Trane Condenser Unit Problem

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Old 06-03-15, 10:14 PM
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Trane Condenser Unit Problem

First off, I'm very mechanically minded and very proficient with automotive A/C.
I'm not so proficient with home central units. My wife has had 3 strokes in the past year so needless to say we're buried in doctors and pharmacies. Here's the problem and any advice is greatly appreciated as I'm trying to avoid if at all possible having to call in a HVAC expert due to potential worst case cost.

Trane XE1100 Condenser unit.
Was working and not used for some months due to cool weather here The thermostat was kept in the OFF position.
When energized neither the compressor nor the condenser fan come on.
The contactor worked but the points looked a bit shabby due to burning so I replaced the contactor with no improvement.

My multimeter shows 120 volts to the fan motor and 2 of the leads going to the compressor show 120 volts each with the T-stat on and the contactor engaged.

When the thermostat is set to COOL and the temp lowered to less than room temps the contactor in the outside condenser operates and the evaporator fan in the house also operates. The compressor and cond. fan do not.
The condenser fan also rotates very freely and I make it a yearly job to thoroughly clean the condenser of dirt, milkweed, and so on.

There were some bad electrical storms and power outages recently but the thermostat was turned off so I assume the comp/fan were not fried by lightning.

I'm leaning toward a capacitor problem (4 of them and hopefully....) or a failed compressor and condenser fan (hopefully not). It struck me as odd that both would fail at the same time and there are no smells of burnt wiring or insulation.

Anyway, I hope I provided enough info to work with and will appreciate any insight on trying to resolve this myself. Thanks in advance.
 
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Old 06-04-15, 08:13 AM
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Are you measuring the voltage at the compressor leads referenced to ground, or between the two compressor leads? The compressor runs from 240VAC, so you really need to measure the AC voltage between the compressor terminals. Be very careful when you do so, as the voltage & current are lethal if the probe slips. Alternatively, you could measure the voltage across the two output terminals of the contactor. Again, measure between the terminals, not to ground.
 
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Old 06-04-15, 07:22 PM
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Yes, I'm very careful when dealing with this kind of thing as I've been nipped a few times in the past including 220. Ouch...

I measured the voltage between each terminal and ground. There are 3 leads to the compressor; orange, red, and black with a blue tracer. Ground to each of them shows about 120 volts.

I went out this evening and checked the voltage between terminals and got the same thing; about 120 volts. The only action is a slight hum from the double post contactor which appears to energize fine and pass voltage through the contacts.
 
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Old 06-04-15, 07:40 PM
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Measuring to ground will not tell us if you have the correct voltage.
Measure L1 to L2. ( Red to Black)

The only 2 options should be 0 or 240 (ish).

Do you have a square D breaker type disconnect by this unit?


If you do, my money is on that.


Is this a Trane XE1100 air conditioner or a Trane XE1100 heat pump?

The model number will tell us if you do not know.

TT = air conditioner
TW = heat pump.


The XE1000, the XE1100 and the XE1200 have the same wiring diagram.
 

Last edited by Houston204; 06-04-15 at 08:02 PM.
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Old 06-05-15, 09:21 AM
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The unit is an XE1100 TT model and yes it has the similar square breaker box outside next to the unit.

I'm on my way out right now but will measure the L1 to L2 this evening and post back with results. Thank you for the info as it is greatly appreciated.

I'm hesitant to suspect the worst on things like this because as a lifelong mechanic I've seen so many automotive problems with perceived major problems that turned out to be something simple.

Being a home central novice it may be a can't see the forest for the trees thing for me. :-)
 
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Old 06-05-15, 11:11 AM
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I decided to take a quick look at it before leaving. I show 0 between L1 and L2 with the breaker on according to my multimeter.

I have a voltage sensor tool that does beep though so could it be assumed that some current is able to pass but the connection is so poor there is not enough current to operate anything?

There appears to be a tiny rectangular clear window on the breaker. I assume that is supposed to glow a bit when current passes through it?
When the breaker is first flipped on it glows weakly for a second and fades.

Will check back later and again, thanks.
 
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Old 06-05-15, 11:49 AM
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You'll need to check voltage at the contactor. You'll see a pair of heavy wires at each end of the contactor, with thinner wires connected to each side of the contactor (typical). The heavy wires at one end of the contactor is the 240VAC input (always present), and the other side is the 240VAC output (voltage present only when the thermostat is calling for cooling). The thinner wires connected to the center are the control signal wires (24VAC) coming from the thermostat. When the thermostat calls for cooling, it sends 24VAC to the outside unit where it becomes the control signal wires going to the contactor. When the 24VAC is present at the contactor, it energizes the contactor (contactor is really a relay), pulling the contactor contact(s) closed, thus passing the 240VAC to the output side which is connected to the compressor and condenser fan. Often times, there are pressure switches and possibly a time delay circuit in series with the 24VAC before it gets to the contactor.

To recap, here's what I would suggest you do (in order):
1) Check for 240VAC at one end of the contactor (across the 2 terminals)
2) With the thermostat calling for cooling, check for 240VAC at the other end of the contactor
3) If 240VAC is present at both ends of the contactor, your contactor and control signal are okay. You have a problem elsewhere.
4) If 240VAC is not present at output side of contactor, check for 24VAC at side terminals of contactor. If not present, check for 24VAC where thin wires come into outside unit (should be wire nuts to connect to wiring inside unit). If 24VAC is coming in to outside unit, one of the switches is open and preventing voltage from reaching contactor.
 
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Old 06-05-15, 04:46 PM
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Square D disconnects fail often enough to make me hesitant to turn one off without informing the owner that that there is a high possibility that they will need a new disconnect if I turn it off.

They are cheap but the installed price isn't.

I would measure for 240 volts to the disconnect if I am not getting 240 volts out of it.
 
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Old 06-05-15, 07:57 PM
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Well, I guess I have cold air again and many appreciative thanks to both of you for the invaluable advice. Apparently the problem was the circuit breaker.

I had made multiple checks in the past few days at the contactor and showed roughly 120ish volts just about everywhere. My thought originally was a bad contactor or circuit breaker but seeing as how voltage was present in a number of places I dismissed the idea.

Today however, when checking it I got 0 readings everywhere. After removing the circuit breaker I checked it with my VOM and found one side was stone dead now. So 15 dollars worth of new breaker later the unit is up and going.

Now the docs and pharmacies where we're on a first name basis get the money instead of a professional HVAC guy....... :-)

Again, the most sincere thanks for your help. It is greatly appreciated.
 
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Old 06-06-15, 04:47 AM
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Glad to hear you found the problem and that we could help. Thanks for reporting back.
 
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Old 06-06-15, 09:40 AM
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Just a final thank you and one additional bit of info. I drilled the rivets out of the old circuit breaker for a look inside as I'm always curious about why something fails.

On the failed side of the breaker one half of the contact points had a small lump on one edge which was apparently caused by long term arcing. Most of the point surfaces were not able to touch due to that tiny (eye of a needle size) lump. I guess that could explain a random triggering of the VOM and not enough contact surface to carry any kind of electrical load at all.

My wife says thanks to you both also.... :-)
 
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Old 06-06-15, 01:36 PM
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Thanks for the autopsy report.

It sounds much like the way start relays fail.
 
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