a computer that generates the 5-V reference

voltage used by the computer’s internal cir-

cuits, including the logic circuits. Its signal,

sometimes referred to as VREF, is also sent

to those sensors that require it.

REF pulse—The basic timing signal sent to the

computer, indicating crankshaft position. This

signal is modified by the computer to set the

actual timing.

Regenerative braking—The process of return-

ing a moving vehicle’s kinetic energy to an

onboard battery when coasting or braking.

This energy is then reused to get the vehicle

moving again when reaccelerating, thus in-

creasing fuel mileage. Brake life is also ex-

tended.

Reluctance—A material that does not demon-

strate the characteristics associated with

magnetism is said to have reluctance.

Resistance—Opposition to the flow of electrons;

measured in ohms.

Rhodium (Rh)—A white, hard, ductile, metallic

element; a member of the noble metals fam-

ily; number 45 on the periodic table.

ROM—Read-only memory.

Scan tool or scanner—A diagnostic tool used to

retrieve from a vehicle’s computer memory

any malfunction codes it has retained. On

earlier systems, each manufacturer used a

scan tool connector and setup unique to that

manufacturer (though there are aftermarket

scan tools with adapters that can read many

of them). With OBD II–compliant vehicles, the

scan tool is standardized and can be used

for the same purpose on any vehicle. Many

scan tools can also be used to get direct

measurements of various sensors and to

exercise various actuators to check on their

effectiveness; also known as functional tests.

Schmitt trigger—A device that trims an analog

signal and converts it to a digital signal.

Self-test—The self-diagnostic portion of the Ford

quick-test in which the computer tests itself

and its circuits for faults.

Semiconductors—Solid-state devices, such as

diodes and transistors, that are formed from

semiconductor materials that have four elec-

trons in the valence ring. Depending upon

design and applied polarity, they may either

conduct electrical current flow or insulate

against electrical current flow.

Sensor—All the inputs and outputs to the computer

are either sensors that provide information or

actuators that do work. Most sensors are sent

a reference voltage that is reduced in a predict-

able way by whatever parameter they monitor.

Some generate voltages of their own.

Serial data—Digitized code known as binary

code, which is a combination of high and low

voltages known as ones and zeroes, used as

a language to communicate between com-

puters or computer components.

Series hybrid—A vehicle that uses a gasoline

engine to power a generator (or motor/gen-

erator) to provide additional electrical energy

to the primary motor/generator to accelerate

the vehicle.

Short-term fuel trim (STFT)—The PCM’s im-

mediate response to the oxygen sensor

signal. When the oxygen sensor indicates

a lean mixture, STFT increases its value to

increase injector on-time. This causes more

fuel to be added to the base fuel calculation

and enriches the air/fuel mixture. When the

oxygen sensor indicates a rich mixture, STFT

decreases its value to decrease injector on-

time. This causes fuel to be subtracted from

the base fuel calculation and leans out the

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Glossary 639

air/fuel mixture. Originally known on General

Motors products as the “Integrator.”

Signal (or voltage signal)—The voltage val-

ues that a computer uses as communication

signals.

Single-point/multipoint fuel injection—Single-

point injection refers to fuel injection using the

equivalent of an electric carburetor. On four-

cylinder engines, this usually involves a single

injector, but on some V-form engines two in-

jectors are combined in one unit. Also called

throttle-body fuel injection. Multipoint fuel in-

jection employs one injector for each cylinder.

Slave—A dependent node on a data bus.

Snapshot—A computer recording of information

from sensors and actuators.

Soft fault—An intermittent fault whose symptom

does not appear to currently exist.

Solenoid—A device with a movable iron core

inside of an electromagnetic winding. The

movable core is usually spring-loaded and

will move against spring tension when the

electromagnet is energized. It creates linear

mechanical action and may be used to lock or

unlock a door or engage a starter drive with

a flywheel. It is often used to operate a valve

to control the flow of air, vacuum, fuel vapors,

or liquid, and may be spring loaded normally

closed or normally open.

Speed density formula—A term referring to

a method of calculating the flow rate and

density of the intake air. The result of the

calculation indicates the air’s ability to evap-

orate fuel.

SPOUT—The Ford Spark Output (SPOUT) signal

comes from the PCM and indicates to the igni-

tion module when to fire the next spark plug.

The exact timing is calculated on the basis of

inputs from all the computer’s sensors.

Star configuration—A configuration of a multi-

plexing network that connects the data bus to

all nodes on the bus in a parallel circuit.

Star tester—A tester designed to test MCU and

EEC IV systems; it activates and deactivates

the self-test procedure and displays a digital

readout of the service codes.

Stepper motor—A type of direct-current electric

motor with the unique capacity to move

an exact number of turns in response to a

computer’s signal. Typically, such a motor

(used to adjust the idle speed bypass opening,

for example) has a range of 256 different

possible positions it can be set to. This allows

the computer to open or close to an exact

position. Such motors are also increasingly

used in some heater door control systems.

Stoichiometric—In automotive terminology,

refers to an air/fuel ratio in which all combus-

tible materials are used, with no deficiencies

or excesses; 14.7 parts air to 1 part fuel, by

weight.

Technical Service Bulletin (TSB)—A repair pro-

cedure issued by the manufacturer that ad-

dresses a particular problem and instructs

the technician on how to perform a proper

repair so as to correct the symptom.

Test ID (TID)—An identification code, usually

given as a hexadecimal value, that identifies a

test relating to a component in a computerized

engine control system. TIDs and their related

information can be displayed using a scan

tool. The related specifications may be looked

up, many times at a manufacturer’s Web site.

Thermac—The term used by General Motors to

identify the heated air inlet system. The term is

a shortened form of thermostatic air cleaner.

Thermistor—A temperature-sensing device

whose resistance changes dramatically as

the temperature changes. Most thermistors

have a negative temperature coefficient (the

resistance goes up as the temperature goes

down).

Thermo-time switch—An electrical switch usu-

ally applied to a bimetallic strip that uses heat

to open or close a set of electrical contacts.

The amount of heat is controlled, so the

time it takes to open or close the contacts is

always controlled.

Thick film integrated (TFI)—A term that identi-

fies the type of chip used in the ignition mod-

ule attached to the side of the distributor for

EEC IV system applications.

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640 Glossary

Throttle-body injection (TBI)—Throttle-body in-

jection uses a single fuel injector in a throttle

body (or a pair of two for V-form engines) that

delivers the fuel to the intake manifold.

Throttle Plate Position Controller (TPPC)—An

IC chip embedded within a Ford PCM, having

the task of controlling the electronic throttle

control (ETC) system.

Torque converter lockup clutch—A computer-

controlled, hydraulically actuated clutch in the

automatic transmission’s torque converter

that locks the transmission’s input shaft to

the engine’s crankshaft for fuel economy. The

computer determines when to send this com-

mand on the basis of sensor inputs.

Torque management—A set of capacities the

engine control system has, allowing it to re-

duce engine torque output under specific

circumstances. Its first strategy is to retard

spark; in more extreme circumstances it can

selectively shut off fuel injectors. The traction

control system employs part of the torque

management system to prevent front-wheel

spin under power when the pavement is slip-

pery.

Transistor—A semiconductor that consists of

three semiconductor elements placed back-

to-back. A transistor operates as an electronic

switch with no moving parts and may be ei-

ther PNP or NPN design.

Trigger—The characteristics that determine the

instant in time that a DSO begins to display a

waveform. These characteristics include trig-

ger source, trigger level, trigger slope, and

trigger delay (horizontal waveform position).

Trip—A key cycle consisting of Ignition On, En-

gine Run, Ignition Off, and PCM Power Down,

during which the enable criteria for a particu-

lar diagnostic test are met and the diagnostic

test is run by the PCM. A trip is used by the

PCM to perform a diagnostic test to confirm a

symptom or its repair.

Two-trip malfunction detection—On some ve-

hicles, the computer waits to record certain

faults until they appear on two separate driv-

ing trips.

Vacuum fluorescent (VF)—A glass tube con-

taining anodes and a cathode. The anodes

are coated with a fluorescent material and

are positioned so they form all of the seg-

ments of alphanumerical characters. As cur-

rent passes through the cathode, it gives off

electrons. If the anodes have positive voltage

applied, the electrons given off by the cath-

ode are attracted to them and strike the fluo-

rescent material, causing it to glow.

Valance ring—The outermost electron shell of

an atom.

Variable cam timing (VCT)—A system that uses

a cam phaser on the front end of the cam-

shaft to change the camshaft’s position rela-

tive to the crankshaft.

Variable Cylinder Management (VCM)—Hon-

da’s name for a variable displacement sys-

tem to improve fuel economy, in which the

PCM disables cylinders when engine load

is light. The selected cylinders are disabled

by turning off the fuel injector and then dis-

abling the ability of the valves to open using a

Honda VTEC-style rocker arm design, reduc-

ing pumping losses by allowing the cylinder

to act as an air spring.

Variable pulse width (VPW)—The term as-

signed to serial data that consists of bits of

information that are of more than one length.

In this case, a “one” may be represented

either as a short high-voltage pulse or as a

long low-voltage pulse. Conversely, a num-

ber “zero” may be represented as a short

low-voltage pulse or as a long high-voltage

pulse. This allows the voltage to switch be-

tween high- and low-voltage values with

every bit of information.

Variable valve timing (VVT)—The ability of a

computer to vary valve timing using an actua-

tor designed for that purpose to get best per-

formance under high load conditions while still

getting good engine response at low rpm.

Vehicle Control Module (VCM)—A General Mo-

tors PCM that, in addition to controlling the en-

gine, transmission, and cruise control systems,

now also controls the antilock brake system.

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Glossary 641

VIN entry mode—A diagnostic mode on a scan

tool that allows the scan tool to recognize the

particular make, model, and year of vehicle as

well as other attributes such as engine identi-

fication and computer protocols using the the

vehicle identification number (VIN), which is

entered into the scan tool. This mode gener-

ally allows the technician to access the most

diagnostic information using a scan tool. Not

all aftermarket scan tools support this mode.

Volt—A unit of measurement of electrical pres-

sure or electromotive force.

Voltage or voltage potential—A measurement

of electrical pressure or electromotive force.

Voltage drop—The voltage that is used up as

current flows through resistance in an electri-

cal circuit.

Voltage drop test—A test that is a more precise

substitute for resistance tests done with an

ohmmeter. A voltage drop test checks the pos-

itive side or negative side of the circuit to mea-

sure precisely how much voltage is used up

by the resistance in that portion of the circuit.

It must be performed while current is flowing

in a completed circuit. Excessive voltage drop

indicates excessive unwanted resistance.

Volumetric efficiency (VE)—A measure of

cylinder-filling efficiency. VE is expressed as

the percentage of atmospheric pressure to

which the cylinder is filled at the completion

of the compression stroke. This efficiency

varies with engine speed; the point of highest

volumetric efficiency (which can rise above

100 percent in some cases) is the point of

highest engine output torque.

Wastegate—A valve or door that when opened

allows the exhaust gas to bypass the exhaust

turbine of a turbocharger. The wastegate is

used to limit turbocharger boost pressure.

Waste spark—An ignition system using one coil

between every complementary pair of cylin-

ders, firing both plugs simultaneously, one at

the end of the compression stroke and the

other at the end of the exhaust stroke.

Water (H

O)—One of the by-products of com-

bustion of a hydrocarbon-based fuel within

the engine.

Wattage—A measurement of total electrical

power. Volts times amps equals watts (known

as Watt’s Law).

WOT—Wide-open throttle.

Zirconium dioxide (ZrO

)—A white crystalline

compound that becomes oxygen-ion conduc-

tive at about 600°F (315°C).

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