Trane - Defrost Problem

Article ID: 149
Last updated: 13 Mar, 2023

Trane Heat Pump: 4TWB, 2TWR, 4TWR

Re: Defrost Problem / Frozen Outdoor Coil

The outdoor unit becomes completely covered in thick, white ice.

Qty Possible Causes:
Bad Coil Sensor
1 Mis-Located Coil Sensor
Inoperative Condenser Fan Motor
Bad Defrost Board
Low Refrigerant
Bad Reversing Valve
Stuck Contactor
Bad Ambient Sensor


1. Check Unit Operation

  • Turn thermostat to the System "Off" position.
    • Confirm that the compressor stops running.
      • Compressor Continues to Run = Stuck Contactor
  • Restart the unit in regular heating mode.
    • Ensure that the compressor and condenser fan are operating normally.
    • No Condenser Fan operation:
      • Make sure unit is not in Defrost Mode
      • Heating Mode & No Outdoor Fan = Bad Fan Motor or Capacitor

2. Test Defrost

  • Normal Operation = LED flashing once per second.
  • While running in the heating mode, jump TEST_COMMON and FRC_DFT on the defrost board.
    • Fan Shuts Off but Reversing Valve Doesn't Switch = Bad Reversing Valve, Coil or Wiring
    • Goes Into Defrost Mode Normally + Stays In Too Long ⇒ Suspect a Misplaced Coil Sensor → Step 3. Check Sensors
    • Goes Into Defrost Mode but Quickly Goes Back Out = Sensor Problem → Step 3. Check Sensors
    • Does Not Go Into Defrost Mode = Bad Defrost Board or Ambient Sensor → Step 3. Check Sensors

3. Check Sensors

  • The coil sensor mounts in a copper sleeve brazed on one of the coil feeder tubes.
  • Ensure that the coil sensor is secure inside the sensor sleeve.
    • It is not uncommon for the wire-tie to break and the sensor to come out of the sleeve.
  • The Coil and Ambient Sensors are10Kohm sensors (see attached Sensor Resistance chart).
  • Compare Coil Sensor resistance to actual coil temperature.
  • Compare Ambient Sensor resistance to actual outdoor temperature.

4. Check Refrigerant Charge

  1. Check "True Suction" and "Discharge" Pressure
    • Watch for premature freezing of the cap tubes and expansion valve = Low Charge or Underfeeding Valve
    • If a liquid line sight glass is installed, observe for bubble in the sight glass = Low Charge
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Article ID: 149
Last updated: 13 Mar, 2023
Revision: 9
Access: Public
Views: 931
Comments: 0
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