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My A/C is not working. I checked the panel and the LED light code told me that there was a fuse blown or a short in the circuit. I checked and confirmed that the fuse had blown. If the fuse blows again, how do I check for a short or broken wire in the circuit between the panel and the thermostat?

Here is the layout:

Control Panel:
   White   Red/Blue   White   Yellow   Orange   Green   (Wires)
     0       0         0        0        0        0
    Hum      Com       W        Y        R        G     (Terminals)
Thermostat:
   Green     Blue     Red     Yellow   White            (Wires)
     0         0       0        0        0
     G        Rc       Rh       Y        W
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2 Answers 2

Use a Network Cable Tester

To find short- and open-circuit conductors in any low-voltage multi conductor cable, I recommend

  1. isolating the cable from all devices (i.e. disconnect it from the furnace/heatpump/AC and thermostat), and

  2. using a network cable electrical tester and RJ45 breakout adapters to test it.

RJ45 Breakout Adapter

You need two of these. One for each end of the cable.

This is one style:

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Here is another:

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Network Cable Electrical Tester

You need one of these 2-part devices. They are very simple and cost about $10.

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Making the Connections

Attach an adapter to each end of the cable under test (CUT). Decide on a mapping between CUT wire colors and pins 1 to 8 marked on the adapter. The mapping can be arbitrary, but you should use the same mapping at both ends.

Connect one part of the tester to the adapter at each end of the CUT. Depending on the adapters and tester, you may need some short network patch cables to do this. If you do use patch cables, make sure they are straight through and not crossover cables.

Performing the Test

To perform the test, you just switch on the tester and confirm that its lights flash according to the expected pattern. You will need to read your tester's manual to determine the expected pattern and interpret what a deviation from that pattern means in terms of short- and open-circuits. This video shows one of these testers in use including the flash pattern for that particular tester.

Limitations

There are, of course, limitations to this method. Here are the ones I can think of off the top of my head:

  • Limited to 8 conductors at a time
  • will not detect shorts to ground via other cabling
  • can be confused by even very low voltages applied to conductors by shorts to energized circuits (although you'll probably still learn that there's something awry)
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The wire colorings of HVAC systems are used as a guide that not everyone decides to follow when installing systems so make sure to document where each wire goes to before disconnecting. This will at minimum get you safely back to where you started. –  Jason Jun 19 '13 at 17:53
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If it is a break 99% of the time it was caused by someone or something. A wire just doesnt break. So inspect anywhere the wire is exposed. If there is nothing obvious I would take the thermostat off the wall and hook it up at the terminal. You just need a need a short wire or wires. See if it works. This will let you know if its the thermostat/ a/c unit or the wire. If you can not run a new wire you can get a wireless thermostat. Its probably going to be a couple hundred dollars though.

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