Applicable Systems
Carrier Transicold Refrigeration Unit
Symptoms
The evaporator coil is icing up. The defrost cycle is not effectively clearing the frost, and airflow through the coil is restricted, reducing cooling capacity.
Possible Causes
- Defrost heater open-circuited or partially failed
- Defrost termination thermostat stuck, ending the defrost cycle too early
- Defrost timer schedule too infrequent for the operating conditions
- Evaporator fan failed, causing coil to operate below freezing with no airflow
Troubleshooting Steps
- Step 1: Check the defrost heater resistance with a multimeter
Replace if open or more than 10% different from rated resistance - Step 2: Test the defrost termination thermostat by checking continuity at cold and warm temperatures
Replace if it does not close at the defrost termination temperature (typically 10-15C) - Step 3: Verify the defrost timer or controller schedule
Increase defrost frequency if frost builds up between cycles (e.g., 3-4 times per day) - Step 4: Check the evaporator fan operation and replace if not running
A failed fan causes the coil to freeze because there is no airflow to carry heat
How to Reset
Reset: After the defrost system is repaired, manually initiate a defrost cycle and verify the coil clears completely.
Common Replacement Parts
- Defrost heater: 240V, 500-1500W, match to evaporator (Verify electrical connections)
- Defrost thermostat: Spare tire type, 10C close (Match to evaporator design)
- Evaporator fan: 230V AC, shaded pole (Match to evaporator model)
Frequently Asked Questions
- Q: How often should defrost run?
Typically 2-4 times per day, 20-30 minutes each, depending on humidity and door openings. The defrost should be just long enough to clear all frost.
- Q: What causes coil icing?
Low airflow (dirty coil, failed fan), insufficient defrost frequency, defrost heater failure, or high humidity infiltration. Check the simplest causes first.
- Q: How do I know defrost is working?
During defrost, the evaporator fan stops, the compressor may stop, and the defrost heater energizes. The coil should be clear of frost within 20-30 minutes.
Source: OEM Service Manual
Category: Refrigeration