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