228 Auto Heating & Air Conditioning
All previous chapters concentrated on HVAC compo-
nents and how various HVAC systems operate. This chapter
begins the discussion of HVAC service. In this chapter, you
will learn how to diagnose refrigeration system problems
and quickly identify defective parts. The seven-step trouble-
shooting process outlined here will enable you to quickly
locate and correct refrigeration system problems. Be sure
you know how to perform every diagnosis and service
procedure in this chapter. You will need all of the informa-
tion presented here to successfully complete the remaining
chapters in this text.
Strategy-based Diagnostics
In the past, it was fairly easy to fi nd and locate a
problem, since most vehicle systems were simple and
common to many, if not all vehicles. As vehicles became
more and more complex, the methods used to diagnose
them became obsolete and in some cases, inapplicable.
Technicians who were accustomed to using the older
diagnostic routines, or no routine at all, began to simply
replace parts hoping to correct the problem, often with
little or no success. Unfortunately, this process was very
expensive, not only to the customer, but to the shop
owner as well.
In response to this problem, a routine involving the
use of logical processes to fi nd the solution to a prob-
lem was devised for use by technicians. This routine is
called strategy-based diagnostics. The strategy-based
diagnostic routine involves the use of a logical step-by-
step process, explained in the next sections. Variations
of strategy-based diagnostics are used in many fi elds
outside of automotive repair. A fl owchart of this process
as recommended by one vehicle manufacturer is shown
in Figure 15-1.
The Importance of Proceeding
Logically
When troubleshooting any refrigeration system or
other HVAC system problem, always proceed logically.
Logic is a form of mental discipline in which you weigh all
factors without jumping to conclusions.
To work logically, the fi rst thing you must know is how
the refrigeration system works and how it affects and is
affected by its components as well as other vehicle systems.
This has been covered in earlier chapters. The knowledge
you have gained can be put to use when a refrigeration
system problem occurs.
The second thing you need is a logical approach. To
diagnose a problem, think about the possible causes of
the problem, and just as important, the things that cannot
cause it. You can then proceed from the simplest things
to check, to the most complex. Do not guess at possible
solutions, and do not panic if the problem takes a little
while to fi nd. If you remember these points, you will be
able to diagnose most refrigeration and HVAC problems
with a minimum of trouble.
The Seven-Step Troubleshooting
Procedure
Troubleshooting is a process of taking logical steps to
reach a solution to a problem. It involves reasoning through
a problem in a series of logical steps. The seven-step process
will, in the majority of cases, be the quickest way to isolate
and correct a problem. Refer to Figure 15-2 as you read the
following sections.
Step 1—Determine the Exact
Problem
Do not expect the vehicle’s driver to tell you what is
wrong in a way that will immediately lead you to the prob-
lem area. Most drivers will state the problem in layman’s
terms, such as “It doesn’t cool,” or “The air conditioner is
noisy.” The fi rst step is to determine the exact problem. You
determine the driver’s exact complaint and its symptoms.
Many times, the complaint has nothing to do with the
HVAC system. This process involves talking to the driver
and road testing the vehicle.
Talking to the Driver
Obtaining information from the driver is the fi rst and
most important part of troubleshooting. Information from
the driver will sometimes allow you to bypass some pre-
liminary testing and go straight to the most likely problem.
In one sense, the driver begins the diagnostic process by
realizing the vehicle has a problem.
Question the vehicle driver to fi nd out exactly what he
or she is unhappy about. Try to get an accurate description
of the problem before beginning work on the vehicle. Try
to translate the driver comments into commonly accepted
automotive diagnostic terms. The easiest way to do this is
with a series of basic questions:
❑
When does the problem occur?
❑
How often does the problem occur?
❑
Does the problem only occur in a certain mode or all
modes?
❑
Do you hear any unusual noises?
❑
Are there any unusual odors?
❑
Does there seem to be any airfl ow when the problem
occurs?
❑
Does air come out of the wrong vents when the prob-
lem occurs?
❑
Did the vehicle have any recent HVAC service, cool-
ing system service, or any other type of repairs?
❑
Did the problem start suddenly, or gradually develop?