232 Auto Heating & Air Conditioning
Diagnosing Intermittent Problems
If the problem does not occur either in the shop or dur-
ing the road test, it is tempting to dismiss it as the owner’s
imagination or as normal vehicle operation, but the prob-
lem may well be real. Intermittent problems are the most
diffi cult to diagnose, because they usually occur only when
certain conditions are met. Intermittent malfunctions can
be related to temperature, humidity, certain vehicle opera-
tions, or in response to certain tests performed by a vehicle
computer. While most problems in the HVAC system are
usually easily spotted, like other vehicle systems, it can
develop problems that occur intermittently.
When dealing with an intermittent malfunction,
always try to recreate the exact conditions in which the
problem occurred. Unfortunately, most drivers do not relate
intermittent problems to external conditions. Intermittent
problems cannot always be duplicated. If a road test of
reasonable duration does not duplicate the problem, it is
time to try other types of testing. It is essential the principles
of strategy-based diagnostics be followed closely when
diagnosing intermittent malfunctions.
Note: On modern vehicles, body computers
control some of the refrigeration, heating,
and air fl ow components. This is true even
on vehicles with manual controls. Before beginning
diagnosis, use a scan tool to retrieve any trouble
codes and perform other diagnostic tasks. Using a
scan tool can save diagnostic time.
Step 2—Check for Obvious Problems
Most of your time in Step 2 will be spent checking for
obvious causes of the problem, including possible causes
that can be easily tested. Visual checks and simple tests
take only a little time, and might save more time later. As
a minimum, open the hood and check the following items
before you start the engine and HVAC system:
❑
Retrofi t label. A retrofi t label indicates the original
refrigerant has been replaced with a substitute. If the
retrofi t was done properly, the service fi ttings should
be different from the originals. Always use a refrigerant
identifi er whether a retrofi t label is present or not.
❑
Service fi ttings. The type and style of the service fi ttings
are the other indication the system may have been ret-
rofi tted to another refrigerant. Note the size, shape, and
location of the high and low side fi ttings. Keep in mind
some vehicles have an additional fi tting that was used
at the factory, and should not be used. Service fi ttings
are also the cause of some refrigerant leaks.
❑
Obvious refrigerant leaks. Since refrigerant oil leaks
out with the refrigerant, leaks can usually be spotted by
the presence of oil at the leak site. Figure 15-4 shows
some typical refrigeration system leak locations.
❑
Belt condition. Check the belt for tightness and condi-
tion. Sometimes you may fi nd the belt is missing. Also
check the condition of the belt pulleys. If the belt is
missing or badly burned, try to turn the compressor by
hand to ensure it is not locked up.
❑
Refrigeration lines and hoses. Check for obvious dam-
age such as frayed rubber or cuts. Also look for kinks
or improper bends in lines and hoses.
❑
Compressor clutch. Check for evidence of slippage,
excessive clearance, and overheating, Figure 15-5. This
check is especially important if the belt is damaged.
Figure 15-5. Make a visual check of the compressor. Typical
problems are a loose and slipping drive belt and signs of refrig-
erant leaks at the front seals or hose fi ttings.
Signs of
clutch
damage
Oil thrown on
nearby parts
Signs of
overheating
Figure 15-4. Leaks can occur at many places in the refrigeration
system. This diagram shows some of the most likely locations on
one kind of vehicle. (Nissan)
Cooling unit Low-pressure
service valve
High-pressure
service valve
Thermal
protector
Compressor
Dual-pressure
switch
Condenser