A/C - Blown Fuse / Low Voltage Short

Air Conditioner

Re: Blown Fuse / Low Voltage Short

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

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 furnace or air handler.
  • No Short --> Step 3. Test A/C 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 A/C Wiring

At the Furnace or 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 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 A/C
    • No Short --> Re-connect the Y control wire, disconnect the Y wire from the contactor and retest.
      • No Short = Bad Contactor 
      • Short = Shorted Pressure Switch Wiring

4. Test Thermostat

• Turn off line-voltage power to the furnace or 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 Y going to the stat.
    • No Short --> Suspect an Intermittent Short in Control Wiring - Trace the control wires & look for spots where control wiring contacts metal.
    • Short On Any Wire= Shorted Control Wire to Stat