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Old 10-13-2016, 10:36 PM   #1
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Atwood Furnace not lighting, AC is Clicking.

Hey Guys,
I have a Atwood 8940 Furnace and a 4 button duo-therm Thermostat. When I switch the thermostat to Furnace, nothing happens at the furnace, but I do get a consistent click, like a relay is turning on and off in the front Roof mounted AC. Any ideas? Thanks!

2000 Monaco Diplomat 38A
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Old 10-13-2016, 11:20 PM   #2
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Is your A/C unit a heat pump? Perhaps the heat setting is calling for the heat pump to come on. This is the manual for your thermostat.
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Old 10-14-2016, 08:53 AM   #3
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Is your A/C unit a heat pump? Perhaps the heat setting is calling for the heat pump to come on. This is the manual for your thermostat.

Thanks for the Manual, I do not have heat strips that I know of anyways. The Control board is a 3106481.009
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Old 10-14-2016, 09:44 AM   #4
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If you look at the various wiring diagrams for different set ups, in all cases the thermostat sends the "furnace on" signal through the A/C. That could be the clicking sound you're hearing, a relay in the rooftop A/C control board. Next step is to check for 12v at the furnace control board. If no 12v, then look for a fuse or bad relay in the A/C. You could also use jumper wires to apply 12v directly to the furnace to check that the furnace is capable of working when it gets 12v.
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Old 10-24-2016, 04:33 PM   #5
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Voltage Test

Hey guys,
This is what I found. This is the board into the AC, and I have no power at the furnace. Anyone have any ideas? What relay controls the Furnace and can you replace just a relay or do you have to replace the whole board. Just a heads up, the 2 AC's work great, the furnace does not work and I head a clicking from the front AC board which is what I have the photo of below. I have the voltage that I get from what I believe is the Furnace output. Does anyone know where P6 goes within the board? Thanks for any help you may be able offer!

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Old 10-24-2016, 08:20 PM   #6
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The AC zone control board just passes on the thermostat signal, though that two is +12v. The actual power for the furnace goes direct to it from it's own circuit & fuse. You may be getting the thermostat "heat needed" signal at the furnace, but the furnace itself lacks power to run. Check the fuse and wire for that.

Basically, the furnace sits there listening for the thermostat signal, which is +12v on the signal line. The furnace board has its own separate 12v source, which powers the board and the fan.
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Old 10-25-2016, 12:26 PM   #7
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The AC zone control board just passes on the thermostat signal, though that two is +12v. The actual power for the furnace goes direct to it from it's own circuit & fuse. You may be getting the thermostat "heat needed" signal at the furnace, but the furnace itself lacks power to run. Check the fuse and wire for that.

Basically, the furnace sits there listening for the thermostat signal, which is +12v on the signal line. The furnace board has its own separate 12v source, which powers the board and the fan.
Thanks Gary, it must be passing the 12 volts through a relay that is clicking? I am not getting any voltage at the furnace board connector and only 1-2 volt jumps every time it clicks on the furnace wires marked blue in the diagram below.
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Old 10-25-2016, 06:21 PM   #8
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Just to be clear, there are two +12 inputs to the furnace board, so I'm not sure which one you are saying has no 12v. The funance power line should be aconstant 12v, and the thermostat wire should have a steady 12v once the relay closes.

If the a/c controller clicks once, odds are that is the relay closing and there should be +12v on the thermostat-->furnace wire. You can jumper +12v from another source to that wire to see if the furnace fan starts up.
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Old 10-25-2016, 09:39 PM   #9
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Not to hi-jack, but I have the exact same issue except I have a model 8535 IV
With a 6 wire tstat. AC and hi/ low fan works on tstat, just won't send 12v to furnace. I have posted in Class A m/h section. This is what I've found after a few days of checks. I can jump the two therm wires at the furnace and have it work as well as jumping the two " blue- white" wires coming out of a/c controller. I checked the 3 amp fuse, all ok. I went inside the a/c controller and found the two blue-white wires on the circuit board. One always has 12v (to furnace) and the other should put out 12v once your tstat asks for it ( moving temp lever up). The one wire will not put out 12v when asked, so I'm planning on buying a new a/c controller circuit board $60 and try it. I can't believe it's the tstat when it works the a/c fine and I have a 2nd tstat in bedroom that gives the same result.
I'll be following your thread to see if you come up with something else. Good luck.
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Old 10-26-2016, 09:59 AM   #10
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Just to be clear, there are two +12 inputs to the furnace board, so I'm not sure which one you are saying has no 12v. The funance power line should be aconstant 12v, and the thermostat wire should have a steady 12v once the relay closes.

If the a/c controller clicks once, odds are that is the relay closing and there should be +12v on the thermostat-->furnace wire. You can jumper +12v from another source to that wire to see if the furnace fan starts up.
The relay continually clicks, and the voltage jumps to 1-2 volts on the blue furnace wires. If it is a bad relay, are you able to change out the relay or is it time for a new board? Thanks.
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