Generac Power Systems 4451 & 4986 Operating instructions

www.guardiangenerators.com
PORTABLES
PO BOX 297 • WHITEWATER, WI 53190
www.guardiangenerators.com
P/N OF7713 REV. A Printed in the USA 6.06
MODELS:
4451 & 4986 (12,500 Watt)
4582 & 4987 (15,000 Watt)
4583 (17,500 Watt)
5209 (15,000 Watt)
5308 (17,500 Watt)
ULTRA SOURCE PORTABLE GENERATOR
DIAGNOSTIC
REPAIR MANUAL
0F7713 Cover-rev.qxd 7/5/2006 4:18 PM Page 1

SAFETY
Throughout this publication, "DANGER!" and "CAUTION!" blocks are used to alert the mechanic to special
instructions concerning a particular service or operation that might be hazardous if performed incorrectly or
carelessly. PAY CLOSE ATTENTION TO THEM.
*
DANGER! UNDER THIS HEADING WILL BE FOUND SPECIAL INSTRUCTIONS WHICH, IF NOT COMPLIED
WITH, COULD RESULT IN PERSONAL INJURY OR DEATH.
*
CAUTION! Under this heading will be found special instructions which, if not complied with, could result
in damage to equipment and/or property.
These "Safety Alerts" alone cannot eliminate the hazards that they signal. Strict compliance with these spe-
cial instructions plus "common sense" are major accident prevention measures.
NOTICE TO USERS OF THIS MANUAL
This SERVICE MANUAL has been written and published by Generac to aid our dealers' mechanics and com-
pany service personnel when servicing the products described herein.
It is assumed that these personnel are familiar with the servicing procedures for these products, or like or
similar products manufactured and marketed by Generac. That they have been trained in the recommended
servicing procedures for these products, including the use of common hand tools and any special Generac
tools or tools from other suppliers.
Generac could not possibly know of and advise the service trade of all conceivable procedures by which a
service might be performed and of the possible hazards and/or results of each method. We have not under-
taken any such wide evaluation. Therefore, anyone who uses a procedure or tool not recommended by
Generac must first satisfy themselves that neither his nor the products safety will be endangered by the ser-
vice procedure selected.
All information, illustrations and specifications in this manual are based on the latest product information
available at the time of publication.
When working on these products, remember that the electrical system and engine ignition system are capa-
ble of violent and damaging short circuits or severe electrical shocks. If you intend to perform work where
electrical terminals could be grounded or touched, the battery cables should be disconnected at the battery.
Any time the intake or exhaust openings of the engine are exposed during service, they should be covered to
prevent accidental entry of foreign material. Entry of such materials will result in extensive damage when the
engine Is started.
During any maintenance procedure, replacement fasteners must have the same measurements and
strength as the fasteners that were removed. Metric bolts and nuts have numbers that indicate their strength.
Customary bolts use radial lines to indicate strength while most customary nuts do not have strength mark-
ings. Mismatched or incorrect fasteners can cause damage, malfunction and possible injury.
REPLACEMENT PARTS
Components on Generac recreational vehicle generators are designed and manufactured to comply with
Recreational Vehicle Industry Association (RVIA) Rules and Regulations to minimize the risk of fire or explo-
sion. The use of replacement parts that are not in compliance with such Rules and Regulations could result
in a fire or explosion hazard. When servicing this equipment, it is extremely important that all components be
properly installed and tightened. If improperly installed and tightened, sparks could ignite fuel vapors from fuel
system leaks.

Table of Contents
Page 1
SAFETY .................. INSIDE FRONT COVER (IFC)
NOTICE TO USERS OF THIS MANUAL ..............................IFC
REPLACEMENT PARTS .....................................................IFC
TABLE OF CONTENTS ...................................... 1-2
SECTION 1:
GENERATOR FUNDAMENTALS ....................... 3-5
MAGNETISM..................................................................... 3
ELECTROMAGNETIC FIELDS .............................................. 3
ELECTROMAGNETIC INDUCTION ...................................... 3
A SIMPLE AC GENERATOR ................................................ 4
A MORE SOPHISTICATED AC GENERATOR......................... 4
SECTION 2:
MEASURING ELECTRICITY ............................. 6-8
METERS .......................................................................... 6
THE VOM ........................................................................ 6
MEASURING AC VOLTAGE .............................................. 6
MEASURING DC VOLTAGE .............................................. 6
MEASURING AC FREQUENCY ......................................... 6
MEASURING CURRENT .................................................... 7
MEASURING RESISTANCE ................................................ 7
ELECTRICAL UNITS .......................................................... 8
OHM'S LAW .................................................................... 8
SECTION 3:
DESCRIPTION AND COMPONENTS ............. 9-15
INTRODUCTION .............................................................. 9
ENGINE-GENERATOR DRIVE SYSTEM ............................... 9
THE AC GENERATOR ....................................................... 9
ROTOR ASSEMBLY .......................................................... 9
STATOR ASSEMBLY ........................................................ 10
BRUSH HOLDER AND BRUSHES ..................................... 10
OTHER AC GENERATOR COMPONENTS ........................ 10
EXCITATION CIRCUIT BREAKER .................................. 10
VOLTAGE REGULATOR .............................................. 11
ADJUSTMENT PROCEDURE ....................................... 11
CIRCUIT BREAKERS ................................................... 12
ROTOR RESIDUAL MAGNETISM ..................................... 12
FIELD BOOST CIRCUIT ................................................... 12
OPERATION ................................................................... 12
STARTUP ................................................................... 12
ON-SPEED OPERATION .............................................. 12
FIELD EXCITATION ..................................................... 12
AC POWER WINDING OUTPUT .................................. 12
BATTERY CHARGE WINDING OUTPUT ....................... 12
10 AMP BATTERY CHARGE WINDING OUTPUT .......... 12
INSULATION RESISTANCE .............................................. 13
THE MEGOHMMETER ................................................... 13
GENERAL .................................................................. 13
TESTING STATOR INSULATION ................................... 13
TESTING ROTOR INSULATION .................................... 13
HI-POT TESTER .......................................................... 13
STATOR INSULATION RESISTANCE TEST .......................... 13
GENERAL .................................................................. 13
TESTING ALL STATOR WINDINGS TO GROUND .......... 14
TEST BETWEEN WINDINGS: ....................................... 14
ROTOR INSULATION RESISTANCE TEST .......................... 15
CLEANING THE GENERATOR ......................................... 15
DRYING THE GENERATOR ............................................. 15
SECTION 4:
ENGINE DC CONTROL SYSTEM .................. 16-27
PRINTED CIRCUIT BOARD .............................................. 16
GENERAL .................................................................. 16
CIRCUIT BOARD CONNECTIONS ............................... 16
DIP SWITCH POSITIONS ............................................. 16
BATTERY ....................................................................... 16
RECOMMENDED BATTERY ........................................ 16
CONTROL PANEL COMPONENT IDENTIFICATION ...... 17-18
OPERATIONAL ANALYSIS .......................................... 20-27
CIRCUIT CONDITION - REST ...................................... 20
CIRCUIT CONDITION - START .................................... 22
CIRCUIT CONDITION - RUN ....................................... 24
CIRCUIT CONDITION - STOP ...................................... 26
FAULT SHUTDOWN ................................................... 27
SECTION 5:
TROUBLESHOOTING FLOWCHARTS .......... 28-36
INTRODUCTION ............................................................ 28
IF PROBLEM INVOLVES AC OUTPUT ............................... 28
PROBLEM 1 -
VOLTAGE & FREQUENCY ARE BOTH HIGH OR LOW .. 28
PROBLEM 2 -
GENERATOR PRODUCES ZERO VOLTAGE
OR RESIDUAL VOLTAGE (2-12 VAC) ...................... 29-30
PROBLEM 3 -
EXCESSIVE VOLTAGE/FREQUENCY
DROOP WHEN LOAD IS APPLIED ............................... 30
PROBLEM 4 -
NO BATTERY CHARGE OUTPUT ................................. 31
PROBLEM 5 -
NO 10 AMP BATTERY CHARGE OUTPUT ................... 31
PROBLEM 6 -
ENGINE WILL NOT CRANK ........................................ 32
PROBLEM 7 -
ENGINE CRANKS BUT WILL NOT START ..................... 33
PROBLEM 8 -
ENGINE STARTS HARD AND RUNS ROUGH ................ 34
PROBLEM 9 -
ENGINE STARTS THEN SHUTS DOWN ........................ 34
PROBLEM 10 -
10 AMP FUSE (F1) BLOWING ..................................... 35
PROBLEM 11 -
UNIT OVERSPEEDS .................................................... 36
PROBLEM 12 -
IDLE CONTROL “RPM DOES NOT DECREASE” ........... 36
PROBLEM 13 -
IDLE CONTROL “RPM DOES NOT INCREASE
WHEN LOAD IS APPLIED” .......................................... 36
PROBLEM 14 -
ENGINE “HUNTS” / ERRATIC IDLE .............................. 36

Table of Contents
Page 2
SECTION 6:
DIAGNOSTIC TESTS ...................................... 37-65
INTRODUCTION .............................................................. 37
TEST 1 - CHECK NO-LOAD VOLTAGE AND FREQUENCY .. 37
TEST 2 - CHECK MAIN CIRCUIT BREAKER........................ 37
TEST 3- TEST EXCITATION CIRCUIT BREAKER ................... 38
TEST 4 - FIXED EXCITATION TEST/ROTOR AMP DRAW ..... 38
TEST 5 - CHECK STEPPER MOTOR CONTROL................... 40
TEST 6 - WIRE CONTINUITY............................................. 41
TEST 7 - CHECK FIELD BOOST ......................................... 41
TEST 8 - DIODE/RESISTOR................................................ 42
TEST 9 - TEST STATOR ..................................................... 43
TEST 10 - SENSING LEADS............................................... 44
TEST 11 - EXCITATION WIRING ........................................ 45
TEST 12 - CHECK BRUSH LEADS...................................... 45
TEST 13 - CHECK BRUSHES & SLIP RINGS ........................ 45
TEST 14 - CHECK ROTOR ASSEMBLY............................... 46
TEST 15 - CHECK LOAD VOLTAGE & FREQUENCY ........... 46
TEST 16 - CHECK LOAD WATTS & AMPERAGE ................ 46
TEST 17 - CHECK BATTERY CHARGE OUTPUT ................. 47
TEST 18 - CHECK 10 AMP BATTERY CHARGE OUTPUT.... 47
TEST 19 - CHECK BATTERY CHARGE RECTIFIER............... 47
TEST 20 - CHECK 10 AMP CIRCUIT BREAKER .................. 48
TEST 21- CHECK 10 AMP FUSE ....................................... 48
TEST 22- CHECK BATTERY & CABLES .............................. 48
TEST 23- CHECK VOLTAGE AT STARTER CONTACTOR ..... 49
TEST 24 - CHECK STARTER CONTACTOR......................... 49
TEST 25 - CHECK STARTER MOTOR................................. 49
CONDITIONS AFFECTING STARTER MOTOR
PERFORMANCE:.......................................................... 49
CHECKING THE PINION: .............................................. 50
TOOLS FOR STARTER PERFORMANCE TEST: ................. 50
MEASURING CURRENT: ............................................... 50
TACHOMETER:............................................................ 50
TEST BRACKET: ........................................................... 51
REMOVE STARTER MOTOR:......................................... 51
TESTING STARTER MOTOR: ......................................... 51
TEST 26 - TEST STARTER CONTACTOR RELAY (SCR)......... 51
TEST 27- CHECK START-RUN-STOP SWITCH ................... 52
TEST 28- CHECK START-RUN-STOP (SW1) WIRING........... 52
TEST 29 - CHECK IGNITION SPARK .................................. 53
TEST 30 - CHECK SPARK PLUGS ...................................... 53
TEST 31 - REMOVE WIRE 18 / SHUTDOWN LEAD ............ 54
TEST 32 - TEST START STOP RELAY .................................. 54
TEST 33- TEST WIRE 167 ................................................. 55
TEST 34 - TEST START STOP RELAY WIRING ..................... 56
TEST 35 - CHECK AND ADJUST IGNITION MAGNETOS .... 56
TEST 36: TEST FUEL SHUTOFF SOLENOID......................... 57
TEST 37: TEST FUEL SHUTOFF SOLENOID VOLTAGE ......... 57
TEST 38: CHECK FUEL PUMP........................................... 57
TEST 39 - CHECK CARBURETION..................................... 58
TEST 40 - VALVE ADJUSTMENT........................................ 58
TEST 41 - CHECK ENGINE / CYLINDER LEAK
DOWN TEST / COMPRESSION TEST ................................. 59
TEST 42 - CHECK OIL PRESSURE SWITCH AND WIRE 86 .. 60
TEST 43: CHECK START STOP RELAY (SSR)....................... 60
TEST 44: TEST STARTER CONTACTOR RELAY (SCR) .......... 61
TEST 45: CHECK WIRE 15 CIRCUIT.................................. 61
TEST 46: CHECK WIRE 14 CIRCUIT.................................. 61
TEST 47: CHECK FUEL SHUTOFF SOLENOID..................... 61
TEST 48: CHECK HOURMETER ........................................ 62
TEST 49: CHECK WIRE 15B ............................................. 62
TEST 50: CHECK WIRE 167 ............................................. 62
TEST 51: CHECK WIRES 11S & 44S.................................. 62
TEST 52: CHECK IDLE CONTROL SWITCH (SW2).............. 63
TEST 53: CHECK IDLE CONTROL WIRING ........................ 63
TEST 54: CHECK IDLE CONTROL TRANSFORMERS ........... 64
TEST 55: CHECK TR1 & TR2 WIRING ............................... 64
TEST 56: CHOKE TEST ................................................... 65
SECTION 7:
DISASSEMBLY AND EXPLODED VIEWS ..... 66-71
MAJOR DISASSEMBLY ..................................................... 66
GENERATOR – FIGURE A ................................................ 68
FRAME, HANDLE & WHEELS – FIGURE B ......................... 70
SECTION 8:
ELECTRICAL DATA......................................... 72-79
WIRING DIAGRAM 12.5 & 15 KW (UNITS WITHOUT
HOURMETER) – DRAWING NO. 0E0228 .......................... 72
ELECTRICAL SCHEMATIC 12.5 & 15 KW (UNITS
WITHOUT HOURMETER) – DRAWING NO. 0E0229-A....... 74
WIRING DIAGRAM 12.5 & 15 KW
(UNITS WITH HOURMETER) – DRAWING NO. 0D4609-D.. 76
ELECTRICAL SCHEMATIC 12.5 & 15 KW (UNITS WITH
HOURMETER) – DRAWING NO. 0D6297-A ...................... 78
WIRING DIAGRAM 17.5 KW UNITS –
DRAWING NO. 0G0731 .................................................. 80
ELECTRICAL SCHEMATIC 17.5 KW UNITS –
DRAWING NO. 0G0733 .................................................. 82
WIRING DIAGRAM, 17.5 KW MANUAL
TRANSFER SWITCH – DRAWING NO. 0G1065 ................ 83
INTERCONECTION DRAWING – 17.5 KW GENERATOR .... 84
SECTION 9:
SPECIFICATIONS & CHARTS ....................... 86-87
GENERATOR SPECIFICATIONS.......................................... 86
ENGINE SPECIFICATIONS ................................................. 86
ENGINE SPEEDS AND VOLTAGE SPECIFICATIONS ............. 86
TORQUE SPECIFICATIONS................................................ 87
TRIM TORQUE SPECIFICATIONS....................................... 87

Section 1
GENERATOR FUNDAMENTALS
MAGNETISM
Magnetism can be used to produce electricity and
electricity can be used to produce magnetism.
Much about magnetism cannot be explained by our
present knowledge. However, there are certain pat-
terns of behavior that are known. Application of these
behavior patterns has led to the development of gen-
erators, motors and numerous other devices that uti-
lize magnetism to produce and use electrical energy.
See Figure 1-1. The space surrounding a magnet is
permeated by magnetic lines of force called “flux”.
These lines of force are concentrated at the magnet's
north and south poles. They are directed away from
the magnet at its north pole, travel in a loop and
re-enter the magnet at its south pole. The lines of
force form definite patterns which vary in intensity
depending on the strength of the magnet. The lines
of force never cross one another. The area surround-
ing a magnet in which its lines of force are effective is
called a “magnetic field”.
Like poles of a magnet repel each other, while unlike
poles attract each other.
Figure 1-1. – Magnetic Lines of Force
ELECTROMAGNETIC FIELDS
All conductors through which an electric current Is
flowing have a magnetic field surrounding them. This
field is always at right angles to the conductor. If a
compass is placed near the conductor, the compass
needle will move to a right angle with the conductor.
The following rules apply:
• The greater the current flow through the conductor,
the stronger the magnetic field around the conductor.
• The increase in the number of lines of force is
directly proportional to the increase in current flow
and the field is distributed along the full length of
the conductor.
• The direction of the lines of force around a conduc-
tor can be determined by what is called the “right
hand rule”. To apply this rule, place your right hand
around the conductor with the thumb pointing in
the direction of current flow. The fingers will then be
pointing in the direction of the lines of force.
NOTE: The “right hand rule” is based on the “cur-
rent flow” theory which assumes that current
flows from positive to negative. This is opposite
the “electron” theory, which states that current
flows from negative to positive.
Figure 1-2. – The Right Hand Rule
ELECTROMAGNETIC INDUCTION
An electromotive force (EMF) or voltage can be pro-
duced in a conductor by moving the conductor so that
it cuts across the lines of force of a magnetic field.
Similarly, if the magnetic lines of force are moved so
that they cut across a conductor, an EMF (voltage)
will be produced in the conductor. This is the basic
principal of the revolving field generator.
Figure 1-3, below, illustrates a simple revolving field
generator. The permanent magnet (Rotor) is rotated
so that its lines of magnetic force cut across a coil of
wires called a Stator. A voltage is then induced into
the Stator windings. If the Stator circuit is completed
by connecting a load (such as a light bulb), current
will flow in the circuit and the bulb will light.
Figure 1-3. – A Simple Revolving Field Generator
Page 3

Section 1
GENERATOR FUNDAMENTALS
A SIMPLE AC GENERATOR
Figure 1-4 shows a very simple AC Generator. The
generator consists of a rotating magnetic field called a
ROTOR and a stationary coil of wire called a STATOR.
The ROTOR is a permanent magnet which consists
of a SOUTH magnetic pole and a NORTH magnetic
pole.
As the MOTOR turns, its magnetic field cuts across
the stationary STATOR. A voltage is induced Into
the STATOR windings. When the magnet's NORTH
pole passes the STATOR, current flows in one direc-
tion. Current flows in the opposite direction when the
magnet's SOUTH pole passes the STATOR. This con-
stant reversal of current flow results in an alternating
current (AC) waveform that can be diagrammed as
shown in Figure 1-5.
The ROTOR may be a 2-pole type having a single
NORTH and a single SOUTH magnetic pole. Some
ROTORS are 4-pole type with two SOUTH and two
NORTH magnetic poles. The following apply:
1. The 2-pole ROTOR must be turned at 3600 rpm to produce
an AC frequency of 60 Hertz, or at 3000 rpm to deliver an AC
frequency of 50 Hertz.
2. The 4-pole ROTOR must operate at 1800 rpm to deliver a 60
Hertz AC frequency or at 1500 rpm to deliver a 50 Hertz AC
frequency.
S
TAT
O
R
R
O
T
OR
MA
G
NETI
C
FIEL
D
Figure 1-4. – A Simple AC Generator
CURRENT VOLTAGE
ONE CYCLE
0180 360
(+)
(-)
Figure 1-5. – Alternating Current Sine Wave
A MORE SOPHISTICATED AC GENERATOR
Figure 1-6 represents a more sophisticated generator.
A regulated direct current is delivered into the ROTOR
windings via carbon BRUSHES AND SLIP RINGS.
This results in the creation of a regulated magnetic
field around the ROTOR. As a result, a regulated volt-
age is induced into the STATOR. Regulated current
delivered to the ROTOR is called “EXCITATION” cur-
rent.
S
TAT
OR
BR
US
HE
S
12
0
V
12
0
V
S
LI
P
RIN
GS
AC
OU
TP
U
T
DC
CU
RRENT
S
TAT
OR
24
0
V
Figure 1-6. – A More Sophisticated Generator
See Figure 1-7 (next page). The revolving magnet-
ic field (ROTOR) is driven by the engine at a con-
stant speed. This constant speed is maintained by a
mechanical engine governor. Units with a 2-pole rotor
require an operating speed of 3600 rpm to deliver a
60 Hertz AC output.
Generator operation may be described briefly as fol-
lows:
1. Some “residual” magnetism is normally present in the Rotor
and is sufficient to induce approximately 7 to 12 volts AC Into
the STATOR's AC power windings.
Page 4

Section 1
GENERATOR FUNDAMENTALS
2. During startup, printed circuit board action controls the START/
STOP RELAY to deliver battery voltage to the ROTOR, via the
brushes and slip rings.
a. The battery voltage is called “Field Boost”.
b. Flow of direct current through the ROTOR
increases the strength of the magnetic field
above that of “residual” magnetism alone.
3. “Residual” plus “Field Boost” magnetism induces a voltage into
the Stator excitation (DPE), battery charge and AC Power wind-
ings.
4. Excitation winding unregulated AC output is delivered to an
electronic Voltage Regulator, via an Excitation Circuit Breaker.
a. A “Reference” voltage has been preset into
the Voltage Regulator.
b. An “Actual” (“sensing”) voltage is delivered
to the Voltage Regulator via sensing leads
from the Stator AC power windings.
c. The Regulator “compares” the actual (sens-
ing) voltage to its pre-set reference voltage.
(1) If the actual (sensing) voltage is great-
er than the pre-set reference voltage, the
Regulator will decrease the regulated cur-
rent flow to the Rotor.
(2) If the actual (sensing) voltage is less
than the pre-set reference voltage, the
Regulator will increase the regulated current
flow to the Rotor.
(3) In the manner described, the Regulator
maintains an actual (sensing) voltage that is
equal to the pre-set reference voltage.
NOTE: The Voltage Regulator also changes the
Stator excitation windings alternating current (AC)
output to direct current (DC).
5. When an electrical load is connected across the Stator power
windings, the circuit is completed and an electrical current will
flow.
6. The Rotor's magnetic field also induces a voltage into the
Stator battery charge windings.
a. Battery charge winding AC output is deliv-
ered to the battery charge rectifiers (BCR)
which changes the AC to direct current
(DC).
b. The rectified DC is then delivered to the
units battery and battery charge outlet, to
maintain the battery in a charged state.
Page 5
ENGINE -
DIRECT DRIVE
CB2
BCR2
BCR1
BCR1 & BCR2 = BATTERY CHARGE RECTIFIER
FIELD BOOST FROM
START/STOP RELAY (SSR)
CB1 CB2 = EXCITATION CIRCUIT BREAKER
12V DC
OUTLET
10A STATOR
BATTERY CHARGE
WINDING
STATOR
BATTERY CHARGE
WINDING
STATOR
DPE
WINDING
STATOR
POWER
WINDING
STATOR
POWER
WINDING
ROTOR
VOLTAGE
REGULATOR
Figure 1-7. – Generator Operating Diagram

Section 2
MEASURING ELECTRICITY
METERS
Devices used to measure electrical properties are
called meters. Meters are available that allow one
to measure (a) AC voltage, (b) DC voltage, (c) AC
frequency, and (d) resistance in ohms. The following
apply:
• To measure AC voltage, use an AC voltmeter.
• To measure DC voltage, use a DC voltmeter.
• Use a frequency meter to measure AC frequency in
“Hertz” or “cycles per second”..
• Use an ohmmeter to read circuit resistance, in
“ohms”.
THE VOM
A meter that will permit both voltage and resistance to
be read is the “volt-ohm-milliammeter” or “VOM”.
Some VOMs are of the “analog” type (not shown).
These meters display the value being measured by
physically deflecting a needle across a graduated
scale. The scale used must be interpreted by the user.
“Digital” VOM's (Figure 2-1) are also available and
are generally very accurate. Digital meters display the
measured values directly by converting the values to
numbers.
NOTE: Standard AC voltmeters react to the
AVERAGE value of alternating current. When
working with AC, the effective value is used. For
that reason a different scale is used on an AC
voltmeter. The scale is marked with the effective
or “rms” value even though the meter actually
reacts to the average value. That is why the AC
voltmeter will give an incorrect reading if used to
measure direct current (DC).
Figure 2-1. – Digital VOM
MEASURING AC VOLTAGE
An accurate AC voltmeter or a VOM may be used to
read the generator's AC output voltage. The following
apply:
1. Always read the generator's AC output voltage only at the unit's
rated operating speed and AC frequency.
2. The generator's Voltage Regulator can be adjusted for correct
output voltage only while the unit is operating at its correct
rated speed and frequency.
3. Only an AC voltmeter may be used to measure AC voltage. DO
NOT USE A DC VOLTMETER FOR THIS PURPOSE.
*
DANGER!: GENERATORS PRODUCE HIGH
AND DANGEROUS VOLTAGES. CONTACT
WITH HIGH VOLTAGE TERMINALS WILL
RESULT IN DANGEROUS AND POSSIBLY
LETHAL ELECTRICAL SHOCK.
MEASURING DC VOLTAGE
A DC voltmeter or a VOM may be used to measure
DC voltages. Always observe the following rules:
1. Always observe correct DC polarity.
a. Some VOM's may be equipped with a polar-
ity switch.
b. On meters that do not have a polar-
ity switch, DC polarity must be reversed by
reversing the test leads.
2. Before reading a DC voltage, always set the meter to a higher
voltage scale than the anticipated reading. If in doubt, start at
the highest scale and adjust the scale downward until correct
readings are obtained.
3. The design of some meters is based on the “current flow”
theory while others are based on the “electron flow” theory.
a. The “current flow” theory assumes that
direct current flows from the positive (+) to
the negative (-).
b. The “electron flow” theory assumes that cur-
rent flows from negative (-) to positive (+).
NOTE: When testing generators, the “current flow”
theory is applied. That is, current is assumed to
flow from positive (+) to negative (-).
MEASURING AC FREQUENCY
The generator's AC output frequency is proportional
to Rotor speed. Generators equipped with a 2-pole
Rotor must operate at 3600 rpm to supply a frequency
of 60 Hertz. Units with 4-pole Rotor must run at 1800
rpm to deliver 60 Hertz.
Correct engine and Rotor speed is maintained by an
Page 6

Section 2
MEASURING ELECTRICITY
engine speed governor. For models rated 60 Hertz,
the governor is generally set to maintain a no-load fre-
quency of about 62 Hertz with a corresponding output
voltage of about 124 volts AC line-to-neutral. Engine
speed and frequency at no-load are set slightly high
to prevent excessive rpm and frequency droop under
heavy electrical loading.
MEASURING CURRENT
CLAMP-ON:
To read the current flow, in AMPERES, a clamp-on
ammeter may be used. This type of meter indicates
current flow through a conductor by measuring the
strength of the magnetic field around that conductor.
The meter consists essentially of a current trans-
former with a split core and a rectifier type instrument
connected to the secondary. The primary of the cur-
rent transformer is the conductor through which the
current to be measured flows. The split core allows
the instrument to be clamped around the conductor
without disconnecting it.
Current flowing through a conductor may be mea-
sured safely and easily. A line-splitter can be used
to measure current in a cord without separating the
conductors.
Figure 2-2. – Clamp-On Ammeter
Figure 2-3. – A Line-Splitter
NOTE: If the physical size of the conductor or
ammeter capacity does not permit all lines to be
measured simultaneously, measure current flow
in each individual line. Then, add the individual
readings.
IN-LINE:
Alternatively, to read the current flow in AMPERES,
an in-line ammeter may be used. Most Digital Volt
Ohm Meters (VOM) will have the capability to mea-
sure amperes.
This usually requires the positive meter test lead to be
connected to the correct amperes plug, and the meter
to be set to the amperes position. Once the meter is
properly set up to measure amperes the circuit being
measured must be physically broken. The meter will
be in-line or in series with the component being mea-
sured.
In Figure 2-4 the control wire to a relay has been
removed. The meter is used to connect and supply
voltage to the relay to energize it and measure the
amperes going to it.
1.00 A
BATTERY
+- RELAY
Figure 2-4. – A VOM as an In-line meter
MEASURING RESISTANCE
The volt-ohm-milliammeter may be used to measure
the resistance in a circuit. Resistance values can be
very valuable when testing coils or windings, such as
the Stator and Rotor windings.
When testing Stator windings, keep in mind that the
resistance of these windings is very low. Some meters
are not capable of reading such a low resistance and
will simply read CONTINUITY.
If proper procedures are used, the following condi-
tions can be detected using a VOM:
• A “short-to-ground” condition in any Stator or Rotor
winding.
• Shorting together of any two parallel Stator wind-
ings.
• Shorting together of any two isolated Stator wind-
ings.
• An open condition in any Stator or Rotor winding.
Page 7

Section 2
MEASURING ELECTRICITY
Component testing may require a specific resis-
tance value or a test for INFINITY or CONTINUITY.
Infinity is an OPEN condition between two electrical
points, which would read as no resistance on a VOM.
Continuity is a CLOSED condition between two elec-
trical points, which would be indicated as very low
resistance or “ZERO” on a VOM.
ELECTRICAL UNITS
AMPERE:
The rate of electron flow in a circuit is represented
by the AMPERE. The ampere is the number of elec-
trons flowing past a given point at a given time. One
AMPERE is equal to just slightly more than six thou-
sand million billion electrons per second (6.25 x 1018).
With alternating current (AC), the electrons flow first
in one direction, then reverse and move in the oppo-
site direction. They will repeat this cycle at regular
intervals. A wave diagram, called a “sine wave” shows
that current goes from zero to maximum positive
value, then reverses and goes from zero to maximum
negative value. Two reversals of current flow is called
a cycle. The number of cycles per second is called
frequency and is usually stated in “Hertz”.
VOLT:
The VOLT is the unit used to measure electrical
PRESSURE, or the difference in electrical potential
that causes electrons to flow. Very few electrons will
flow when voltage is weak. More electrons will flow as
voltage becomes stronger. VOLTAGE may be consid-
ered to be a state of unbalance and current flow as
an attempt to regain balance. One volt is the amount
of EMF that will cause a current of 1 ampere to flow
through 1 ohm of resistance.
-+
AMPERE - Unit measuring rate of
current flow (number of electrons
past a given point)
OHM - Unit measuring resistance
or opposition to flow
VOLT - Unit measuring force or
difference in potential
causing current flow
Conductor of a
Circuit
Figure 2-5. – Electrical Units
OHM:
The OHM is the unit of RESISTANCE. In every circuit
there is a natural resistance or opposition to the flow
of electrons. When an EMF is applied to a complete
circuit, the electrons are forced to flow in a single
direction rather than their free or orbiting pattern. The
resistance of a conductor depends on (a) its physical
makeup, (b) its cross-sectional area, (c) its length,
and (d) its temperature. As the conductor's tempera-
ture increases, its resistance increases in direct pro-
portion. One (1) ohm of resistance will permit one (1)
ampere of current to flow when one (1) volt of electro-
motive force (EMF) is applied.
OHM'S LAW
A definite and exact relationship exists between
VOLTS, OHMS and AMPERES. The value of one can
be calculated when the value of the other two are
known. Ohm's Law states that in any circuit the current
will increase when voltage increases but resistance
remains the same, and current will decrease when
resistance Increases and voltage remains the same.
VOLTS
(E)
AMPS
(I)
OHMS
(R)
Figure 2-6. – Ohm's Law
If AMPERES is unknown while VOLTS and OHMS are
known, use the following formula:
AMPERES = VOLTS
OHMS
If VOLTS is unknown while AMPERES and OHMS are
known, use the following formula:
VOLTS = AMPERES x OHMS
If OHMS is unknown but VOLTS and AMPERES are
known, use the following:
OHMS = VOLTS
AMPERES
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