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Heat Pump - Blown Fuse / Low Voltage Short - Cooling

Article ID: 296
Last updated: 16 Nov, 2017

Heat Pump: Cooling

Re: Blown Fuse / Low Voltage Short

Indications: A Blown Low Voltage Fuse or a Bad Transformer on the Air Handler


Qty Possible Causes:
  Shorted Control Wire To Stat
1 Shorted Control Wire To H/P
Bad Contactor
Shorted Pressure Switch Wiring
Bad Defrost Board
Bad Air Handler Board
Bad ECM Blower Motor
Shorted Sensor
Bad Thermostat

CHECKOUT:

1. Install Mini-Breaker

  • Turn off line-voltage power to the air handler or furnace.
  • Remove the blown fuse.
  • Install a resettable low voltage breaker (if not already on the transformer) in the fuse holder.

2. Isolate the Thermostat

  • At the furnace or air handler, disconnect the control wiring to the thermostat. (Leave all other low voltage wiring connected.)
  • Turn line-voltage power back on to the air handler.
    • No Short --> Step 3. Test Heat Pump Wiring
    • Short --> Disconnect R control wire at the outdoor unit & retest.
      • No Short = Bad Defrost Board or Shorted Sensor
      • Short = Shorted Control Wire to H/P

3. Test Heat Pump Wiring

At the Air Handler, use a jumper wire to test each output as follows:

  • Connect R to G = Energize fan relay.
    • No Short --> Proceed to next test.
    • Short --> Disconnect the low voltage wiring to the motor & retest.
      • No Short = Bad ECM Motor
      • Short = Bad Air Handler Board
  • Connect R to O = Energize reversing valve.
    • No short --> Proceed to next test.
    • Short --> Disconnect O control wire at the outdoor unit & retest.
      • No Short = Bad Reversing Valve Solenoid
      • Short = Shorted Control Wire to H/P
  • Connect R to Y = Energize compressor. (Wait until compressor contactor energizes.)
    • No short --> Proceed to next test.
    • Short --> Disconnect Y control wire at outdoor unit & retest.
      • Short = Shorted Control Wire to H/P
      • No Short --> Re-connect the Y wire and disconnect the gray wire from the "M" terminal on the defrost board and retest.
        • No Short = Bad Contactor 
        • Short = Shorted Pressure Switch Wiring
  • Connect R to X/L - Lockout Indication
    • Short = Shorted Control Wire to H/P

4. Test Thermostat

  • Turn off line-voltage power to the air handler.
  • Re-connect the control wiring to the thermostat.
  • Set the thermostat System switch to "Off" and the Fan switch to "Auto".
  • Turn on line-voltage power to the air handler.
    • No short --> Step 5. Test Thermostat Wiring
    • Short --> Remove the thermostat from the sub-base and retest.
      • No Short = Bad or Mis-Wired Thermostat
      • Short = Shorted Control Wire to Stat

5. Test Thermostat Wiring

  • At the Air Handler, use a jumper wire to test each output as follows:
    • R to G going to the stat.
    • R to O going to the stat.
    • R to Y going to the stat.
    • R to L going to the stat.
      • No Short --> Suspect an Intermittent Short in Control Wiring - Look for spots where control wiring contacts metal.
      • Short On Any Wire= Shorted Control Wire to Stat
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Article ID: 296
Last updated: 16 Nov, 2017
Revision: 5
Access: Public
Views: 607
Comments: 0