No power to Rh wire and EVAP drain appears not clogged.

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Old 05-28-20, 03:09 PM
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No power to Rh wire and EVAP drain appears not clogged.

Hi All,

This season when I started using the AC for the first time, only after a few minutes, the Nest thermostat displayed an e74 error (no power to rH wire) about 3 days ago. With help of google, was pointed toward clogged air filters or/and clogged EVAP drain. I have a Trane horizontal unit in the attic. Here is what I found so far:
  1. - My home has two air inlets in upstairs hallway (non downstairs), and both were missing air filters - I installed new disposable air filters for now.
  2. - Found the two drain pipes outside the house, got a shop vac and connected it for 3 minutes, and nothing came out. I'm guessing its because AC never run for long enough to generate any condensation?
  3. - I pulled off the Nest to measure voltage and there is no voltage present between the blue (c) wire and any other wires.
  4. - My fan has been stuck ON since the AC broke down - I don't really mind it, as its very quiet and nice to have a little circulation at night. I can turn it off with a switch in the attic if needed.

Appreciate any help with next steps. Thanks.
 

Last edited by SMGOYAL; 05-28-20 at 03:45 PM.
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Old 05-28-20, 04:31 PM
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Welcome to the forums.

Need more info !!

From your description..... you have a horizontally mounted Trane gas furnace....... is that correct ?
If yes..... you have blown 3A fuse on the furnace control board. When that happens the fan stays on continuously to let you know there is a problem. The control board will be in the blower section.

It would be easy to just replace the fuse but you may have a short somewhere in your wiring. If that is a furnace and it worked normally all winter long and just failed now..... I'd check the wiring out to the condenser. That's typically where the shorts are when the system fails in A/C mode.
 
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Old 05-28-20, 05:02 PM
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Pete, thanks for the response. Yes, it is a Trane gas furnace, horizontally mounted. Little more info on the e74 error: I've seeing the error intermittently for the last 6-7 months, those were all heating months. Now the first time we used AC, error seems to be a hard error - and fan is stuck on.

I will try to locate the fuse on the Trane unit, do you know if its an old glass type fuse? Mine is a THN-A- Series. Guess I'll try replacing the fuse and if it blows again, then we have a short.
 
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Old 05-28-20, 05:08 PM
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The info I needed was on the furnace. I don't recognize that model number.
While you're up there digging around..... take off the burner access door too.
The model number is long and is listed on the ID tag on the side wall near the burner.

Typically plastic automotive ATC fuses are used.
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Old 05-28-20, 05:53 PM
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Ok, I was able to get up in the attic and take some pictures, took an access panel off as well. I didn't see a fuse in there, but maybe I'm not looking in the right place.
 
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Old 05-29-20, 11:28 AM
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Anyone spot the furnace control board and the 3A fuse? Thanks.
 

Last edited by PJmax; 05-29-20 at 06:27 PM. Reason: changed to instant notification
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Old 05-29-20, 06:07 PM
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That Trane installer guide is ancient. I don't think that's for your furnace. It looks completely different.
Come to think of it..... your furnace is not a Trane. It's a Consolidated Furnace MBA 080 NH4RX.

Anyway..... go to your last picture. The control box is in the upper right corner. That's where the fuse would be if that board has one. I'm looking at the replacement board and it has a fuse. I don't think yours has the fuse.

The board is an ST9141A. Is there a red blinking light on it ?
Do you have a meter ?

If yes..... check for 24vAC at the blue and yellow wires in the upper left of the board.
They are labeled as C and X on the board.

If ok...... check for 24vAC on the R and C terminals on the right side


 
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Last edited by PJmax; 05-30-20 at 12:25 PM.
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Old 05-29-20, 06:28 PM
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I changed this thread to instant notification for you so that you get quicker help.
 
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Old 05-30-20, 11:11 AM
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Hi Pete, Thanks so much for your help. On the left side the C and X terminals have 26 V AC. On the right side the R and C terminals have no power. There is a LED but itís green and has a single slow blink. According to the diagnostic label on the control board, single flash means ďburner limit circuit open.Ē

Youíre right I donít see a fuse around the control board. Does that mean a control board now? If yes what do you think caused it to fail.
 
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Old 05-30-20, 12:24 PM
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You're doing good. Not the control board. Looks like an open safety switch.
I don't have your actual wiring diagram so I'm using the generic setup.
The link below has the module diagram.

I up dated my previous picture. You'll now see three colored dots.
Shut down the power. Using an ohmmeter and leaving the plug attached .......
Check from green common to pink/primary limit.
Check from green common to blue/burner limit.
One will show open. Follow the wire from that plug to that sensor. It will have two wires on it. If it has a little red button between the terminals..... press it in. See if that restores normal operation. You should have normal LED and 24v back on R and C.

If you can't get your leads into the wire side of the plug..... note the wire colors, pull the plug off the board and check to the pins. The dot colors I used have nothing to do with the actual wire colors.

ST9141A tech
 
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Old 05-30-20, 02:09 PM
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Ok, green common to pink/primary and green common to blue/burner terminals both had continuity with power off - confirmed 3 times. I did follow each of the wires to see where they go. There was a sensor with two wires and what looks like a push button, tried to push it but it doesn't go down. Other switch/sensor had no pushbuttons. Attaching pictures of what I found. Thanks.
 
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Old 05-30-20, 03:27 PM
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Did you check the one with the push button for continuity ?
Be sure it shows close to 0 ohms.

You can also check it with a voltmeter with the system powered..... there should be 0v AC between the leads.
 
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Old 05-30-20, 05:00 PM
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I checked the sensor with 'push' button with power off and it was 0 ohms. Should I be able to push that button in? it seems to be fixed in place.
 

Last edited by SMGOYAL; 05-30-20 at 05:54 PM.
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