91
Over-the-Road Mechanical Diagnosis
If the desired box temperature cannot be obtained, any of
the following may be indicated:
1. EXCESSIVE HEAT LOAD. An excessive heat load
on the system will be caused by too many, or exces-
sively long, stops with the doors open. Excessive heat
loads will also be caused by loose doors, loose body
panels, warm loads and poor insulation.
2. DIRT ON COILS. Dirt on the condenser or evaporator
coils acts as an insulator reducing the capacity of the
unit.
3. INCORRECT BELT TENSION. If the drive belt is not
correctly tensioned, the compressor will not be driven
at the proper speed, and unit efficiency will be reduced.
By contrast, too much tension will place an additional
load on the bearings causing rapid wear.
4. SHORTAGE OF REFRIGERANT. Shortage of refrig-
erant reduces the capacity of the unit. Find and remedy
the cause of the shortage and recharge the system. DO
NOT operate the unit if there is an indication of low
charge. DO NOT operate below 30 F (-1 C) box tem-
perature if the refrigerant level is below the sight glass
on the receiver tank.
5. FAULTY EXPANSION VALVE ADJUSTMENT. High
superheat settings will starve the evaporator causing
low suction pressure. Low superheat settings will flood
the coil causing high suction pressure. The superheat
setting should be adjusted ONLY by a trained refrigera-
tion service technician.
The superheat setting is 8 F (4.4 C) at 0 F (-18 C) box
temperature.
6. EXCESSIVE OIL. Too much compressor oil in the
system may result in lower than normal suction pres-
sure as well as lowered capacity.
7. MOISTURE IN THE SYSTEM. Symptom: Expansion
valve freeze-up—will not refrigerate. Usually this can
be checked by warming the expansion valve with either
a hand or hot towels to see if the valve opens. Evacuate
the system in the same manner used during installation.
Install a new drier.
8. EXPANSION VALVE LOSES ITS CHARGE. If the
expansion valve loses its charge, the valve will close
causing the system to go into vacuum. Replace the
valve.
9. AIR IN THE SYSTEM. Air is not condensable. Its
presence in the system increases head pressure. When
the compressor is stopped, air will gather at the high
point of the high side. Evacuate the system.
10. TEMPERATURE OF THE LIQUID LINE. During
normal operation, the liquid line will be slightly
warmer than the surrounding air. An extremely hot line
indicates either a shortage of refrigerant or a lack of a
liquid seal at the receiver outlet. A cold line indicates a
restriction, and some flashing takes place in the liquid
line sight glass.
11. DIRTY OR WET DRIER. If the outlet line of the drier
is colder than the inlet line, the drier is either saturated
with moisture or is dirty and must be replaced.
12. DIRT IN THE EXPANSION VALVE SCREEN.
Recover the refrigerant charge, remove the screen and
clean. If the moisture is in the refrigeration system, it
will collect at the expansion valve and freeze. This is
indicated by abnormally low suction pressure. Clean
the system, replace the drier, evacuate the system pres-
surize and check for leaks. If no leaks are found, charge
the system.
13. ICE ON THE EVAPORATOR COIL. Run the unit
through a defrost cycle to remove the ice.
14. AIR FLOW. Do not load product directly in front of the
air return or discharge. Ensure that the fan is correctly
positioned in the orifice to achieve maximum air flow.
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