Ameristar Heat Pump: M4HP4

Re: Defrosting Problem / Outdoor Coil Frozen

The M4HP4 uses a "Hybrid" Demand Defrost control board with a timer.

The board uses an Ambient (outdoor) Sensor, a Liquid Line Sensor and an adjustable 30 or 60 min. timer to determine when defrost should occur.

CHECKOUT:

1. Check Contactor 

• If the outdoor unit is not operating → Step 2. Check Outdoor Fan
• If the outdoor unit is running, check for 24VAC across the contactor coil.
  • No Power + Unit Running = Stuck Contactor

2. Check Outdoor Fan

• If the outdoor unit is not operating, set the thermostat to call for heat.
• Inspect the outdoor unit and make sure the outdoor fan is operating.
  • Fan Operating → Step 3. Test Defrost Cycle
  • No Fan ⇒ Check for 230VAC to the fan motor on the Black wire (at the contactor) and the Black wire on [FAN] at the Defrost Board.
    • No Power = Bad Defrost Board or Unit in Defrost
    • Power Present + No Fan = Bad Condenser Fan Motor or Bad Capacitor ⇒ Test Capacitor

3. Test Defrost Cycle

• Force the unit into defrost by setting SW3-1 to ON.
• The unit should enter a Defrost Mode and terminate when the T3 sensor reaches 77 deg or after 10 min.
  • Defrost Cycle Initiates & Exits Normally → Step 5. Check Refrigerant Pressures
  • Unit Does Not Enter Defrost or Enters Defrost & Exits too Quickly → Step 4. Check Liquid Line Sensor 
• Set SW3-1 back to OFF after a manual defrost.

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 terminal plug from the Defrost Board.
• Disconnect the Black [T3] connector from the Defrost Board .
• Set the multi-meter on 200K (not 200) 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
  • If sensor reads correct but terminates defrost too soon ⇒ Look for Mis-Placed Coil Sensor
    • Sensor should be between the O/D Expansion Valve and the Condenser Coil on one of the feeder lines (typically a short stub of 3/8" pipe at the coil).

5. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
  • Blue --> "True Suction" Port (Not the Suction Line)
  • Red --> High Pressure (either refrigerant line)
• Monitor pressures as the system operates.
• Do Not Attempt to add freon while the coil is frozen!

• Allow the system to operate for at least 5-10 minutes.
• Low Pressure Issues
  • Low Suction + Low Discharge + Expansion Valve Frosting = Low Refrigerant Charge or Bad Expansion Valve
    • 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 O/D Expansion Valve

6. Check Defrost Timer

• Unlike most "Demand-Defrost" controls, the M4HP4 Defrost Board has and adjustable timer.
• Check SW3-3 on the Defrost Board.
  • If everything else checks-out normal ⇒ Change the Defrost Timer to 30 Min by setting SW3-3 to ON.