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Old 07-30-2020, 02:55 PM   #1
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Verify bad AC compressor /no turn on


I have a a Trane XL1200 with what looks like a bad compressor and would like to verify before system replacement. System tripped the breaker with this heat we are having. I reset the breaker but the compressor did not start. Buzzzing with the power on, but the compressor does not start. I ohm out the compressor leads by disconnecting at the contactor and start cap with reading below.

Start Cap , Run, Cap ,contactor and some were replaced. Should I check these measurements at the compressor to be sure ?

Leads to Ground

Run to grd 2.6 ohm
Start to grd 0.6
Com to grd 1.3


Between Windings
S - R 2.6
R - C 1.1
S - C 2.0
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Old 07-30-2020, 03:03 PM   #2
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Re: Verify bad AC compressor /no turn on


Make sure the wires going to the compressor haven't been damaged.

You can ohm out the compressor and check resistance to ground directly.

If there's any resistance to ground when checking directly, the compressor is bad.

The winding resistance readings are off too - S to R should equal R-C + S-C.

Exercise a lot of caution disconnecting the wires from the compressor, terminals can blow out and release refrigerant.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

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Old 07-30-2020, 04:06 PM   #3
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Re: Verify bad AC compressor /no turn on


Best to remove the wires from the compressor and check directly from the compressor terminals to ground.
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Old 07-31-2020, 08:45 PM   #4
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Re: Verify bad AC compressor /no turn on


Preparing to to take a look hopefully tomorrow. I'll inspect the whole length of each wire, but would think the problem is most likely in the terminal box on the compressor. If needed, can the connectors on the wire at the compressor be replaced with typical crimp spade terminals? What about the compressor side, is there a special compressor terminal repair kit required.


I had heard that some will not check the resistance at the compressor if the winding shows a short, since the burnt wires at the compressor normally fail open. Can there be other type of failures that cause a short like debris?


Quote:
Exercise a lot of caution disconnecting the wires from the compressor, terminals can blow out and release refrigerant.
Is there thing that would give a sign that is it is too risky such as a lot of corrosion or oil leak ?

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Old 07-31-2020, 09:20 PM   #5
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Re: Verify bad AC compressor /no turn on


Quote:
Is there thing that would give a sign that is it is too risky such as a lot of corrosion or oil leak ?
no - oil all over = failed already refrigerant lost.

I would just be careful/gentle and not have your face near the connections.
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Old 08-01-2020, 06:17 AM   #6
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Re: Verify bad AC compressor /no turn on


Wear safety glasses.
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Old 08-01-2020, 07:06 AM   #7
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Re: Verify bad AC compressor /no turn on


Yes they sell compressor terminal kits. When you ohm the compressor make sure you do it with the wires disconnected
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Old 08-02-2020, 10:58 PM   #8
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Re: Verify bad AC compressor /no turn on


With the wires disconnected ,at the compressor initially I found R = 2.1 , Com = 0 and S = 0. I cleaned/brushed the terminals and area of some dirt and debris sprayed with Deoxit. I also cleaned each wire terminal . Now all terminals at the compressor = 0.

Between the terminals RS = 2.1, RC= 0.6, SC=1.5.

The terminals look ok with little corrosion. The S terminal was slightly loose, but it not affect the readings.

No compressor wire had shorts.

So, it looks like the electrical side of the compressor is good? I have not had a chance to check at the wires when connected.

Previously, when I tried to start the system the start cap smoked out. However this did have a burned smell prior to the restart when I first checked the system. So, may it was on its way out. I have since replace the start cap, run cap and contactor, but have not started the system.

I got some other feedback that the C circuit of the compressor should be on the switched side of the contactor . Shown in red on the Trane wire diagram. However, the system has always been wired with the C circuit on the power feed (unswitched side ) of the contactor and is wired this way on the Trane Wiring Diagram. Shown in yellow in Trane Diagram. Thoughts?
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Verify bad AC compressor /no turn on-train_wiring_diagram.jpg  

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Old 08-02-2020, 11:31 PM   #9
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Re: Verify bad AC compressor /no turn on


Did you also ohm to ground?

The windings may ohm out okay despite a short to ground.

Both sides of the contactor are hot, line 1 and line 2 are interchangeable, don't alter the wiring. No real "common" side, doesn't matter which side is switched as it's all live when off due to feeding through the motor windings.

Contactor in that diagram looks like a 2-pole in which case, both sides get switched off regardless.
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Old 08-03-2020, 03:17 AM   #10
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Re: Verify bad AC compressor /no turn on


The wiring from factory/on the diagram is how it is suppose to be.


Show use an amp meter and turn it back on, and see if it draws LRA.
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Old 08-03-2020, 12:29 PM   #11
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Re: Verify bad AC compressor /no turn on


I wired it up per factory spec. I powered up without the thermostat off, and could here a soft buzzing at the compressor. I turned the thermostat on and the fan came on, but not the compressor. My meter does not have a clamp to measure LRA, I see if I can get one.
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Old 08-03-2020, 01:39 PM   #12
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Re: Verify bad AC compressor /no turn on


Would testing the start relay (CSR in diagram) be worth a shot ? I do not here the clicking on start up. What would be the best way to test ? It has continuity (1) SC to (2)RC with the power off, since this is a NC relay. I would think it needs to be checked with voltage to see if the contacts open.
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Old 08-03-2020, 03:24 PM   #13
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Re: Verify bad AC compressor /no turn on


If its contacts don't open, the compressor would still start.
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