Outside central air unit won't turn on. - http://www.hvacmechanic.com/ Forums
Original message
| Jay10
 |  "Outside central air unit won't turn on."
, posted Wed 13 Apr 23:19  
Hello all,
I don't know much about central air, but here's what's happening. I turn the thermostat to cool, and the temp down to make sure it should be turning on. The inside blower comes on, but the outside unit does not. The outdoor unit is a Lennox HS 18.
I have checked breakers, etc. and there IS power to the outdoor unit (don't know measurement though, I just tested with simple AC tester). I also know that the thermostat is sending power to the outdoor unit because the relay switches to different position.
The unit then just hums (sounds like from relay). The fan won't start and neither will the compressor. I checked wires into and out of capacitors (of what I think are the capacitors) and seems like there is power everywhere. Again, I don't know how much, tester just detects AC current, it could just be picking up from other wires.
I don't know what the problem is. Also don't know where to test and for what kind of voltage to test for. Is there some kind of fuse or breaker or switch on the unit that I'm overlooking or don't know about?
Any suggestions or help of any kind is greatly appreciated. Hope I supplied enough information, this is the best I can do.
Thank-you.
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Replies:
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| tinner73
| "Re(1):Outside central air unit won't turn on." , posted Thu 14 Apr 16:24
call for service 220v is a lot of power.
union trained in Chicago
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| pacserve
| "Re(1):Outside central air unit won't turn on." , posted Thu 14 Apr 09:19
iF YOU HAVE 24 VOLTS AND 220 VOLTS TO CONTACTOR AND CONTACTOR PULLS IN AND NOTHING WORKS CHANGE CONTACTOR.
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| | thief
| "Re(2):Outside central air unit won't turn on." , posted Fri 15 Apr 15:09
I am experiencing the exact same problem, I have removed the panel and checked all the wires. The fuse is good at the panel, and when I check the voltage at the a/c unit (white wire to ground) I am only reading 110 volts. I have read alot about this contactor thing, but need to be able to locate it, and help or other suggestions would be appreciated.
Thanks
Thief
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| | Freon
| "Re(3):Outside central air unit won't turn on." , posted Fri 15 Apr 17:27
A contactor is a big single throw (like a light switch-on/off), double pole(two contacts connected at once) relay. It passes the 220 line voltage from the line side to the compressor and fan circuit. The relay coil is activated by the 24 volt thermostat circuit. So two things have to happen for a condenser to run. First you need 24 volts at a functioning contactor coil to cause the contacts to 'make' the connection. Then you need good contacts for a solid connection. Due to the current flow, contactor contacts do 'pit' from sparking at contact. There's also a spring counteracting the coil magnetic field(from the 24 volt thermostat circuit) so when the coil is deactivated the spring separates the contacts and the fan and compressor are no longer powered.
This is a very dangerous place to try and learn. 220 volts will kill you very quickly. Use gloves and rubber soled shoes if you must venture there. For most testing you can pull the main 220 volt disconnect so there's not 220 volts at the condenser. Be careful and if uncertain, call a professional. You're worth more alive than dead to your family.
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| | thief
| "Re(4):Outside central air unit won't turn on." , posted Sun 17 Apr 14:42
Ok after somemore tinkering I now I have 220 V at the contactor, and 24 Volts from the stat. When I flip the switch to cool, it blows the fuse at the panel. The unit makes like a humming sound. The fan blades spin freely, so I dont think its seized.
Any other thoughts, or info I can post to help diagnose.
Thanks
Thief
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| | Xenos
Webmaster 
| "Re(5):Outside central air unit won't turn on." , posted Sun 17 Apr 14:55
Does it trip right away or is there a like a 3 to 5 second delay? if there is a delay it may be a siezed compressor. however if it's instant I wouls expect a short or ground.
P.S. the fan still could lock up when powered if there is play in the bearing.
Xenos.
The best way to escape a problem is to solve it.
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| | thief
| "Re(4):Outside central air unit won't turn on." , posted Fri 15 Apr 21:42:
Thanks for your response (and the tip for the 220V). I have worked around residential electricity so am very comfortable with that aspect of the unit, just not sure why I am only reading 110V at the unit. Where in the unit can I check to see if I have 24V. Also, when I flip the stat to cool I can here the contactor make a clack sound, but nothing else happens, fan dont spin. What else could I check?
Thanks
Thief
[this message was edited by thief on Fri 15 Apr 22:00] |
| | Freon
| "Re(5):Outside central air unit won't turn on." , posted Sun 17 Apr 15:36
There should be a red and black lead from the disconnect box to the condenser connecting at the contactor. Between the red and black you should measure 220 volts AC. From the red to the condenser case you should get 110 volts; same for the black to the case. If you're not getting those voltages, go to the breaker box and do the same test on the red and black wires attached to the circuit breaker. Use the breaker case for the 110 volt test.
The clack sound you hear when you call for heat/AC is the contactor being activated. However it may not be making a solid connection. If all voltages are good, then Xenos has the best answer. Reread his post. A frozen compressor may be helped with a hardstart kit.
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| | thief
| "Re(6):Outside central air unit won't turn on." , posted Sun 17 Apr 16:23
First off thanks so much guys for taking the time to walk me through this. I have done all the voltage checks you requested and they are all correct. The unit is fine until I go switch from heat to cool at the thermostat, then I get that humming sound at the A/C unit and no fan spinning (cant remember if the blower started at the furnace). I then shut the disconnect outside so the humming stops, go back in and check the fuse and it is blown. The fuse is two 15 amp cartridge fuses and the same one always blows. I am going to try and get below the unit and make sure the blue/black and red wires from the otherside of the contactor that lead to the compressor are not chewed or frayed.
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| | Freon
| "Re(7):Outside central air unit won't turn on." , posted Sun 17 Apr 16:49:
Cartridge fuses are reasonably slow-blow so I suspect the compressor may be locked. If you want to try and check the compressor for a lock situation and not a short, here's what you should do. First pull the disconnect. Then disconnect the compressor wires from the contactor--mark them carefully!!. Get your ohm meter and check the following:
Run to common = lowest reading. You are measuring the run winding only.
Start to common = medium reading. You are measuring the start winding only.
Run to start = highest reading. You are reading both windings.
(Run to common) + (Start to common) = (Run to start)
also S winding should be 3 to 6 times the value of R winding.
Example = If C to R is 1 Ohm, C to S needs to be at least 3 Ohms.
[this message was edited by Freon on Sun 17 Apr 16:58] |
| | thief
| "Re(8):Outside central air unit won't turn on." , posted Sun 17 Apr 22:53
I have checked all the connections and wire conditions and it all looks good. I replaced the 15 amp fuse cartridge and tried it again. This time I left the disconnect off, went inside and turned the stat to cool, the blower motor came on in the furnace, went outside, looked at the unit and I could see the contactor was making contact, and I could here a slight buzzing sound, I am assuming thats the coil getting 24volts. I then flipped power on the disconnect, the fan blade made 1 1/2 revolutions and then stopped. I went back to the panel, checked the fuse and it was blown again. Could I not try and see if the fan motor is gone (by removing the motor and applying just 110v)??
I read what you posted above, Freon, about checking the compressor for lock. There are only two wires to the compressor right?? Red and blue/black? After that I lose you on what to check, how do I know which is run, and which is start?? I dont think its possible to post pictures in the forum, but my email is thief@sympatico.ca if I could email you I would take a few pictures of the contactor that I have and maybe point out a few things. The unit is a Tempstar.
Thanks again for walking me through this, I would think that a slight spin of the fan is a good sign (or at least I am hoping).
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| | Freon
| "Re(9):Outside central air unit won't turn on." , posted Sun 17 Apr 23:19
Most likely the fan isn't your problem and your compressor has shorted/locked. You need to call a pro and have him check it. Possibly he can add a hard start capacitor/relay and get it going if it's locked but that's only a short term fix. If it's an old condenser, consider a new one. If it's relatively new, thing about a new compressor only. Sadly the cost of either option will be similar.
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| | thief
| "Re(10):Outside central air unit won't turn on" , posted Mon 18 Apr 14:23
Just one more thing before I break down and call a tech (I hate doing that, for things that I could do myself), should I not check and make sure the capacitor is good? I will check the diagram tonight on the inside panel of the unit and see how many microfarads the cap should be good for and then I will check it with my meter ( I am pretty sure it checks microfarads). Is it not possible that it is gone?
Thanks
Thief
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| | Mastertech
| "Re(2):Re(10):Outside central air unit won't t" , posted Mon 18 Apr 15:58
If it trips the breaker in less then a sec then its shorted. Most likely a shorted compressor. Locked up compessor normally take a few sec before tripping breaker. At 15 amps that must be really small unit. But next step is to call a tech so u dont hurt yourself or damage the unit in more just incase its just a shorted wire.
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| | thief
| "Re(10):Outside central air unit won't turn on" , posted Mon 18 Apr 10:59
Thanks for all your help, I will call a professional and have them take a look at it. Hope its not too expensive, will post back my results.
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