AC thermostat blank, no compressor either

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Old 07-22-14, 06:03 PM
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AC thermostat blank, no compressor either

Gents, hope someone can help me fix my GF's AC Have a Trane system, about 11-12 years old. One morning the thermostat was blank and no cold air. Breakers were NOT tripped, but cycling them a few times gets the thermostat digital display to come on. But when the AC kicks on, it cuts out a few seconds later. Repeat the cycling, and it does the same. Compressor will run when manually pushing the contactors in. If i watch the contactors while the thermostat is activated, they go in and out (chattering) a few times then it all goes dead. Breakers are NOT tripping. I am obviously resetting something? Thats as far as my limited experience took me Any ideas? And thanks in advance for your time and help.

ps, there WAS some clogging of the condensate drain a month ago, but its been fixed since using a shop vac on it. No water in the tray of the indoor unit now.

Taystat 570 thermostat
Trane XR12 outside unit
 
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Old 07-22-14, 07:56 PM
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Does it do this if you disconnect the 2 stat wires to the condenser outside?


If it stops doing this when you disconnect these 2 wires, do you see any shorts in the low voltage wires to the pressure switches?
 
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Old 07-23-14, 06:44 AM
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All wiring looked ok, no signs of insects getting in it or anything chewing on it. I wiggled and tugged everything i could to see if it made a difference. Kind of confused. If i disconnect the stat wires, how will i turn it on? I have a background in automotive DC electricity, so in other words, i know just enough to know i don't know what i am looking at
 
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Old 07-23-14, 07:24 AM
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Wondering if T-stat batteries are very weak?
Is the low control voltage sub-par?
 
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Old 07-23-14, 07:29 AM
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From what i read, i don't THINK it has batteries. But i would LOVE to be proven wrong
 
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Old 07-23-14, 07:52 AM
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i don't THINK it has batteries
The owner's manual says it has no batteries.

TRANE TAYSTAT TAYSTAT 570 OWNER'S MANUAL Pdf Download.
 
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Old 07-23-14, 09:11 AM
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If you remove the thermostat, you can turn the system on by "jumping" some of the thermostat wires together. To turn the A/C on, connect the Red, Yellow, and Green wires (or the wires going to those terminals on the thermostat) together. Red is the 24VAC from the transformer, Yellow is the control signal for the compressor/condenser (outside unit), Green is the control signal for the air handler (indoor blower).
 
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Old 07-23-14, 10:04 AM
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I was talikng about disconnecting the 2 low voltage wires outside that carry the 24 volt signal from your thermostat.

The problem is rarely the thermostat itself but looking at your thermostat in your link it is higher on my list of possible causes.
 
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Old 07-23-14, 11:02 AM
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Some great info, thanks guys! Going there after work. One more question, then i should leave you alone till tomorrow How do i remove the thermostat to look behind it? I pulled and pried lightly, but didn't want to break it. At my house i have 6 5000 BTU window units. When one fails, i pick up another on sale for around $80 So this (GF's nice house) is new to me.....
 
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Old 07-26-14, 09:20 AM
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OK, some new info. Switched out the stat with an identical one on the homes second AC unit. No change. Where the stat wires come into the compressor panel, i tied yellow to red, compressor kicks on, contactors chatter, system goes dead. Green light in computer board stops flashing, stat goes blank. It resets in time, or cycling the breakers does it faster. I thought i was looking at the transformer under the contactors, but not so. So i have no idea WHERE to check for 24V? I do have 24 at the stat. was going to shotgun it, and change the capacitor, the contactors and the transformer, but since i don't SEE a transformer, guess i should diagnose a little more
 
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Old 07-26-14, 09:32 AM
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Ok..... you're bouncing all over the place. We need to determine if the problem is inside or outside before throwing un-needed parts at the repair.

I'm assuming you have a gas furnace when you mention control board with blinking light. A make and model could be helpful. You should see a wiring terminal strip in there with C, R, G, Y, W, etc. You are going to need to identify the wiring that comes from the t'stat and the wiring that goes to the condensor outside.

We're going to need to disconnect the outside unit temporarily and check for 24vac between R and C and between Y and C.
 
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Old 07-26-14, 09:55 AM
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Its a Trane XR12. No gas. Computer board is about 5 by 5. Going to try and post pics.
 
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Old 07-26-14, 10:50 AM
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Where the stat wires come into the compressor panel, i tied yellow to red, compressor kicks on, contactors chatter, system goes dead.

Sounds like a contactor problem on that heat pump.
Do you have an ohm meter?

That computer board is a defrost control.
 
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Old 07-26-14, 10:59 AM
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I do....Thanks for sticking with me.....
 
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Old 07-26-14, 11:16 AM
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Turn off power to the indoor unit and outdoor unit, unplug the 2 low voltage wires to the contactor and get an ohm reading across the low voltage connections at the contactor.
 
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