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Old 03-06-2021, 08:21 PM   #1
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HVAC not working

Neither the furnace nor the AC worked on this last trip. Luckily the weather cooperated and I didn't need the AC. BUT- - the furnace was needed. Again, luckily I carry a small electric heater and that served us nicely.
We have a VEGAS 24.1 (2014) and both are controlled by the same Coleman thermostat. I arranged to have a new thermostat delivered to home so that I could work on it when we returned. Simple fix - I figured since the stat is common to both systems. Wrong !! So AFTER I replaced it and still nothing worked, my wife explained to me that there must be something else wrong ( now why didn't I think of that??).
Checking for 12V at the stat revealed that to be the problem- no 12V - good
neg. ground. All fuses in the converter check OK with a VOM. All fuses in the forward battery control center under the driver's seat also check good- even the 25A fuse marked HVAC.
Jumping 12V from the converter to the tstat allows the AC to work from the tstat- but no furnace. That confirms the lack of 12V at the tstat to be the problem. But now I'm stopped and I'm hoping that one of you - my very best friends in the whole world!- can help me out here. Where do I go now?
Every thing else in the coach works OK. What's going on here? TIA, Ed.

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Old 03-06-2021, 08:26 PM   #2
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THOR #3952
Your furnace should have a reset button on it. It may say on/off. Try cycling it on and off.
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Old 03-06-2021, 08:31 PM   #3
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Frustrating, isn’t it!? Alas, I think your choices are limited. You can run a new fused 12V line - although that would leave you forever with an oddball/orphan system - or you can find the other end of the present wire, tracing it from the fuse to your thermostat until you find the break. It’s unlikely to be the wire itself so check it’s connection at your fuse panel. I don’t understand your comment that the AC worked with your temporary test repair but not the furnace. Does that mean you have two separate problems? Good luck.
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Old 03-06-2021, 09:04 PM   #4
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Your thermostat receives its 12 VDC from the Furnace. The furnace On/Off Reset switch must be on and there must be 12 VDC applied to the switch from the "Appliances" fuse in the HOUSE fuse panel that also contains the breakers for 120 VDC items.
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Old 03-06-2021, 09:06 PM   #5
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See if your furnace has a switch like this one. If it does, cycling it of and on and see if it fixes it.
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Old 03-06-2021, 09:50 PM   #6
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If 16Ace27 hadn't told me where the tstat 12V comes from (the furnace) I'd still be pulling panels (and what little hair I have left). The reset switch was in the reset (ON) position, but the crimp job on the ring terminal on the switch was not tight. The wire was out of the terminal. A new ring terminal and a proper crimp put 12V to the tstat and the AC works.
Thank You for that, 16Ace27.
Now I'm left with a furnace that doesn't work. Tomorrow I will try to find out why the blower won't come on. I know that nothing happens until the fan pushes enough air to make the sail switch. So, first things first in the AM. Right now, I'm late for the daily 4PM meeting across the street where we discuss how to fix ALL the world's problems. Sometimes we even have an adult beverage or three to loosen tongues.
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Old 03-06-2021, 09:52 PM   #7
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Originally Posted by 16ACE27 View Post
Your thermostat receives its 12 VDC from the Furnace. The furnace On/Off Reset switch must be on and there must be 12 VDC applied to the switch from the "Appliances" fuse in the HOUSE fuse panel that also contains the breakers for 120 VDC items.


Now THAT is good to know. And, of course that explains the OPs issue. Guessing he’ll find it quickly with this info. And I’ll remember it too in case it’s ever needed.
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Old 03-06-2021, 11:38 PM   #8
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Originally Posted by Seabee View Post
If 16Ace27 hadn't told me where the tstat 12V comes from (the furnace) I'd still be pulling panels (and what little hair I have left). The reset switch was in the reset (ON) position, but the crimp job on the ring terminal on the switch was not tight. The wire was out of the terminal. A new ring terminal and a proper crimp put 12V to the tstat and the AC works.
Thank You for that, 16Ace27.
Now I'm left with a furnace that doesn't work. Tomorrow I will try to find out why the blower won't come on. I know that nothing happens until the fan pushes enough air to make the sail switch. So, first things first in the AM. Right now, I'm late for the daily 4PM meeting across the street where we discuss how to fix ALL the world's problems. Sometimes we even have an adult beverage or three to loosen tongues.
That switch is also a reset switch. It could have been in the on position and still not have 12 volts which is why “I said to cycle it off and on”.
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Old 03-07-2021, 01:33 AM   #9
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That switch is also a reset switch. It could have been in the on position and still not have 12 volts which is why “I said to cycle it off and on”.
Yes, I got that- and cycling the switch is how I saw the loose wire. My thought, too, was that if it was a simple "on/off" switch it likely wouldn't be labeled "Reset". It was just ASKING to be cycled. You directed my attention to it- the labeling could only be seen with an inspection mirror so Thanks for that.
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Old 03-07-2021, 01:59 AM   #10
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So now it's on to troubleshooting the furnace! Here's a simplified schematic for the furnace:


You can see where the ON/OFF/RESET switch supplies the thermostat in the diagram's "blue" wire, and then the thermostat feeds that +12 VDC back into the furnace to the motor relay to start the motor spinning the fan. There also has to be +12 VDC on the "red" wire from the switch to the relay in order for the fan to run. That relay is on the control board and commonly fails due to moisture contamination which requires the control board to be replaced.

Wire colors in the diagram may not match your installation.
If you do end up determining the control board needs to be replaced, replace it with a Dinosaur brand board with a 3 year warranty, not the OEM Atwood board which is prone to fail.

This should get you started troubleshooting the furnace. Give us some feedback on what you find and ask questions if you need to.
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Old 03-07-2021, 05:43 PM   #11
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T/S confirmed it's a bad board. Thanks for that schematic- fast and easy.
How do I confirm the correct P/N for the Dinosaur replacement board?
I'm pretty sure I have the Atwood P/N from the manuals in the MH- 31501. Is there a cross reference chart for Dinosaur or are they all the same?
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Old 03-07-2021, 07:23 PM   #12
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Quote:
Originally Posted by Seabee View Post
T/S confirmed it's a bad board. Thanks for that schematic- fast and easy.
How do I confirm the correct P/N for the Dinosaur replacement board?
I'm pretty sure I have the Atwood P/N from the manuals in the MH- 31501. Is there a cross reference chart for Dinosaur or are they all the same?
This is the one I put in my furnace 1.5 years ago when the relay on my board crapped out.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Dinosaur has cross reference charts on their website but they can be hard to use. If not sure, give them a call tomorrow.

https://www.dinosaurelectronics.com/UIB_Chart.htm
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Old 03-07-2021, 08:28 PM   #13
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I believe the one you used is the one I need- but to be sure- I'll call them tomorrow.
I had the board out to find a mfr part #. No luck there- but the fried animals on the back of the board probably contributed to the failure. I usually keep one of those bug repellant wrist bands in the reefer compartment, water heater compartment and furnace.
I must not have put one in this furnace compartment. They are in the other two. I'll keep updating the post.
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Old 03-11-2021, 01:18 AM   #14
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Great help from everyone and I appreciate every suggestion. That schematic helped out a lot and was the thing that pointed directly to the control board. I spoke to Customer service at DINOSAUR and he confirmed and I ordered the same board 16Ace27 used.
The instructions for the new board were a little confusing, so another call put me straight. What he said, essentially, was to ignore the directions in this case (and only this case) and go wire for wire - just make sure not to swap the power connector with the blower connector.
So, thanks for the help, and I now have an operating HVAC system. Maybe I can help someone else in the future. Ed
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