Carrier 69NT40-541-505, Operation And Service Manual, Page: 89 / 143

6–3 T-368
6.6 UNIT WILL NOT DEFROST PROPERLY
Condition Possible Cause Remedy/Refer-
ence Section
Will not initiate defrost
automatically
Defrost timer malfunction (Cd27) Table 4–5
Loose terminal connections Tighten
Defective wiring Replace
Defrost temperature sensor defective or heat termination
thermostat open
Replace
Heater contactor or coil defective Replace
Will not initiate defrost manually
Manual defrost switch defective Replace
Defrost temperature sensor open 7.21
Initiates but relay (DR) drops out Low line voltage 3.3
Initiates but does not defrost
Heater contactor or coil defective Replace
Heater(s) burned out 7.13
Frequent defrost Wet load Normal
6.7 ABNORMAL PRESSURES (COOLING)
Condition Possible Cause Remedy/Refer-
ence Section
High discharge pressure
Condenser coil dirty 7.10.1
Condenser fan rotating backwards 7.11
Condenser fan inoperative 7.11
Refrigerant overcharge or noncondensibles 7.7.1
Discharge service valve partially closed Open
Suction modulation valve malfunction 7.17
Low suction pressure
Suction service valve partially closed Open
Filter-drier partially plugged 7.12
Low refrigerant charge 7.7.1
Expansion valve defective 7.16
No evaporator air flow or restricted air flow 7.13
Excessive frost on evaporator coil 6.6
Suction modulation valve malfunction 7.17
Suction and discharge pressures
tend to equalize when unit is
operating
Heat exchanger defective Replace
Compressor valves defective 7.8
Compressor cycling/stopped Check
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