INTRODUCTION TO SYSTEMS TROUBLESHOOTING
OBJECTIVE 6
DESCRIBE A SIX STEP TROUBLESHOOTINGSEQUENCE
Effective troubleshooters always use a well-organized approach to solve a
problem. This increases productivity by reducing system downtime. The following
list shows a 6-step process for troubleshooting a system.
Step 1 - Identifythe Problem or Symptom
Always identify the details of the problem in the system before trying to solve
it. Careful observation of the machine will give you many clues to what might be
wrong.
The problem should be described in terms of what the machine or system
is not doing correctly (e.g. the mold does not open) and in terms of the specific
output devices (e.g. cylinder 2 does not retract). It is also a good idea to observe the
machine's operating conditions, such as air temperature, machine speed, and time
of day. These conditions often will give you clues about the source of the problem.
Problems can fall into several types. The most common is an output device,
such as an actuator, which does not tum on or off when it is supposed to. This can
cause the machine to either stop in the middle of the sequence, tum on an actuator
out of sequence, or not stop the machine when it should. This is primarily the type
of problem you will troubleshoot in this course.
Another type of problem is one where either the speed, position, force or
acceleration of the actuator varies from the specification. These problems take a
somewhat different approach than the one you will use here.
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Step 2 - Determine if the Problem is in the Power Circuit or the Control
Circuit
First, test the main power (Ll, L2, and L3) and the control power at the
secondary of the control transformer to see if the power is on. If power is off,
you have a power problem. If power is on, you may still have a power problem.
To determine if you have a power problem, identify the device that operates the
actuator. In most electric motor control circuits, the device is the motor starter. You
should check the status of the device (motor starter's coil). If the device status is
correct, then the problem is in the power side. If the device status is incorrect, the
problem is in the control side. Step 2 of the troubleshooting process can save you
a great deal of time. You eliminate problems with half of the system with one test.
POWER PROBLEM:
BAD MOTOR CONTACTOR
T1
2
2
3
4
5
'E'
CONTROL PROBLEM:
BAD PUSHBUTTON .
F
2
Figure 17. Power vs. Control Circuit Problems
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Step 3 - Isolate the Problem to a Component Using an Organized
Troubleshooting Method
In the next objective, you will learn about four methods you can use. Each
method will enable you to find the problem most quickly for certain types of
circuits. All of them will find the problem eventually.
V
COM
CR1-A
2
---1
1----'
O.L:S
CR1-B
3
---t
5
1---------- . . . .
Figure 18. In-Circuit Signal Testing
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Step 4 - Test the Suspected Component Out-of-Circuit
Sometimes even the best troubleshooter can reach the wrong conclusions.
Before you spend money and time replacing a component, test the component
itself out-of-circuit to determine if it is truly bad.
Figure 19. Out-of-Circuit Testing
Step 5 - Repair or Replace the Component
Most troubleshooters replace components immediately, rather than repair the
component, in order to get the machine running as quickly as possible.
Step 6 - Test the System
In most cases, there is only one problem in a system. However, things like heat
and increased current from a bad component will sometimes damage other components. A good troubleshooter will always test a system after a replacement/repair
to make sure that all of the problems are gone.
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OBJECTIVE 7
DESCRIBE FOUR METHODS OF SYSTEMS LEVEL
TROUBLESHOOTING AND GIVE AN ADVANTAGE OF EACH
Systems level troubleshooting is a methodical approach to isolating the cause
of a problem in a system to a particular component. The following list shows four
of the most common methods of troubleshooting systems. The particular method
used depends on two factors, the type of system and the type of problem. You
might use any or all of these methods in any troubleshooting situation.
• Symptom and Cause Method
• Output-Back Method
• Half-Split Method
• Shotgun Method
Symptomand Cause Method
In this method of troubleshooting, the troubleshooter isolates circuits or
components in a system according to whether or not they could cause the symptoms you observe. By first testing only in the areas most likely to cause the known
symptom, a troubleshooter reduces repair time.
This method is very fast if you see a symptom for which you know there are
only a few causes. This is why it is good to observe the machine's operation carefully before trying to isolate the problem.
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Output-Back Method
In this method of troubleshooting, as shown in figure 20, the troubleshooter
starts testing the outputs of a system and systematically works back toward the
inputs of the system until the cause of the problem is isolated. The output-back
method of troubleshooting is very effective in ladder logic circuits. It is usually a
slower method but is a sure method that will lead you to the problem.
NOTE
Figure 20 shows only the control portion of the circuit. In addition, since
two motors are controlled by this circuit, two sets of overloads are needed.
2
2
4
3
START
STOP
PB1
1 PB3 3 _J.._
5
-:;;:
4
2
2
PUMP
ON
LUBE
MOTOR
3
J--"'-2
...... 5
4
A
TROUBLESHOOTING
SEQUENCE
START
HERE
Figure 20. Output-Back Method
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Half -Split Method
The half-split method of troubleshooting assumes that measurements taken at
a point before the location of the problem will be normal and the measurements
taken at a point after the location of the problem will not be normal. In this method
of troubleshooting, the troubleshooter continually tests at a point half way between
a known good test point and a known bad test point until the problem is isolated.
This method is particularly useful for quickly isolating problems in large
circuits or in systems which have multiple types of circuits. In fact, step 3 of our
recommended troubleshooting sequence, where you test to see if the problem is in
the power or control circuit is actually the half-split method. As a standard procedure, you are using the half-split method in conjunction with another method. An
exception would be when you see a symptom for which you recognize the cause.
2
2
4
3
5
-==-
BEGINNING
OF LOGIC
4
1CR-A
'
2
/
G
/
1CR-B
3
1
·2
1CR
2
'
O.L:S
2
5
1TR
4
2
2,3
PUMP
ON
LUBE
MOTOR
5
A
TIMER
O.L.'S
2
6
COMPRESSOR
MOTOR
SYMPTOM:
MOTOR COIL
IS NOT ON
Figure 21. Half-Split Method
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ShotgunMethod
In the shotgun method of troubleshooting, the troubleshooter randomly tests
every component or connection in a suspect area. The shotgun method can save
time if there are only a few components to test or the problem is intermittent.
However, it is usually the worst method to use and will be the slowest if the circuit
is large.
OBJECTIVE 8
DESCRIBE FIVE TYPES OF IN-ctRCUIT COMPONENT TESTS
In the previous objective, you learned four methods a troubleshooter can use to
decide in what order to test the components. What was not discussed, however, was
how to actually test the circuit components during this troubleshooting process.
The following list shows five types of tests you can use to test a component. Just as
with the troubleshooting methods, you may use any or all of these tests in a single
troubleshooting situation.
-Five senses - One way oflocating component failure is with your five senses.
Many times, defective components will produce physical evidence of their
failure. Shorted components can overheat and produce smoke or a burnt
appearance. A mechanical device, such as a motor or solenoid, may make
an unusual noise when it fails. It is always good to visually inspect a system.
•Measurements - The most common method of isolating circuit and component problems is to take voltage, current, and resistance measurements. The
type of electrical measurement used by the troubleshooter will depend on
the circuit and the suspected problem. You will use these electrical measurements in the upcoming skills section of this LAP.
-Substitution - If a replacement part is readily available, it is common to
substitute a known good part for a suspected bad one. This approach is sometimes faster than taking measurements or analyzing the circuit. You should
only use this method when you are fairly certain the circuit will not damage
the new component.
•Component/circuit bridging - If the function of a component or circuit is
simply to conduct current flow, one way of isolating the problem area is to
jump or bridge the suspected bad area. Temporarily bridging a relay contact,
for example, will allow current to flow around the contact. If the symptom
then disappears, the contact may not be working properly.
•Component/circuit elimination - If a troubleshooter suspects that a circuit
or component is providing an improper path for current, disconnecting the
circuit may change the symptoms in a manner that will lead to the problem.
For example, a component or circuit may connect the system to ground at the
wrong point and cause shorts to occur. By disconnecting this short, the rest of
the system should operate again.
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OBJECTIVE 9
DESCRIBE HOW TO TEST AND ANALVZE CIRCUIT SIGNALS
During step 4 of the troubleshooting sequence, you make a series of test
measurements on the circuit to isolate the problem to a particular component.
These measurements do not directly test the components, they test the electrical
signals between them. In testing these signals, the troubleshooter looks for points
in the circuit where the signal is not what it is supposed to be. By knowing what
effect each component has on the circuit, the troubleshooter can determine which
component is likely the cause of the symptom and should be removed for out-of
circuit testing.
In a ladder logic control circuit, the signal measured is usually voltage. This is
measured by attaching one meter/tester lead to the low side of the circuit and the
other lead to the point where you want to measure the voltage. For example, the
multimeter in figure 22 is measuring the voltage at point A. The voltage at other
points in the control circuit can be measured in a similar manner.
O.L.
F
L1
O.L.
L2
O.L.
F
L3
T1
T2
T3
1CR·A
PUMP
ON
2
1CR·B
3
5
PUMP
MOTOR
Figure 22. Control Circuit Signal Measurement
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On the power side of the circuit, the measured signal is also voltage. However,
the voltage is usually measured between two of the power legs rather than referencing the signal to ground. For example, figure 23 shows a voltage measurement
between two legs, Ll and L2.
2
2
5
-==
Figure 23. Power Circuit Signal Measurement
Although the signal measurements made during the troubleshooting process
are all voltage measurements, the way in which you interpret the measurements
depends on what type of components are wired to the point you are measuring.
Also, components in one part of the circuit can cause effects in other parts of the
circuit. The following paragraphs explain some of the common effects a component will have on a circuit both when it is good and bad.
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Output Control Devices
Control circuit output devices include lamps and coils. The coils can control
other internal devices like timers, control relays, and counters. They also control
external devices like motor starters and fluid power solenoid valves.
Since these components have a certain resistance, you should get a full control
voltage reading across the coil, as shown if figure 24, if the coil is good. If the coil
is open, you will still get this same reading. But, you can tell that the device is bad
if it is getting voltage and it is not operating (e.g. closing its contacts). You must
inspect the complete output device to determine this.
F
O.L.
F
O.L.
T1
L1
T2
L2
O.L.
T3
L3
~ VAC ~
2
2
1
STOP
PB3
3
QD
4
4
F
2
--=2.. PUMP
ON
Figure 24. Measurement of Coil Voltage
If the coil is shorted, the entire circuit will be short circuited when you try to
energize the coil and there will be no voltage anywhere. This will often result in a
blown fuse. If you suspect this, disconnect this rung of the circuit and try to restore
power. If it comes back on, there is a short circuit.
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Input Control Devices
Control input devices include manual switches, relay contacts, and automatic
input switches. These devices consist of one or more normally open and normally
closed contacts which control the flow of electricity to the output devices. If a
switch is good, you should measure full control voltage at the output terminal of its
switch contacts when the switch operator is in one position and zero voltage when
the operator is in the other position. This assumes that you have control voltage at
the input terminal of the contact.
For example, if PB lin figure 25 is good, you should measure control voltage at
its normally open contacts when PB 1 is pressed and no voltage when it is released.
If it is bad, the contacts will not change when the operator is actuated. Either they
will stay closed (you will measure control voltage all the time) or they will stay
open (you will not get any voltage in either actuator position).
F
O.L.
F
O.L.
F
O.L.
L1
L2
L3
T1
T2
T3
Figure 25. Measurement of Switch Voltage at Output Terminal
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For any switch, you can test it by measuring the voltage at its output terminal
when it is actuated and not actuated. Then apply the previously described rules. If
you get a change in voltage when it is actuated, the switch is good. If you get no
change at the output terminal of a switch when actuated, the switch may be bad.
However, another possibility is that the switch is good but a device upstream has
failed to an open condition.
For example, if the output of PB 1 in figure 26 is tested, and there is no change
with the switch actuated, the next step is to check the input to PB 1. If voltage is
present on the input side of PBl, as measured in figure 26, it indicates that the
upstream switch PB3 is good (NC contacts are closed).
Since no voltage passes through PBl (NO contacts) when it is actuated, the
pushbutton switch is bad.
F
O.L.
L1
O.L.
L2
F
L3
1
O.L.
T1
T2
T3
STOP
PB3
Figure 26. Measurement of Switch Voltage at Input Terminal
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Power Devices
Power devices include 3-phase devices such as motors, overload devices,
motor contactors, and safety switches (e.g. fusible disconnects or circuit breakers).
It also includes single-phase devices such as transformers and fuses, which are
used to provide control power.
To test each device on the 3-phase power side, you will have to make three
measurements at each point. For example, the power to the motor is first tested by
measuring Tl to T2, as shown in figure 27. Then you must measure Tl to T3 and
T2 to T3. If power is being supplied to the motor, you should measure full 3-phase
voltage for each of the three measurements. If you get full voltage at each set of
motor terminals and the motor still does not run, the motor has a problem.
If you do not get full voltage in each case, check the components upstream.
When only one power leg is lost, your voltmeter should read significantly less
than 208 VAC but not zero. To determine which leg it is, make all three terminal
measurements (e.g. Tl-T2, Tl-T3, T2-T3). Only one will measure full voltage and
two will not. For example, if Ll in figure 27 is lost, you will get 208 VAC only
when you measure T2 to T3. You will get a much lower voltage when you measure
Tl to T2 and Tl to T3. The leg common to both low voltages is the bad leg.
2
2
3
4
5
'Si'
Figure 27. Measurement of Motor Signals
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The overload relay, motor contactor, and safety switch are checked like
switches, except you have to make three power leg measurements to test the output
signals of each device. The outputs of the motor contactor are checked at points
Bl, B2, and B3. The inputs are checked at B4, B5, and B6, as shown in figure 28.
The outputs of the overload relay are checked at points Cl,C2, and C3. Its
inputs can be checked at C4, C5, and C6. The outputs of the safety switch are
checked at points Al, A2, andA3 and its inputs are checked atA4, A5, andA6.
If a power device is working correctly, the voltage measurements should be
the same across its three output terminals. The safety switch and motor contactor
should measure line voltage at the output terminals when actuated, assuming that
there is voltage at the input terminals. The overload relays should measure line
voltage if they are not tripped, which is normal operation. In the opposite state,
each device's output terminals should measure zero voltage.
FUSIBLE
DISCONNECT
(SAFETY SWITCH)
T1
A1
A2.
A3
2
2
5
-==
Figure 28. Measurement Points in a Power Circuit
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SKILL 5
PERFORM AND ANALVZE CIRCUIT SIGNAL TESTS
Procedure Overview
In this procedure, you will perform signal tests on a circuit to determine
if a component is working correctly or not. At this point, you are not going
to be concerned about the methodology used to decide the order in which to
make the tests. You are only trying to develop your ability to recognize when
a signal measurement indicates a good component or not. To do this, you will
use the single fault mode of the 890-FTS 1 Troubleshooting System or manual
fault plugs.
o 1. Perform the following substeps to set up the 85-MT5 trainer.
A. Make sure the panels are still arranged on the trainer, as shown in Skill 2.
B. Make sure the troubleshooting system is off.
C. Perform a lockout/tagout on the safety switch.
D. Perform an electrical safety check.
E. Make sure the green ground wires connect all station panels to the equipment ground.
F. Mount the motor on the motor base and connect the motor to the Motor
Connection station.
G. Wire the motor connection station for low voltage operation and connect
it to equipment ground.
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H. Set up the circuit shown in figure 29.
I. Make sure you connect terminal 5 of the control transformer secondary to
a green earth ground.
The bolded test points (B through G) are used to identify measurement
locations used later in this skill. They are not marked on the 85-MTS
trainer. The terminal names that are not bolded (i.e. Al, A2, 21, 22) are
marked on the trainer.
Before you test this circuit, go to step 2 and set up the troubleshooting
system.
0.L.
T1
L1
T2
L2
0.L.
T3
L3
1
STOP
PB3
4
2,3,i_
1CR
5
2
5
3
44
A1
B
4
1
21
C1
1CR
22
6
OTOR
ON
LAMP 1
c
CR
5
21
G1
22
G
LAMP2
Figure 29. Motor Control Circuit
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