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Old 05-12-2011, 08:13 AM   #1
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Lennox A/C Won't Start


Hi!

I've run into a problem with my AC. It's a new house and this is the first time we've ever tried to run it. I've placed a service call, but not sure how long it will take to get fixed. It would be nice to know what's going on with it.

My problem is that when you switch the thermostat to 'cool', the fuse on the board blows. If you replace the fuse, you can switch the blower on and the heat on, but just not the A/C. Once you do, it blows the fuse.

I did a little research last night and they say that it might be a pinched thermostat wire. Just for kicks, last night, I connected the thermostat to the furnace via short 3' wires. The fan worked when I switched it on, but when I turned the a/c on, instead of a fuse blowing, it did nothing. The LED's both blinked at the same time, signaling that the unit was receiving power.

I had the electrician come yesterday and he said everything was receiving power properly and he checked the continuity of the wires, and everything was fine.

I was hoping to get some ideas of maybe what I can look for if the HVAC people don't show for a week.

Thanks!

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Old 05-12-2011, 09:30 AM   #2
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Lennox A/C Won't Start


1.) If you have small gator jumper clips (purchasable at radio shack or any electronics store) then at the furnace itself you can jump from y terminal to the r terminal to see if the ac comes on. All you are doing by jumping at the control board in the furnace is bypassing the stat. If the ac comes on this way, your stat is bad.

2.) Go out to the condenser and pull off the service panel and see if the contactor is pulled in while on call for cool with jumper wires hooked to the y and r terminals in the furnace. If the fuse is not blown and you've jumped it properly then low voltage power (24 volts) should be sent outside and the condenser should be running so you might not even need to pull of the service panel but if the unit is not running then do so. Check the low voltage wire connections to make sure the wire nuts are not loose or have fallen off if the contactor is not pulled in.

Be careful at the contactor, it is 240 volts on high side so do not go sticking your hands or anything in the service area on any electrical components, just look and see if the contactor is pulled in and all connections are tight, both high and low voltage. If you do need to do any work in there such as fix a loose or fallen off wire nut or tighten a screw on the contactor (HIGH VOLTAGE), turn the system off completely by way of breakers FIRST and verify with a meter that there is no voltage or you could end up seriously hurting yourself and your system.

Let me know.

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Old 05-12-2011, 09:38 AM   #3
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Lennox A/C Won't Start


Thanks for the response.

I'll have to give those a shot. I've been doing more reading on it and my next step was wanting to bypass the thermostat, just to see if it's the wiring or the stat. This will be of great help!

On the AC unit itself, how do you access the electrical panel? Is there one or two screws by the charging lines that you undo and does a panel pop off? Do you have to take the entire cover off?

I've been trying to read the manuals for the furnace and I have a feeling that the wiring is wrong. Bypassing the stat wiring will confirm (or not) this. Plus, I found a posting that said that if the fuse blows, it might be a kink or slice in the wiring.

Thanks again for the quick response!
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Old 05-12-2011, 09:43 AM   #4
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Lennox A/C Won't Start


I think I may have just answered my own question... This web site here...

http://factoidz.com/how-to-replace-a...ing-contactor/

Shows that the panel does come off. So, I'm going to assume that there's a few screws that hold an access panel that's located on the unit. I'll have to check this out.

Thanks!
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Old 05-12-2011, 10:09 AM   #5
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Lennox A/C Won't Start


Quote:
Originally Posted by wiz561 View Post
Thanks for the response.

I'll have to give those a shot. I've been doing more reading on it and my next step was wanting to bypass the thermostat, just to see if it's the wiring or the stat. This will be of great help!

On the AC unit itself, how do you access the electrical panel? Is there one or two screws by the charging lines that you undo and does a panel pop off? Do you have to take the entire cover off?

I've been trying to read the manuals for the furnace and I have a feeling that the wiring is wrong. Bypassing the stat wiring will confirm (or not) this. Plus, I found a posting that said that if the fuse blows, it might be a kink or slice in the wiring.

Thanks again for the quick response!
Let me hunt some pics down for you of some Lennox condensing units.
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Old 05-12-2011, 10:22 AM   #6
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Lennox A/C Won't Start


It could be like this:



or like this



or like this



which are pretty much the same, the service panel being on the corner held on by a few 5/16" screws.

Although this is not a Lennox, this is what it would look like standing in front of it. Most brands are designed the same.


And this is what it looks like on the inside.


The yellow arrow is pointing to the contactor which when has both high and low voltage ( low voltage when calling for cool, high voltage always) the little black plastic bar/button is pulled in. The red arrrow is the low voltage connection from the furnace/stat.

Last edited by Doc Holliday; 05-12-2011 at 10:24 AM.
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Old 05-12-2011, 11:13 AM   #7
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Lennox A/C Won't Start


Thanks for the pictures. It's good to know to stay away from the yellow arrows.

I have a funny feeling that the morons just wired it up wrong, or there's kinks in the wire.

Thanks again!
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Old 05-13-2011, 08:03 AM   #8
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Lennox A/C Won't Start


Thanks for the great information. I went home last night and fiddled around with it. I bypassed the thermostat by jumping the terminals and things started working. Turns out that the thermostat was wired wrong!

We called the HVAC guy the night before and he called back yesterday and said he would be out at night. He did swing by and confirmed that it was wired wrong and that was what the problem was. He then checked the coolant levels and we're now comfortably cold.

Thanks for the great information and help!!! As usual, it's great to be able to troubleshoot and fix things on your own!

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