No power to thermostat/furnace.

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  #1  
Old 06-01-13, 11:17 PM
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No power to thermostat/furnace.

My Honeywell circular non-programmable thermostat went blank the other day. It would not display or turn on the AC/fan. After initially just thinking I'd have to change the tstat, I did some googling and realized that is probably not the issue. I pulled the face of the tstat and used a multimeter to test R-Y, R-G, and R-W, and all read 00.00. However, I have very little experience with a multimeter. I have a horizontal mount gas fired high efficiency furnace in my crawlspace. I seem to remember that when it was working, there would always be a light that I could see through a little glass circle on the blower enclosure. That is no longer lit. Can't find manufacturer on unit, but may be a Goodman. Any help would be much appreciated. Here is a list of things that may help you help me:
.
- Checked 5 amp fuse on control board and it's fine
- it does not have a float switch. It was emptying onto the rock in my crawlspace, until I encapsulated the space and ran the condenser and furnace drains into the sump pit.
- I reset all breakers
- The local power control (light switch) for the furnace is turned on.
- The furnace is about (8) years old



 
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Old 06-02-13, 03:27 AM
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Welcome to the forums.

In the pic below I have drawn two red arrows pointing to the leads on your transformer.
Using your voltmeter..... you should measure 120 vac on the white and black leads (left side) and 24 vac on the orange and looks like brown wire (right side).

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Old 06-02-13, 05:58 AM
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remember to push in the door switch when testing for power.
 
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Old 06-02-13, 07:49 AM
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Thank you for the clear instructions. I'm getting 124V on the left side and 00.00 on the right.
 
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Old 06-02-13, 04:28 PM
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blown transformer....................................
 
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Old 06-12-13, 02:58 PM
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I just received the new transformer and installed it. The thermostat immediately turned back on. Thanks for all your help on that. Unfortunately, I now think that was just a symptom of another problem. Now, when I turn the AC on, the compressor does not start. The thermostat just flashes a snowflake and the outside unit does not kick on. I reset the breaker for the unit with no luck. When I turn the thermostat to heat, it works fine. Any ideas?
 
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Old 06-12-13, 03:33 PM
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Wait 5 minutes............
 
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Old 06-12-13, 03:45 PM
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The unit is also a Goodman, by the way. I've had the AC on for about a half hour and turned the fan on for about the last 15 minutes.
 
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Old 06-12-13, 05:32 PM
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Anyone have any easy things to check? It just hit 96 here. Thanks in advance.
 
  #10  
Old 06-12-13, 05:34 PM
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Is the contactor pulled in in the outdoor unit? Do you have power out there?
 
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Old 06-12-13, 06:50 PM
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Of course it's pouring out now, so I'll have to put off checking it. How exactly do I check the contactor? I have to pull the shroud off the AC unit, right?
 
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Old 06-13-13, 12:21 AM
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Jumper R, Y, and G at the t-stat.

If the a/c comes on, you have a t-stat problem.

Otherwise you have an a/c problem.

The t-stat may have a delay - turn it to cool and wait a while
 
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Old 06-17-13, 01:27 PM
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Okay, I have power going to the contactor. When I push the contactor in, the fan turns on and the condenser starts. I have left the tstat turned to AC for the past hour and a half, so I don't think it's due to a delay. Does it sound like just a bad contactor or could it still be the tstat? The contactor is a GE CR453CB3HBAFG, if that helps. Thanks.
 
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Old 06-17-13, 01:37 PM
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You need to check for 24vac to the contactor coil from the indoor unit/thermostat. There should be 2 small wires coming into the condensing unit.
 
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Old 06-17-13, 04:28 PM
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Checked the tabs on the side of the contactor and I'm not getting 24vac with the tstat set to cool. Since the wires are coming in to the unit, is it safe to say that it's the tstat after all?
 
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Old 06-17-13, 04:31 PM
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no, there could be a pressure switch inline or a wire off at the indoor unit, you need to track back from the contactor.
 
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Old 06-17-13, 09:53 PM
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Okay, I checked for a pressure switch and there isn't one between the AC unit and the furnace. I checked the connections at the air handler and everything seems fine. I changed the tstat to rule that out. The AC still doesn't come on, but the blower on the furnace comes on now when I switch to cool.
 
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Old 06-17-13, 10:10 PM
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In your hijacked pic below..... you'll see I circled the brown cable that goes to the compressor. The red and white wires go to the compressor thru that cable. Check for 24 VAC right there.

If good.....go out to compressor and find where that brown cable enters the unit. The red and white conductors will probably be connected with wire nuts. Check again there for power.

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Old 06-18-13, 04:26 PM
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Well, when I went to check the red and white wires at the furnace board, I found that the transformer was blown again. Does that narrow down the possible causes or does put me at square one?
 
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Old 06-18-13, 05:00 PM
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That 5A fuse on the board is to protect the transformer and board from wiring shorts. It's technically to large to protect the transformer. So you may have had a short and that short knocked out the transformer. I would change that fuse to a 3A.

If I remember...... your heat and inside blower were working fine. If I had to make an educated guess..... I'd say that wire to the compressor I asked you to check is shorted to itself or ground.

Remove the red from the Y terminal and the white from the C terminal. Using the Rx1 scale on your meter check from each wire to ground.
 
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Old 06-18-13, 07:44 PM
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I don't have much experience with a multimeter, so I'll tell you what I did and you can tell me how wrong I was. First of all, I don't have an Rx1 setting on the meter. I posted a pic of the meter I have below. I set the meter to Ohms and touched one probe to white and the other to a metal bracket I have mounted to the ceiling in the crawl (pictured). I did the same for red. The reading was just bouncing all over. I then did the same on the setting to the right of Ohms (continuity?) and had the same result. I kept it on that setting and touched one probe to white and the other to red and the multimeter beeped, indicating continuity, I guess. Does that tell you anything?

 
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Old 06-18-13, 07:49 PM
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Sorry, whenever I touched to the bracket, the multimeter did nothing. When I touched to the red and white at the same time, it bounced all over the place on both settings. I mistyped in the previous post.
 
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Old 06-18-13, 09:18 PM
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The meter has automatic ohms setting. Use ohms setting. Touch meter leads together. You should read around 1-2 ohms.

You didn't measure anything from white and red to ground......if that bracket is grounded.....can't tell by picture. That would mean no short to ground in wiring.

Connect the meter leads to the red and white wires. You can't touch the leads with your hands. What do you read in ohms ?

We are trying to see if the AC or its wiring is causing a short.
 
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Old 06-18-13, 11:05 PM
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The reading jumped around quite a bit, but if I kept the leads still and waited a few seconds, the reading would settle at about 1.6. Keep in mind that I'm using a multimeter with probe leads and not clips, so anytime there is slight movement, I'm getting a big jump in number. However, I'm confident that 1.6 - 2.0 is accurate.
 
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Old 06-19-13, 12:40 AM
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Did you check the meter leads shorted to see what the meter and leads read ?

My guess is you will see almost the same thing which means you may have a short in your wiring or a defective contactor.
 
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Old 06-19-13, 07:12 AM
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I think I'm going to buy a contactor and another transformer. If I change the fuse to a 3A, do you believe that will protect the components so if the short happens again, I can start over with a new fuse, rather than a new transformer?
 
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Old 06-19-13, 07:33 AM
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On a related note, the contactor area in the AC unit had a lot of spider webs and bugs in it. I wanted to give it a good cleaning, so I tried to pull the cut off switch mounted near the unit. I ended up pulling the enclosure off of my house. Is there a technique to pulling one of these out? Also, it would be fairly easy to change out the red/white wires that go from the control board to the AC unit. Should I just change those out?

 
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Old 06-19-13, 08:18 AM
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OK... enough is enough. Its time to call in a professional. You are doing more damage than good .
 
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Old 06-20-13, 08:18 PM
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For those interested, the problem was the contactor. Thanks for the help.
 
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Old 06-21-13, 01:19 AM
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Thanks for letting us know. I would replace that fuse with a 3A one.

It appears that the vinyl siding is fairly new on the house. That floating service disconnect is a common site. The workman pry the unit off the wall and then allow it to float there. The screws are usually just laying around inside the box. I would try to remount that box or have it done. Power must be turned off at the main panel before opening that box.
 
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