WSH - Defrost Problem / Outdoor Coil Frozen

Trane Rooftop Heat Pump: WSH

Re: Not Defrosting / Frozen

This model uses a Reliatel RTRM module to control defrost.

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 in normal heating mode.
  • Fan Operating → Step 2. Test Defrost Cycle
  • No Fan ⇒ Check for high voltage power to the fan motor(s) on the Purple and Black wires.
    • Power Present + No Fan Operation = Bad Fan Motor or Capacitor ⇒ Power-off the unit and test the capacitor(s).
    • No Power = Bad Fan Relay or Unit in Defrost

2. Test Defrost Cycle

• Ensure that the LED on the RTRM is lit and not flashing.
  • Flashing = Test Mode or Fault
  • No Light = Bad RTRM Module or No 24VAC Power ⇒ Check for 24VAC between "R" and "Com".
• On the brown terminal strip on the right-hand of the control cabinet, use a jumper to Jump "Test 1" and "Test 2" until the unit enters Test Mode.
  • The LED on the RTRM Module should start flashing and the Indoor Blower Motor should energize (Test Mode - Step 1).
• Each time you jump the Test terminals will advance to the next Step.
• Advance through Steps until the unit enters Defrost.
• If a Step is unavailable (doesn't exist or is in a fault condition), it will skip to the next Step.

• Unit enter and stays in defrost but the coil does not thaw → Step 3. Check Refrigerant Pressures
• ​​​​​Unit enters Defrost Mode but exits quickly → Step 4. Check Liquid Line Sensor

3. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
• Monitor pressures as the system operates in normal heating mode.
  • ► Be aware that the suction pressure will run low if the outdoor coil is frozen.

• Low Pressure Issues
  • Low Suction + Low Discharge ⇒ 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 Heating Expansion Valve 

4. Check Liquid Line Sensor

• Turn-off power to the unit.
• Disconnect the J2 plug (top-left) from the RTRM Module.
  • ​​​​​​​The Liquid Line (coil) Sensor should plug into pins 3 & 4.
• Set the multi-meter on 200K Ohms.
• Test resistance of the 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 (Outdoor) Sensor

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