Turning on AC either resets thermostat or blows fuse

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Old 05-17-18, 06:10 PM
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Turning on AC either resets thermostat or blows fuse

I have an 10-year-old American Standard Freedom 90 HVAC system, with an outdoor AC compressor. My thermostat (Honeywell RTC6580WF 1001) stopped being able to turn the AC on (it could still turn the fan and heat on), so I called out an HVAC guy. He couldn't find anything wrong with the contactor (but he replaced it with a sealed one) or the cap on the compressor, and upon further investigation he was able to get the AC to start by manually shorting the R, G, and Y wires behind the thermostat. From that he determined that the thermostat was bad.

I replaced the thermostat with a new one (same model), but when it tries to start the AC it immediately turns off for a few seconds (it's powered by the C wire) and then turns back on. It sits there blinking that it's waiting for the compressor protection timer, and then after 5 minutes it tries to start the AC again and reboots again and the cycle repeats.

I pulled it back off the wall and tested the voltages, and I'm getting a little over 26V between R and all the other wires. However, with the thermostat off, if I short R and Y, it now blows the 5A fuse on the furnace's control board. I've tried this twice, with the same result.

Any ideas before I spend another $150 on a service call?
 
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Old 05-17-18, 06:17 PM
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Welcome to the forums.

Your tech missed the problem. It's a short in the line going out to the condenser. As a guess..... it's between the house and the unit. It could also be one of the wires touching a sharp piece of metal inside the condenser.

To test..... locate the splice/wirenuts where the thermostat cable from the house connects to the condenser. It may be behind a wiring access door where the main power wiring enters. You can power down the outside unit for testing. Remove the wirenuts and open the splices. Now put stat to cool and see what happens.

If the fuse doesn't blow... it's a shorted wire inside the condenser.
If the fuse blows.... look closely at the cable outside. Animals chew on them as well as weedwackers.
 
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Old 05-17-18, 06:42 PM
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I can check that in the morning. In the meantime, I disconnected the line going to the condenser from the control board in the furnace and it only measures 1 or 2 ohms. I assume that there is supposed to be more resistance across this if it isn't shorted?
 
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Old 05-17-18, 06:57 PM
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1 or 2 ohms is considered a short.
It should be at least 15-50 ohms depending on make of contactor.

If not automatic ohms..... use the lowest ohms setting. Rx1 typically.
 
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Old 05-17-18, 07:34 PM
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I got curious and went out tonight. Found an intermittent short across the control wires at the compressor end. Pulled out the wire and found this:



The short is too far back to splice it inside the compressor housing. Any suggestions for an outdoor splice, or should I splice in a new wire inside the house and run it outside?
 
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Old 05-17-18, 11:02 PM
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Someone was very hungry. I'd replace it and splice it inside the house.
 
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