M09010 05/02 Air Conditioning System M9-27
for HFC 134a Refrigerant
TROUBLESHOOTING
PRE-DIAGNOSIS CHECKS
If the system indicates Insufficient cooling, or no cool-
ing, the following points should be checked before pro-
ceeding with the system diagnosis procedures.
NOTE: If the truck being serviced is a Model 930E, be
certain the Rest Switch in the cab is ON. Place the GF
Cutout Switch in the CUTOUT position. (Refer to Fig.
3-1, Page E3-2, Propulsion System, for switch
location.)
PREPARING FOR DIAGNOSIS
Successfully servicing an air conditioning system,
beyond the basic procedures outlined in the previous
section, requires additional knowledge of system test-
ing and diagnosis.
A good working knowledge of the manifold gauge set is
required to correctly test and diagnose an air condition-
ing system. An accurate testing sequence is usually
the quickest way to diagnose an internal problem.
When correctly done, diagnosis becomes an accurate
procedure rather than guesswork.
• Compressor Belt - Must be tight, and aligned.
• Compressor Clutch - The clutch must engage. If it
does not, check fuses, wiring, and switches.
• Oil Leaks - Inspect all connection or components
for refrigeration oil leaks (especially in the area
of the compressor shaft). A leak indicates a
refrigerant leak.
• Electrical Check - Check all wires and connec-
tions for possible open circuits or shorts. Check
all system fuses.
Note: Some systems use different safety devices in the
compressor circuit to protect the compressor. Check
the thermal fuse, the low pressure cutout switch, high
pressure cutout switch or trinary pressure switch if
equipped.
• Cooling System - Check for correct cooling sys-
tem operation. Inspect the radiator hoses,
heater hoses, clamps, belts, water pump, ther-
mostat and radiator for condition or proper
operation.
• Radiator Shutters - Inspect for correct operation
and controls, if equipped.
• Fan and Shroud - Check for proper operation of
fan clutch. Check installation of fan and shroud.
• Heater/Water Valve - Check for malfunction or
leaking.
• System Ducts and Doors - Check the ducts and
doors for proper function.
• Refrigerant Charge - Make sure system is prop-
erly charged with the correct amount of refriger-
ant.
PRELIMINARY STEPS
The following steps outline the correct procedures nec-
essary to prepare the truck and the system for testing
and diagnosis:
1. Correctly connect the manifold gauge set to the
system. Refer to the connection and purging pro-
cedures outlined in this section.
2. Run the engine with the air conditioning system
on for five to ten minutes to stabilize the system.
3. With the engine and the system at normal operat-
ing temperature, conduct a Performance Test as
outlined in this section.
SYSTEM PERFORMANCE TEST
This test is performed to establish the condition of all
components in the system. Observe these conditions
during testing:
1. Start engine and operate at 1200 to 1500 RPM.
2. Place fan in front of condenser to simulate normal
ram air flow and allow system to stabilize.
3. Place a thermometer in air conditioning vent clos-
est to evaporator.
4. Evaluate the readings obtained from the gauges
to see if they match the readings for the ambient
temperature.
As preliminary steps to begin checkout of the system,
perform the following:
1. Close all windows and doors to the cab.
2. Set air conditioning system at maximum cooling
and blower speed operation.
3. Readings on the two manifold gauges should be
within normal range, adjust for ambient tempera-
ture.
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