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Old 10-22-2013, 09:22 PM   #1
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Atwood 8535 furnace diagnostics...HELP!

I have a dead furnace and cannot figure out the problem...I think....I hope I don't know what the problem is. Without complete AS wiring schematics I am guessing on some things which may be leading me down the wrong path.

Status: I have heat pump operation. Last night I had no furnace and I found the breaker tripped in the furnace compartment. Reset and it ran for a test cycle. I now have nothing going on...no lights in the board and no operation. I have 12V and can make the fan run. I HAVE NO CONTINUITY ON THE TWO BLUE THERMOSTAT WIRES, ON THE HARNESS END OF THE CONNECTOR WHEN THE THERMOSTAT IS SET TO HEAT AND 90*.
I fear this is where my problem is, but I am not sure how the electronic thermostat behaves. An old mechanical thermostat is just a switch that completes the circuit....not sure about this one.

Tests so far: I have continuity on all four of the telephone wires from the thermostat to the A/C unit. The A/C side of the telephone line disappears up into the roof top junction box. There are two blue wires which come back down inside from the rooftop which have continuity when the furnace is commanded on and they are open when furnace is commanded off. They are butt connected to two blue wires which disappear between the curbside skins in a bundle with a lot of other wires. This bundle reappears at the floor behind the microwave. The two blue wires in that bundle are different colors/stripes than the ones at the A/C unit and connect to the furnace connector....but I am used to AS not following any wiring conventions.

When I cut the wires at the furnace connector and at the A/C harness end and run a jumper I can get not continuity on EITHER BLUE WIRE in the wall and ceiling. This makes me believe I don't understand the chassis wiring harness.

I was of the belief that I have an open in the wall/ceiling in a blue wire....BUT BOTH? AT THE SAME TIME?????? I find this too coincidental.

I am questioning myself:

What is the wiring routing? What does the thermostat need and to allow the 12V pass through for the furnace to verify sail switch and limit switch status to the board?

What am I missing?
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Old 10-22-2013, 11:12 PM   #2
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Man's good friend Google says: http://www.atwoodmobile.com/manuals/...0SP%201.08.pdf
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Old 10-23-2013, 06:47 AM   #3
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Thanks, I have that manual, but unfortunately it tells you nothing about the thermostat, nor AS wiring. I fear my problem or at least the circuit logic info I need to properly diagnose lies outside the furnace and the very basic info in the Atwood manual. I would like mor info relative to the Atwood board logic though.
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Old 10-23-2013, 08:54 AM   #4
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Well, as usual, sleeping on it brings clarity....and my worst fear.
(disregard the PWM controller circuit for independent blower control...that was the subject of another thread and not a factor here)

This morning I ran test pigtails from the "thermo" wires at the connector just above the Thermostat in the attached diagram. I have 13.4 volts at the connector and at the overhead A/C connection, and at the thermostat, and back to the A/C connection.
However, back at the "thermo" test lead at terminal 5 on the attached diagram, I only have 1.3 volts. I then ran a jumper from my terminal 5 test lead to the overhead A/C connection FROM the thermostat, supplying a known 13.4 volts to terminal 5. And the furnace and thermostat runs as designed.

So....The wire which supplies thermostat controlled voltage from the A/C unit connection to terminal 5 is mostly broken somewhere. It runs between the ceiling skins, down the wall above the stove and exits at the floor.

How would you experts suggest I run a new wire, short of removing counters, padded ceiling, and skins?
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Old 10-23-2013, 10:07 PM   #5
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Update and a successful repair. Dan at AS, got out some line drawings and recommended I drill through the rib here:

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and fish to a 1" hole drilled in the corner of the closet here:

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See the fish tape? Can you believe I hit the hole with the tape on the first try? Sometimes it's good to be lucky.

This is the hole in the rib in the A/C cutout with grommet installed and wire installed.

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This is the hole in the closet corner with grommet and wire installed.

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This is the finished product in the closet.

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I then ran the new wire down the existing conduit for the closet light, under the closet floor and under the galley counter to the furnace.

All works as new now!

All in all, after the expert "hidden obstacles" advice for proper fishing, it wasn't too bad a job. About 2 hours.

I have found AS, particularly Dan, to be extraordinarily helpful with two separate difficult issues this week alone. I am very impressed with their willingness to work with owners on technical advice.
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