TROUBLESHOOTING AND TESTING THE AIR
(All Except M915A2 and M916A1)
cool spots exist, replace the receiver-drier.
Before testing the operation of the air conditioning
system, make the following checks:
Make sure the refrigerant compressors
drive belt is not damaged and is correctly
tensioned. Also check the compressor
mountings for tightness.
Check for broken, burst, or cut hoses; also
check for loose fittings on all parts.
Check for road debris build-up on the
condenser coil fins. Using air pressure and
a whiskbroom or a soapy spray of water,
carefully clean off the condenser; be careful
not to bend the fins.
Check the color of the moisture indicator
sight glass. If the color is a deep cobalt blue,
the refrigerant charge is dry. If the indicator
is not blue, the system is contaminated with
moisture; recover the refrigerant, evacuate
the system, replace the receiver-drier, and
add a full refrigerant charge.
If there is not enough airflow, make sure that
leaves or other debris has not entered the
fresh air ports under the windshield. If debris
has entered, it could clog the fins of the
evaporator core, and block airflow.
Also, be sure that all ducts are connected to
the dash louvers and that the air-control
flaps in the heater housing are moving
properly (this requires removal of the right
and center dash panel).
Following is a brief description of symptoms or
conditions that could exist if something goes
wrong with a refrigerant part.
The receiver-drier is normally at outside
temperature. To the touch, the entire length of the
unit should be the same temperature. If noticeable
A blockage at the inlet of the unit will cause high
head pressures; outlet blockages will cause low
head pressures and little or no cooling.
If the moisture indicator is pink or white (showing
that the system is wet), the receiver-drier is
saturated with moisture and must be replaced.
C O O L I N G S Y S T E M
Although they are not physically connected, there
is a close tie between a vehicles air conditioner
and its cooling system. Poor air conditioner
cooling can be the result of a problem in the
If the cooling system does not work correctly, the
heat of the engine will rise to abnormal levels. The
added heat will transfer to the air conditioner,
other underhood parts, and maybe make its way
into the cab. The added heat makes it necessary
for the air conditioner to work harder and at the
same time, it reduces the air conditioners ability
to cool down the air in the cab. Also, if the water
regulating valve isnt closing all the way, heat will
enter the cab, giving the impression that the air
conditioning system is not working.
Refer to the engine cooling section for cooling
Problems that start in the expansion valve show
up as follows: when stuck closed, the evaporator
coil and the expansion valve will be at outside
temperature; when stuck open, both the coil and
the valve will be extremely cold with frost or ice
Because the expansion valve channels are very
small, blockages in the system tend to be found
here (the valve is very sensitive to contamination).
Usually, the contaminant is water; less than a
drop of water is all it takes to make the valve
inoperative. When water reaches the valve, the
extreme cold that results from the pressure drop
freezes the water, forming a block of ice in the
valve. After the system shuts down and the valve
warms up, the ice melts, and the valve operates
again, only to freeze up when moisture returns.