234 Auto Heating & Air Conditioning
for proper pressures and temperatures. Some of the test
procedures do not apply to every system. Always make
sure you obtain and use the manufacturer’s procedures
and specifi cations for function and performance tests.
The functional test can be performed without gauges
or a refrigerant service center. Start the engine and allow
it to run for fi ve minutes. Then, perform the functional test
steps outlined in the service information.
To make the performance test, shut off the engine,
make sure the transmission is in Park or Neutral, and set
the parking brake. Attach gauges or a refrigerant service
center as shown in Figure 15-7. Ensure the high and low
side hoses are attached properly.
Caution: If there is no refrigerant in the
system, do not attempt the performance
test. Instead go to Step 4.
Once the gauges are attached, check static pressure. A
normally charged system will have 70-125 psi (482-861 kPa)
when it has been inactive for about one hour. If the gauges
show low or no pressure in the system, you can be sure there
is a leak somewhere. Be sure the hoses do not contact any
moving parts.
Install a temperature gauge in the vent nearest the
evaporator, Figure 15-8. Then start the engine and set it to
run at approximately 1500 to 2000 rpm (this will vary by
manufacturer). Turn the HVAC control panel settings to the
maximum cooling position and set the temperature switch
to the maximum cold position. Turn the blower speed
switch to the high position and open the front windows.
Check the compressor clutch to make sure it is engaged.
If the clutch does not engage, shut off the HVAC system and
engine and check the clutch, relay, switches, and wiring.
Basic electrical system checks were outlined in Chapter 4.
If the compressor clutch engages, allow the refrigera-
tion system to operate for about fi ve minutes to stabilize the
gauge readings. Monitor the cooling system gauge or light
to ensure the engine does not overheat. Observe the fan
clutch or fan motor(s) and ensure they are operating and
moving air through the condenser and radiator.
Caution: If the cooling system fans are not
operating, or if the high side pressure
exceeds 325 psi (2467 kPa), stop the
performance test immediately and determine the cause.
Go through all the steps outlined in the service infor-
mation for testing the system. A typical performance test
chart is shown in Figure 15-9.
Note: Gauge readings will vary with tem-
perature, humidity, system design, and type
of refrigerant. For this reason, you should
always refer to the manufacturer’s specifi cations
before deciding the refrigeration system is defective.
Normal Operation
If the system is operating normally, high side pressure
should be between 150 and 300 psi (1034 to 2067 kPa) on a
R-134a system. R-12 high pressures are usually somewhat
lower, about 250 psi (1723 kPa). Low side pressures should
be between 30 and 40 psi (208 and 276 kPa).
To get a rough estimate of the desired high side
pressure, multiply the ambient temperature by 2.2 and by
2.5. If the outside air temperature is 80°F, for example,
the high side pressure should be between 176 to 200 psi
(80 × 2.2 = 176 and 80 × 2.5 = 200). This method does
not work with the metric system, so metric temperatures
and pressures must be converted to U.S. customary
equivalents before calculating this estimate. Note that
actual pressures higher than the estimates obtained by
this method are considered normal, especially when the
humidity is high.
Normal system pressures for each system type are
shown in Figure 15-10. Based on the pressure gauge readings,
you may want to proceed to Step 4 and consult available
diagnostic and trouble shooting charts.
Figure 15-7. This illustration shows a typical refrigerant service
center used for servicing the refrigeration system.
Figure 15-8. A mechanical temperature gauge installed in the
outlet nearest the evaporator provides an accurate reading of
evaporator temperature.