Central air turns on and then off right away

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  #1  
Old 07-17-12, 10:48 AM
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Central air turns on and then off right away

Hello,

This is my first post here and I really hope you can help me out. I bought my first house early in the spring and I couldn't test the central A/C when it was inspected because it was cold out. Anyways, now that it is summer I notice that there is a problem with it. I will try to explain the issue as detailed as I can.
I switch the thermostat to "cool", set the temperature and then turn it on. I can hear the blower motor in the furnace kick on and then after about 10 to 15 seconds the air conditioner will attempt to kick in. At that point I can hear the compressor kick in and the top fan starts spinning. After only a couple of seconds though it stops (compressor and fan) but the furnace fan keeps blowing.
If I turn it off and on a few times it will eventually stay on. Sometimes it will run for a few minutes and then kick off though also.
One other interesting thing I noticed is that if I set the temp to say 75 degrees and let the AC run it will keep running even if it gets down to 70 or less. I have to manually shut it off.
Could anyone guide me to what might be causing this? I am pretty handy with almost everything. I have a multimeter and know how to test current and circuits and stuff.
I would appreciate it. I just had my range and built in microwave go completely dead and the fridge is on its way out also. With just moving in, I am pretty broke and I would love to try and save some money if I can do this myself. Thanks
 
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  #2  
Old 07-17-12, 11:29 AM
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Does your AC run or not ?? you said it only run for a few seconds/minutes, then you said it can cool the room to 70F and keep running.
 
  #3  
Old 07-17-12, 11:43 AM
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Is it a heat pump? When the unit shuts off is the contactor depressing? Its in the condenser outside. Look up a pic
 
  #4  
Old 07-17-12, 12:44 PM
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Clocert: If I can get it to stay running by flipping it on and off a few times then it will continue to cool the house down consistently.

ender2272: I will definitely look into that this evening. I will do a little research on it and take a peek. I will post with the results. Thanks for the pointer.

One other piece to the puzzle is that when it actually stays running it takes a long time to cool down the house. It is cold air blowing through the ducts but it doesn't feel as cold as it could be. I just changed the furnace filter hoping that would help but it didn't.
Also, sometimes at startup it will "clunk". It sounds basically like it tries to kick in and kill right away a couple of times. It will always fail to start up and keep running when this happens. You can feel and hear this sound in the room next to the unit.
Hopefully this information will help also. Thanks
 
  #5  
Old 07-17-12, 04:38 PM
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Check your contactor first as mentioned by ender2272, if contactor is OK, then check capacitor (in the condenser unit), then because you said the air is not cold enough, you will have to call a tech to check the freon charge.
 
  #6  
Old 07-17-12, 07:49 PM
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Thanks for the info clocert. I took a peek at it tonight and after cleaning out the webs, spiders, and other debris I looked around. The first thing I looked for was disconnected wires but I didn't find any. Next I pulled the capacitor and cleaned the connectors and ends on that (they were a little corroded). I put power back and started it up.
It seemed to start up like normal and ran for a bit. I was watching the contactor and then I watched the jumper bar on that jump up and down a couple of times (with some sparkage) and then disengage. I played around with it a bit by turning it off and then leaving it on. It kicked in and out a few times and then stayed off for a while. After a bit though it came on and stayed on.
This is an old Lennox from probably 1995ish. I looked on the web a little and I don't believe it is a heat pump. It is just one of those regular old units you always see.
clocert is there a specific way to test the contactor? Thanks
 
  #7  
Old 07-17-12, 10:20 PM
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IT is very easy to test contactor. The contactor is powered by 24V from your T-stat. Turn the AC on and T-stat call for 'cool', you should have 24V at the contactor terminal (usually the small wires, one on each side), and the button should be pulled in. When room temp reach the set temp, the T-stat should turn off and cut the 24V, the contactor button should spring back up. You need to use a multi-meter to check the 24V at the contacter terminal. (Contactor has high voltage on other terminals, be careful. ) If the contactor does not work this way, get a new one.
 
  #8  
Old 07-18-12, 03:30 AM
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it is likely there is a poor connection on the 24V circuit feeding the contactor. Check all connection and the wire to see if there is any chaffing or other problems.
 
  #9  
Old 07-18-12, 06:37 AM
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Thanks for the info guys; that helps a lot. I will work up from the 24 volt wiring/circuit and go from there. I will do that this evening and post the results.
 
  #10  
Old 07-18-12, 07:43 PM
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I tested it tonight. Long story short I ended up turning the tstat all of the way down and turned on the switch on the tstat also. The furnace blower kicked in but not the A/C. I tested the 24 volt leads going into the contactor but there was no voltage. I also tested the capacitor and there wasn't any voltage coming off of that either.
I let it sit for about 5 minutes and then it decided to start up. I started measuring the leads to the contactor and they held steady at 25 volts. I then put the multi-meter on the capacitor and 2 of the poles had a reading around 300+ volts.
Then with nothing changing, it shut off on it's own again. There wasn't any voltage going to the contactor anymore and the capacitor didn't read anything. I stopped testing there.
Now I believe that there are only 2 parts it could be. The capacitor or this little PC board with what looks like a relay and some resistors on it. The 24 volt (which was reading 25) wires come off of this and go to the contactor.
With this info, which do you think it would be? I sure do appreciate the help. Thanks
 
  #11  
Old 07-18-12, 08:16 PM
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Sounds like it would be that board as the problem. You must have a heat pump and that board is used for heat. You can bypass it easily but you would have to replace the board for heat if it is a heat pump. You can check to see if you have 24 volts going into the circuit board to see if thats tge problem.
 
  #12  
Old 07-19-12, 06:31 AM
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Thanks ender2272. The board has 3 terminals with wires coming off of them. The first terminal has a wire (blue pigtailed into a black one) that runs back inside the unit somewhere. The other two go straight to the contactor, one on each side. One is yellow and one is blue also.
I guess I will have to test again but do you think that the first wire would or should be a constant current to that board? I am going to get a pic of the board also and post it. Thanks again.
 
  #13  
Old 07-20-12, 08:27 AM
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I took another look at it last night and I did notice that there was a solid 25 volts going into that timer delay board but nothing being output from it which is the reason the contactor wasn't closing. I am going to order the replacement board and see if this does the trick; I believe it will. Thanks for all of the guidance to solving this problem. I will post after I get the board to confirm that this is fixed or not.
I am including a picture of the part for any future references.
 
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  #14  
Old 07-20-12, 09:05 AM
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I sure hope it solves the problem. I just wanted you to bypass the board and go straight to te contactor. Run the unit and see if the problem happens again.
 
  #15  
Old 07-20-12, 12:13 PM
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Thanks for the reminder. Yeah I didn't order it yet or anything. It seems pretty straight forward to bypass that and go to the contactor. I will try that first before ordering that part.
 
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