Main burner only lights for short time; pilot cycling on/off

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  #1  
Old 03-07-15, 02:37 PM
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Main burner only lights for short time; pilot cycling on/off

Hi folks,

I have what I think is a bad gas valve and I'm hoping someone here can confirm or suggest what else it could be.

Overall the problem is that main burners don't stay lit. Pilot will light, main burners sometimes light up, but then go out. Most of the time it's just the pilot light cycling on and off.

There's a video at https://www.dropbox.com/s/vhak743g9s...15219.mp4?dl=0. It's a bit long - skip forward to about 3:30 for the important part.

Obviously I've unmounted the mercury switch in the course of messing with things. The other thing I've done here is put an LED+2K resistor from each of the hot/cold terminals from the mercury switch so you can clearly see what's energized.

At 3:32 you see the cold line (left LED) is energized and the pilot lights. Then in a few seconds, the cold line de-energizes, hot line (right LED) energizes, and pilot light goes out. That's what's happening in most of the 3.5 minutes you skipped over.

At 3:41, the hot line energizes while the cold line also stays energized, and the main burner lights up (yay!). But around 3:49 the cold goes off again, pilot & main burner go out immediately with the cold shutting down, and then eventually hot turns off and the cycle of not lighting starts again.

Here's the wiring diagram from inside the door:
https://www.dropbox.com/s/esk0y40rvj...23349.jpg?dl=0

If I'm reading this right, the "RAP" in the bottom right corner should be the mercury switch and the "AGV" right above it is the gas valve. Now, it looks like that "cold" wire should be energized in either of two ways: by RAP connecting terminals 3 & 4 (in the cold position), or by the pressure switch in AGV connecting 3 & 4 due to pressure switch in AGV.

So when I see both lines energized, that's what it *should* do, and when I see just the hot line energized, it means the pressure switch in the gas valve isn't closing the circuit.

The common suspect in how-to's on the web (none of which seem to address the exact symptoms I'm seeing) seems to be the thermocouple/mercury switch, but based on these observations I don't think that's at fault here.

Can anyone confirm? Or have suggestions to diagnose?

Thanks!
 
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  #2  
Old 03-07-15, 02:57 PM
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Can you post the make and model of furnace?

I would be inclined to change the t couple as it seems there is not enough voltage sensed to open gas valve.
 
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Old 03-07-15, 02:58 PM
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Oh, and welcome to the forums
 
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Old 03-07-15, 03:43 PM
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Thanks, lawrosa!

It's a Trane BLD045L936A https://www.dropbox.com/s/n3855mnind...3-pub.jpg?dl=0

And here's the mercury switch:
https://www.dropbox.com/s/f2oa9x38cu...90448.jpg?dl=0

Multimeter shows 27V on the hot/cold terminals when they're active.

Seems like I'm looking at $130-150 for a replacement mercury switch.
https://www.patriot-supply.com/produ...DGERS_3049_115
 
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Old 03-07-15, 06:29 PM
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In looking at the schematic, thank you for posting it, the gas valve is controlled directly from the RAP control. That control gets it's command from the thermal sensor. My gut feeling is that you have a pilot size issue.
 
  #6  
Old 03-07-15, 09:55 PM
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A couple close-ins on the pilot:
https://www.dropbox.com/s/hmegxto2p8...94405.mp4?dl=0
https://www.dropbox.com/s/zzeyw0bqoz...94529.mp4?dl=0

I still think it's the pressure switch in the AGV. Here's a marked up version of the schematic:
https://www.dropbox.com/s/1gxnnzea4e...tated.png?dl=0
In the first part of the cycle, just the pilot valve is opened and the cold side of the merc switch is active. (The top image shows current flow through AGV pilot magnetic coil and down through the RAP's cold terminal. Ignoring for a second that this is AC...) In the 2nd part of the cycle, the RAP switches to the hot side (disconnecting the cold side), which activates the MV coil, but would (without anything else) break the circuit to the pilot coil. BUT, you want the pilot to stay on, so the circuit is completed from the P coil through the pressure switch (instead of the cold side of the RAP).

Unfortunately if that's it I think it means messing with gas parts I probably shouldn't to mess with myself, so next step is getting an HVAC guy to come see it.
 
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