PREVOST H3-41 Maintenance Manual page 925

TROUBLE
Reduced air flow:
a. Dirty or clogged air filter;
b. Evaporator motor inoperative; or
c. Plugged return air ducts.
Frequent starting and stopping on low pressure
control switch.
Compressor intermittently starts and stops.
Non-condensable in the refrigeration system.
Testing condenser pressure.
NOTE
: R-134A pressure varies according to temperature.
Example, for an exterior temperature of 100°F.
Exterior temperature (100°F) + 30°F = 130°F.
Take note: 30°F is added to ambient temperature by definition.
Refer to paragraph "5.8 Temperature & Pressure".
Note the corresponding pressure for a temperature of 130°F, 199.8 psi.
Read the condenser pressure, example 171.9 psi.
171.9 psi & 199.8 psi, the pressure in the condenser is inferior to the pressure corresponding to the
exterior temperature, in this case the condenser pressure may be too low. Check for refrigerant leaks
and add refrigerant if necessary. If the pressure corresponding to the condenser temperature is
superior to the pressure corresponding to the exterior temperature, then the air cooled condenser
pressure may be too high. Most frequent causes are:
Reduced air quantity. This may be due to:
Non-condensable in system;
*
Dirt on the coil;
*
Restricted air inlet or outlet;
*
Dirty fan blades;
*
Incorrect rotation of fan;
*
Fan speed too low;
*
Fan motor going out on overload; or
*
Prevailing winds.
*
Too much refrigerant in system. Remove refrigerant if necessary.
*
SECTION 22: HEATING AND AIR CONDITIONING
external equalizer line, or filter-dryer.
Dirty or iced evaporator coil. Clean air filter
screen. Check return ducts for obstructions.
Check blower motor.
Lack of refrigerant. Check for leaks. Recharge.
Intermittent contact in electrical control circuit.
Compressor valves not in operating position.
Leak on system, system in vacuum in low temp.
Specific symptom, pressure in system will not
correspond to ambient temperature on shutdown.
Only non-condensable will cause this.
(Example: Pressure of idle R-134a system in
80°F (26.6°C) room should be 86.4 psi (595.7
kPa). See temperature chart in this section.)
An evaporator just does a proper cooling job
without sufficient air. Shortage of air can be
caused by the following:
o Dirty filters; or
o Dirty coils.
PA1621 Maintenance Manual All Series | Revised February 2019
CAUSE
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