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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?

EDIT: I think I found the problem: https://photos.app.goo.gl/yeGaGdQPyAbwvw2x8

I heard scurrying when I first approached the compressor, so it's likely the cable became rodent food at some point. It's too far back to splice inside the compressor housing, so I am going to either need to figure out how to do a weatherproof splice or splice it inside the house and run a new wire through the wall.

  • What happens if you measure the resistance from Y to C, then disconnect the compressor from the furnace board and try again? – ThreePhaseEel May 18 '18 at 0:54
  • Sounds like a C wire problem, maybe it's not really using the C wire for power, maybe it formerly leached power through the old contactor, but the new contactor doesn't allow enough current to flow. I'm guessing he switched to an electronic contactor? – Harper - Reinstate Monica May 18 '18 at 1:10
  • I get 1 or 2 ohms from Y to C without disconnecting anything else. @ThreePhaseEel when you say disconnect the compressor from the furnace board, are you referring to the second set of wires attached the the "Y" and "C" terminals? – Zan Hecht May 18 '18 at 1:24
  • @Harper it's a WiFi thermostat, it's definitely using the C wire for power (it has a big sticker on it that you must have a C wire installed for it to work). The new contactor is a sealed white box, as opposed to the exposed voice coil on the old one. – Zan Hecht May 18 '18 at 1:27
  • @ThreePhaseEel When I remove the second set of wires on those terminals, I get about 1600 ohms between "Y" and "C". – Zan Hecht May 18 '18 at 1:31
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The cable to your outdoor unit's shot

You'll have to replace or repair the control cable to your outdoor unit -- it's shorted out, which is what's causing the fuse to blow. (If you can't replace it immediately, some electrical tape to insulate the individual wires should get you back going for now -- as to replacements, I'd recommend running the replacement thermostat cable in some liquidtight flexible metal conduit, given that the rodents in your area pay no heed to 24VAC.

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First I would make sure all the wires are hooked up correctly. Look at the thermostat, what color is hooked to R, Y, C, G,W and whatever else you have. Then go to the furnace and make sure those same wires are hooked up the same at the furnace. There is a cable going to the condensing unit outside from the furnace. One wire should be hooked to Y and the other to C on the furnace. The other end should be hooked to the contractor outside. Also make sure none of these wires are grounding out. Make sure the new contractor is 24 volts at the coil. They make them with 120 and 240 volt coils. You say the contractor is white? The only white ones I have seen are special purpose. Usually they are brown or black. I would be interested in seeing a picture with the label. My guess is something got wired back up wrong with all the fiddling.

  • The colors all match at the thermostat and the control board. I've got a picture of the contactor at photos.app.goo.gl/dEXUeVYoyUJtuZ6o2 . Looks like a Trane RVAH21AEKL (similar to shortyshvac.com/… ). – Zan Hecht May 18 '18 at 2:04
  • That explains the confusion. That is a potential relay and is used to remove the start capacitor from the circuit after a few milliseconds so as not to burn out the motor. It looks really old, it sounded like it was recently replaced – user76730 May 18 '18 at 2:16
  • If that were the problem it would either keep the start capacitor in the circuit too long and burn out the compressor or fail to engage the start capacitor in which case it would either start anyway or hum until the thermal overload in the compressor tripped. – user76730 May 18 '18 at 2:18
  • Oops, you're right. In the darkness I took a picture of the wrong part. The new contactor is black: photos.app.goo.gl/9QR04TI5vCrz9Zpu2 – Zan Hecht May 18 '18 at 2:41
  • However, I think I found the issue: photos.app.goo.gl/yeGaGdQPyAbwvw2x8 I heard scurrying when I first approached the compressor tonight, so my control cable likely became rodent food at some point. – Zan Hecht May 18 '18 at 2:41

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