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Old 06-13-2008, 10:49 AM   #1
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Lennox A/C troubleshooting


Hi all, I just recently purchased my first house and i'm having trouble with the A/C unit outside. First off, it's a Lennox 10ACB24-11P. I'm assuming it was built with the house, in 2001.

Here's my problem. On the outside unit, when the thermostat is set to cool, and the unit turns on, after about five minutes or so, the fan will cut off, and the compressor will continue to run. Here's a list of things we've tried to troubleshoot the problem:

-replaced fan motor
-replaced cap
-cleaned contactors
-ran old motor with new cap
-disconnected compressor from power and still watched the fan stop running.
-ran separate caps for the compressor and fan, and still had the fan cut out.

i'm out of ideas. Any help would be great.

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Old 06-13-2008, 01:52 PM   #2
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Lennox A/C troubleshooting


Contact a Lennox HVAC dealer in your area? Chances are they've probably seen stuff like this before and will be able to help you out (or at least give you some sort of quote).

Of course, that's not exactly "DIY", but if you're all out of ideas, might be the best bet.

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Old 06-14-2008, 06:24 PM   #3
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Lennox A/C troubleshooting


I have seen this before your fan is over heating and going off on thermal over load contact tech support of the equip with the model and serial ready and find out the RIGHT size motor and where it can be ordered say you would prefer a direct replacment and not a rescue motor
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Old 06-15-2008, 10:46 AM   #4
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Lennox A/C troubleshooting


Where are you losing power? Take voltage reading where the motor leads are connected when the fan stops running.

Measure fan amperage when the fan IS running, compare to the motor rating tag on the motor.

Measure the amperage at the disconect when both are running, compare to rating tag on the condensor.

Disconnect power to the condensor when the fan quits. Is the motor hot or cool?

Last edited by 8 Ball; 06-15-2008 at 10:54 AM.
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Old 06-15-2008, 09:32 PM   #5
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Lennox A/C troubleshooting


Be sure you have the correct replacement motor for that unit. I had that same type of problem with my Lennox after replacing the fan motor. Brought the motor back to dealer and got another one, and did the same thing. Finally when I mentioned that the fan seemed to be turning faster that I remember it used to, the actually looked up the replacement motor, instead of just grabbing a 1/4 hp replacemenet motor. Turns out that the correct motor was indeed a 300 rpm slower motor, with a higher torque rating to handle the high pitch in the particular fan blade for my model. Exactly as geo fan said; the motor had insufficient torque, so it was under extra strain, and would overheat and shut down.
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Old 06-17-2008, 08:27 AM   #6
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Lennox A/C troubleshooting


Ran another test last night with another new cap, and original motor. put meter on contactor, showed a steady 238 vac, with occasional 239. still retained 238vac after fan kicked off. took much longer to do, either on account of it not being as hot, or some other reason.

i'm starting to lean towards the motor is crap, and the one we got to replace it was the wrong one....
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Old 06-17-2008, 08:04 PM   #7
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Lennox A/C troubleshooting


Voltage sounds fine, however, amperage tells the tale. If your not too frustrated by now, measure the amperage then compare it to the nameplate rating FLA on the motor. Anything over the FLA will cause the motor to overheat and trip its internal overload.
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Old 06-18-2008, 09:15 AM   #8
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Lennox A/C troubleshooting


that will be the next thing i check.... luckily, it's been in the mid 70's here all week so I haven't been hurting for air.
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Old 06-20-2008, 03:14 PM   #9
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Lennox A/C troubleshooting


My brother owns an Air Conditioning Company In Tampa FL
I bet he would know, since he is an A/C Contractor.
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Old 06-23-2008, 01:34 PM   #10
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Lennox A/C troubleshooting


i had an HVAC specialist come over last friday and look at it. He declared it was the motor, and said that more than likely i had been given a bad replacement motor, so I have a new one on the way. another update when i get this motor installed...
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Old 06-26-2008, 09:17 AM   #11
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Air's been running healthy for the past 15 hours now. it's nice to finally have working A/C. It was in fact the motor the whole time. Apparently the replacement was bad?
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Old 06-27-2008, 06:16 AM   #12
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Thanks for the closure. Was a great post. Glad everything is working well.
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Old 06-22-2010, 01:02 PM   #13
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Make sure its the right replacement

years ago ran into the same thing, (keeprite unit ) needed ball bearing mtr
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Old 06-22-2010, 03:46 PM   #14
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I hope he got it working in the 2 years since he started this thread.
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Old 06-22-2010, 03:48 PM   #15
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Lennox A/C troubleshooting


ha ha, these old threads that get bumped...gotta pay close attention to the dates. it got me too.

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