HVAC Electrical Troubleshooting: A Systematic Approach

Troubleshooting Is Not Guessing

Troubleshooting is not replacing parts until the equipment
starts working.

A good technician determines what the equipment should be
doing, takes measurements, and follows the circuit until
finding the point where what should be
happening and what is happening are no
longer the same.

Remember:

Every electrical circuit has four basic parts:

Source → Path → Switch → Load

A complicated HVAC system may have many switches, loads,
relays, safeties and control devices, but the basic electrical
circuit has not changed.

When you become confused by a schematic, go back to the basics.

Where is my source? What is my path? What switches have
to be closed? What is my load?

Safety Before Troubleshooting

Electrical troubleshooting sometimes requires voltage
measurements on energized equipment. Electricity can cause
serious injury or death, so do not treat energized testing
casually.

Your meter should be the first tool out of your tool bag
and the last tool you put away.

Know what voltage you expect before taking a measurement.
Use a meter and test leads properly rated for the circuit.
Make sure the leads are plugged into the correct meter
terminals.
Set the meter to the correct function before testing.
Verify meter operation on a known source before relying
on its reading.
Never assume a circuit is de-energized because a switch
is off.
Never assume the function or safety of a conductor from
its color alone.
De-energize the circuit before resistance or continuity
testing.
Do not disconnect or move conductors on energized
equipment unless the procedure specifically requires it
and you are trained to perform that work safely.

If you do not understand the circuit, cannot identify the
expected voltage, or cannot safely reach the test points,
stop.

There is nothing wrong with stopping and looking at the
schematic again.

There is something very wrong with guessing around an
energized circuit.

The Troubleshooting Process

You can use the same basic process on most electrical
troubleshooting calls.

1
Understand the complaint

2
Know the sequence of operation

3
Identify source, path, switches and load

4
Verify the source voltage

5
Follow the circuit

6
Find where expected operation changes

7
Test the suspected component

8
Determine why it failed

1. Understand the Complaint

The first step in troubleshooting is not opening the
equipment and grabbing your meter.

First determine what is actually wrong.

A complaint such as “The air conditioner doesn’t
work” does not give you enough information.

Ask questions and observe the equipment.

Does the indoor blower run?
Does the outdoor fan run?
Does the compressor run?
Does the thermostat display anything?
Did the equipment stop suddenly?
Is the problem intermittent?
Was someone working on the equipment recently?

Your job is to turn a general complaint into a specific
problem that you can troubleshoot.

2. Know the Sequence of Operation

Before you can determine what is wrong, you have to know
what is right.

The sequence of operation tells you what
should happen and in what order it should happen.

Consider a simple cooling call.

Thermostat calls for cooling
↓

Control circuit is completed
↓

Contactor coil energizes
↓

Contactor closes
↓

Line voltage reaches the outdoor loads
↓

Compressor and condenser fan operate

If the outdoor equipment does not operate, somewhere in that
sequence something did not happen.

Your job is to find where.

Without knowing the sequence of operation, you are guessing.

3. Find the Four Parts of the Circuit

Source

Where does the electrical energy come from?

On a line-voltage circuit this may be the electrical
service, breaker, disconnect or fuse.

On a 24-volt control circuit the source is normally
the secondary side of a transformer.

Path

How does current get from the source, through the
circuit, and back to the other side of the source?

Wires, terminals, connectors and circuit-board traces
can all be part of the path.

Switch

What controls current through the circuit?

Thermostats, relay contacts, contactors, pressure
switches and safety controls are all examples of
switches.

Load

What is supposed to do the work?

Motors, compressors, heaters, relay coils, contactor
coils and other devices that use electrical energy
are loads.

4. Verify the Source

Always prove that the circuit has the proper source voltage.

If you are troubleshooting a 120-volt circuit, do you actually
have approximately 120 volts?

If you are troubleshooting a 240-volt circuit, do you have
approximately 240 volts across the two lines?

If you are troubleshooting a 24-volt control circuit, does
the transformer actually have the expected secondary voltage?

Do not assume.
Measure it.

You can waste a lot of time troubleshooting a circuit that
cannot possibly operate because its source voltage is missing.

5. Follow the Circuit

This is where your schematic and your meter work together.

The schematic is your map.

Your meter tells you where you are on that map.

Start at a point you have proven is good and move toward
the load.

Do not jump randomly from component to component.

If you have the expected voltage at point A and should have
the same voltage at point B, but do not, ask yourself:

What is between point A and point B?

It may be a switch, relay contact, safety, connector, terminal,
or section of wire.

You have now narrowed the problem down to a much smaller
section of the circuit.

6. Let the Meter Tell You What Is Happening

A meter reading only helps you when you understand what the
reading should be.

Closed switch
Approximately 0 volts across it

Open switch
Approximately source voltage across it

Working load
Its intended operating voltage across it

Open circuit
No current flows

Be careful with the statement that a load has voltage.

Finding voltage across a load does not
automatically prove that the load is good.

If a motor has the proper voltage across it and should be
running but is not, you have learned something important.
You have largely proven the circuit feeding the motor.

Now you need to investigate the motor and the components
associated with it.

7. Use the Hop-Scotch Method

One of the easiest ways to find an open in a circuit is to
work through the circuit one test point at a time.

I call this the Hop-scotch method.

Start with your meter set correctly for the voltage you are
testing. Keep one meter lead at the appropriate known
reference point and move the other lead through the circuit.

Check before and after each switch, contact, safety or other
component.

You are looking for the point where the reading changes from
what you expect.

Think of it as walking through the circuit.

Do not jump from the beginning to the end.
Take one step at a time.

Line Voltage and Control Voltage

Line-Voltage Troubleshooting

Start at the source and follow the circuit toward
the load.

Breaker → Disconnect → Contactor
→ Load

If you have correct voltage entering the disconnect
but not leaving it, concentrate on the disconnect.

If you have correct voltage entering a closed
contactor but not leaving it, investigate the
contactor and its connections.

If the proper voltage reaches the load but the load
does not operate, investigate the load.

Control-Voltage Troubleshooting

Control circuits often look more complicated because
several switches and safeties may be wired in series.

Do not let the number of components confuse you.

The transformer secondary is your source. The wiring
is your path. Thermostats, relays and safeties are
switches. A relay or contactor coil may be your load.

Start at the transformer and follow the circuit.

Where does the voltage stop?

WORKED EXAMPLE

The Contactor Will Not Energize

The complaint is that the outdoor unit will not run.

You inspect the equipment and find that the contactor is
not pulling in.

Do you replace the contactor?

No.

You do not know that the contactor is bad.

Step 1 — Know What Should Happen

When the thermostat calls for cooling, control voltage should
eventually reach the contactor coil.

The energized coil should pull in the contactor. The closed
contacts should then provide line voltage to the outdoor loads.

Step 2 — Verify Control Voltage

Check the transformer secondary.

Do you have the expected control voltage?

If not, stop moving toward the contactor. The control circuit
does not have its source.

Determine why.

Step 3 — Test the Contactor Coil

If the source is good, measure across the contactor coil while
the system is calling for cooling.

If the proper voltage is across the coil but the contactor
does not pull in, the contactor becomes a strong suspect.

If there is no proper voltage across the coil, replacing the
contactor probably will not fix anything.

The problem is somewhere else in the control circuit.

Step 4 — Follow the Control Circuit

Look at the schematic.

What switches and safeties have to be closed before voltage
can reach the contactor?

Test them in order.

Eventually you will find the point where the circuit stops
behaving as it should.

That is troubleshooting.

You followed the evidence instead of changing parts.

Prove the Component Is Bad

This is one of the most important habits you can develop.

Before replacing a component, prove that it is bad.

If a relay does not energize, does its coil have the
correct voltage?
If a transformer has no secondary voltage, does it have
the correct primary voltage?
If a motor does not run, does it have the proper voltage?
If a contactor does not pull in, does its coil have the
correct voltage?
If a switch should be closed, what voltage do you measure
across it?

Ask yourself:

Does this component have the correct input?

Is it producing the expected output?

If the input is wrong, the component may be perfectly good.

The problem may be somewhere before it.

Do Not Stop at the Failed Part

Your job is not necessarily finished when you find the
failed component.

Why did it fail?

Suppose you find a blown fuse.

Replacing the fuse may restore power, but a fuse is designed
to protect the circuit. Why did it open?

A burned connection may have been caused by a loose terminal.

A failed transformer may have been overloaded by a problem
in the control circuit.

A tripped breaker may indicate a short circuit, ground fault,
or excessive load.

Do not stop thinking just because the equipment starts
working again.

Common Troubleshooting Mistakes

Replacing Parts Without Testing

Sometimes you will guess correctly.

That does not make guessing a good troubleshooting
method.

Ignoring the Sequence

If you do not know what should happen next, you cannot
determine why it did not happen.

Trusting Wire Color

Wire colors are useful. They are not proof.

Someone may have changed the wiring before you arrived.

Resistance Testing With Power On

Resistance and continuity measurements are performed
on de-energized circuits with components isolated as
necessary.

Using the Wrong Reference Point

Voltage is a difference in electrical potential
between two points.

Make sure your meter leads are positioned to answer
the question you are actually asking.

Resetting a Breaker and Walking Away

A breaker normally opens for a reason.

Find the reason.

Electrical Troubleshooting Quick Reference

What You Are Testing	Typical Result	What It Tells You
Closed switch	Approximately 0 V across switch	Switch is passing voltage
Open switch	Approximately source voltage across switch	Switch is interrupting the circuit
Working load	Expected operating voltage across load	Load is receiving its intended voltage
Open circuit	0 A	No complete path for current
Closed switch, power off	Very low resistance	Switch has continuity
Open switch, power off	OL / infinite resistance	Switch has no continuity

Actual readings depend on the circuit and equipment being
tested. Always compare your measurement to the schematic,
equipment specifications, and the expected sequence of operation.

When You Troubleshoot, Follow the Same Process

Understand the complaint.
Know the sequence of operation.
Identify the source, path, switches and load.
Verify the source voltage.
Follow the circuit with your meter.
Find the point where the circuit stops behaving
as expected.
Test the suspected component.
Prove the component is bad before replacing it.
Determine why it failed.
Verify proper operation after the repair.

At first this may seem slower than guessing.

It isn’t.

As you gain experience, you will move through these steps
quickly. More importantly, you will understand why you are
taking each measurement and what the answer means.

Use the schematic.
Use your meter.
Follow the circuit.
Do not guess.

Continue Learning

Electrical troubleshooting depends on understanding the
fundamentals. If any of the concepts on this page are
unfamiliar, review the electrical training material before
trying to memorize a troubleshooting procedure.

HVAC/R & Electrical

Basic Electrical Training Manual