4TWR3 - Defrost Problem / Outdoor Coil Frozen

Trane Heat Pump: 4TWR3

Re: Not Defrosting / Frozen

The 4TWR3 models use a Demand Defrost control board.

The board uses an Ambient (outdoor) Sensor and Liquid Line Sensor to determine the proper conditions for defrost to occur.

CHECKOUT:

1. Check Outdoor Fan

• Inspect the outdoor unit and make sure the outdoor fan is operating.
  • Fan Operating → Step 2. Test Defrost Cycle
  • No Fan ⇒ Check for 230VAC to the fan motor from the defrost board fan relay (Black) to the capacitor "C" (Purple).
    • No Power = Bad Control Board or Unit in Defrost
    • Power + No Fan Operation = Bad Fan Motor or Capacitor

2. Test Defrost Cycle

• Ensure that the Lite-Port LED is lit or flashing.
  • No Light = Bad Defrost Control or No 24VAC Power ⇒ Check for 24VAC between "R" and "B". (Unit will operate but not defrost without 24V power.)
• Jump the TEST_COMMON to FRC_DFT pins to force the unit into defrost.
• Defrost Sequence
• Compressor & outdoor fan shut-off and the reversing valve energizes.
• Compressor re-starts after approx. 20 seconds (outdoor fan stays off).
• Unit stays in defrost until liquid line sensor reads 47 or 70 deg. (See Step 6. Defrost Termination Temperature)
• Outdoor fan restarts for several seconds.
• Compressor shuts-off & the reversing valve de-energizes.
• Outdoor unit restarts after approx. 20 seconds.
  • Defrost Cycle Initiates & Terminates Normally → Step 3. Check Refrigerant Pressures
  • Unit Enters Defrost & Exits Quickly → Step 4. Check Liquid Line Sensor
  • Unit Will Not Initiate Defrost → Step 5. Check Ambient Sensor

3. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue --> "True Suction" Low Pressure
  • Red --> High Pressure (either refrigerant line)
• Monitor pressures as the system operates.

• If a Liquid-Line Sight Glass is installed, observe for bubbles in the sight glass (after the system has been operating for at least 5 min.)
  • Bubbles = Low Refrigerant Charge
• Low Pressure Issues
  • Low Suction + Low Discharge + Bubbles in Sight Glass + Expansion Valve Frosting = Low Refrigerant Charge
    • Try adding refrigerant through the Low Pressure gauge.
      • Suction and Discharge Pressures should gradually increase as refrigerant is added.
      • If the Discharge Pressure increases but the Suction stays low or drops = Bad Expansion Valve
  • Low Suction + Low Discharge + Clear Sight Glass + Expansion Valve Frosting = Bad Expansion Valve 

4. Check Liquid Line Sensor

• Disconnect high voltage power from the unit by pulling the service disconnect or turning the breaker off.
• Disconnect low voltage from the defrost board by removing the Red, "R" wire.
• Disconnect the plug from the defrost board connected to "COIL" and "AMBIENT".
• Set the multi-meter on 200K Ohms.
• Test resistance of the COIL sensor.
• Use Resistance Chart to determine sensed temperature.
• Compare sensed temperature to actual temperature.
  • If sensed temperature is more than 5 deg. higher or lower than actual temperature = Bad Liquid Line Sensor

5. Check Ambient Sensor

• Disconnect high voltage power from the unit by pulling the service disconnect or turning the breaker off.
• Disconnect low voltage from the defrost board by removing the Red, "R" wire.
• Disconnect the two wires from the defrost board connected to "COIL" and "AMBIENT".
• Set the multi-meter on 200K Ohms.
• Test resistance of the AMBIENT sensor.
• Use Resistance Chart to determine sensed temperature.
• Compare sensed temperature to actual temperature.
  • If sensed temperature is more than 5 deg. higher or lower than actual temperature = Bad Ambient Sensor
  • Ambient Temperature reading less than 50 deg and the board will not initiate defrost = Bad Defrost Board

6. Defrost Termination Temperature

• The defrost control board has an adjustable termination temperature.
• If everything tests correctly but the unit still has defrost issues, you can increase the defrost termination temperature.
• As shipped from the factory, the termination temperature is 47 deg.
• Clipping J2 (above the Test pins) changes the termination temperature to 70 deg. (Longer defrost cycle)