33
2019 Amazon P80 Package Delivery Vehicle
Ambient Temperature vs. Relative Humidity
Figure 6
Typical louver temperatures
as influenced by the combi
-
nation of Ambient Tempera-
ture and Relative Humidity
PROBLEM B-2,
A/C system not providing cool air
POSSIBLE CAUSE #1
- Loss of refrigerant
CORRECTIVE ACTION
- Verify the presence of 1.75 pounds of refrigerant R134a. If the A/C sys-
tem is either low, or completely empty, a search will be required for leakage. Replace and repair as
required.
POSSIBLE CAUSE #2
- Compressor clutch not engaged
CORRECTIVE ACTION
- Confirm that the system is fully charged. With the ignition switch turned
to
Accessory
, the blower switch turned to the highest speed and the A/C (blue) rocker switch en-
gaged, proceed as follows:
Step 1
Check for the presence of 12 volts at the compressor. If yes, make certain that the com-
pressor clutch is fully grounded. If the ground circuit is intact, the compressor clutch has, most
likely, failed and requires replacement.
Step 2
If no voltage is present at the compressor clutch, refer to the
Wiring Diagram
and check for
voltage at each of the system pressure switches. [
Note
: The low pressure switch is located on the
suction hose near the firewall and opens on a pressure drop to 8 psi. The binary (high/low) pressure
switch is located on the liquid hose near the receiver/drier and opens on a pressure rise to approx
-
imately 400 psi or a pressure drop to 28 psi.] With the system fully charged, there should be conti-
nuity through both switches. Replace pressure switches as required.
Step 3
If no voltage is present at the pressure switches, check for voltage at the thermostat (locat-
ed externally on the HVAC housing). Assuming that the evaporator coil is fully warmed to ambient
conditions, the thermostat should be a closed circuit. [
Note
: The thermostat circuit opens when the
evaporator coil surface temperature drops below 32 degrees F; this action prevents the accumula-
tion of ice on the evaporator coil surface.] If the thermostat presents an open circuit, replacement
is required. Thermostat replacement requires the complete removal and disassembly of the HVAC
housing. When replacing thermostat, take special care to not kink the capillary tube. Also, install
the new capillary tube into the coil's fin surface in the same location as the original.
POSSIBLE CAUSE #3
- Perceived lack of cooling due to extreme conditions
CORRECTIVE ACTION
- Note that extremely high humidity can reduce the effectiveness of the
evaporator. The
Ambient Temperature vs. Relative Humidity
chart
(Figure 6) illustrates how very
high humidity can raise the louver temperatures. This test is best performed with the doors and wid-
ows open, the blower turned to the highest speed and the A/C system fully engaged. Run the en-
gine at 1500 rpm and allow time for the system to stabilize. Measure the discharge air temperature
at one of the dash louvers and compare to the data on Figure 6.
Summary of Contents for MT45 2019
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