SOLVED: furnace and AC fan won't turn on

[rejesterd]

SOLVED: see my response below for details. In short, replacing the thermostat fuse fixed the issue.

Hi All,

I have a 1997 Terry 30g RV with an Atwood 8531-III DCLP furnace and a Coleman 7300 air conditioner. I bought it from the original owner about 5 years ago. My current problem is that neither the furnace nor the AC fan will turn on (they worked fine last season). Even if I try running just the fan without AC, it doesn't turn on. I hear no noise whatsoever when I flip the switch on the thermostat (i.e. no humming like it's a stuck fan or something).

I checked the circuit breaker for the AC, and it's getting 118V. I checked the 15amp fuse for the furnace and it's fine (I tried replacing it twice just to be sure). However, I noticed that the furnace circuit itself is getting only 0.2V. I verified this with a multimeter by putting both red & black connectors into the slot where the 15A fuse goes. After I saw only 0.2V there, I temporarily switched the black wire going into the fuse slot with the one for the fridge (since the fuse for the fridge is also 15A). After switching the wires, I still saw 0.2V on the moved furnace circuit. So I know that my fuse panel isn't the problem, nor is the main circuit breaker for the AC. I also reset the GFI outlet in the bathroom just to make sure that's not related. Finally, I made sure the cranking battery was connected and charged.

So I'm pretty sure this is a grounding issue in the furnace circuit or one or more components along this circuit are just broken. My problem is that I can't tell where the black wire from the fuse panel ends up, so I'm a little lost at this point. Does it go directly to the control board for the furnace or somewhere else? I see no black wires when I open up ceiling port that goes to the wiring for the AC, and it's difficult to see the furnace wiring without taking the blower motor out.

Also, there's no indicator light for this thermostat, so I can't tell if it's even getting power. According to the manual, there's a 2A fuse for the thermostat somewhere, but I couldn't find it. However, I did notice that the red wire coming form the back of the thermostat was connected to a different (thinner) red wire running inside the wall using a plastic cap connector. So I'm wondering if the 2A fuse is just missing, and maybe my thermostat just got fried as a result.

Should my next step be to investigate the thermostat? I'm just not sure if that could possibly cause the furnace circuit to see only 0.2V instead of the expected 12V. I'm also wondering if there's a way to test the thermostat without cutting any wires going into it.

Thanks for reading and any advice.

[rejesterd]

Edit: After learning a bit more about the wiring, I do think that moving the yellow wire was necessary.

Replacing the thermostat fuse definitely solved the original problem of the AC & fan not working. What caused the fuse to blow in the first place is still unknown.

Once the thermostat was working again, I then discovered the additional issue with the blower not cutting out after 2 minutes. That issue was solved by moving the yellow ground wire (for the relay) to a different connection on the furnace housing. So the other ground connection must have been too corroded.

[drsteve]

That's really weird. Hopefully Chris Bryant will chime in again and tell us what's up here. As an RV tech he knows more about this stuff than 99% of the rest of us.

[rejesterd]

drsteve wrote:
The furnace blower is supposed to continue to run after the thermostat shuts off the burner, until the heat exchanger cools off. Does it shut off immediately now? Because it shouldn't.

You probably need to reconnect the wire to where it was.

No, it takes the expected 2 minutes for the blower to fully shut off after the heat cuts out. Before, I waited at least 4 minutes and it was still going. So everything looks good.

[drsteve]

The furnace blower is supposed to continue to run after the thermostat shuts off the burner, until the heat exchanger cools off. Does it shut off immediately now? Because it shouldn't.

You probably need to reconnect the wire to where it was.

[rejesterd]

SOLVED

Thanks again all. I think I've fixed it. Turns out that the 2A fuse in the thermostat was blown. I replaced it, and I was then able to operate the AC and furnace. However, I noticed that the blower wasn't turning off after waiting 3-4 minutes (this was after I put the thermostat switch to the OFF position).

So I decided to look at the wiring going to the relay on the blower motor. The wiring looked slightly different than the picture that Chris posted. The difference was the very top yellow wire that's connected to the furnace housing. You can see there are 2 slots side-by-side. Mine was plugged into the right one. So I moved it to the left one (to look like the above picture), and now it all seems to work fine. Both AC and furnace kick in and turn off automatically according to the temperature setting on the thermostat.

I don't know why it would've worked ok before if the wiring was incorrect. I'm thinking maybe just that particular connector (the right one) became more corroded than the other (causing a bad connection) over time. I'm going to review the wiring diagram from the manual again, but it appears to be working correctly now.

Thanks to Chris and others for the responses.

[rejesterd]

drsteve wrote:
That's not how you do it... check the voltage by touching the red probe to the hot side of the fuse and the black one to ground. You are simply placing the meter in series with whatever load is present on the circuit. This doesn't measure the voltage.

This is another thing I was confused about, because what you say makes sense and I did do what you said. When I put the black multimeter connector on the neutral bar (ground) in the fuse box, I get 13V at the furnace fuse.

However, I also get 13V when I touch the red & black connectors from the multimeter to the fuse connections for the fridge (in series). Only the furnace circuit shows 0.2V when I do that. So that's another reason why I'm pretty sure the fuse is good, and the issue is somewhere along the furnace circuit.

[rejesterd]

Chris Bryant wrote:
OK- the blue wire is the thermostat wire, it will not have power unless the thermostat is getting power and calling for heat. You need to measure the red wire, and the power on both sides of the breaker in the furnace.

The black wire likely hooks to the furnace outside and the left (looking from the outside), and from there goes to the breaker.

Yes, mine looks just like that with 2 red wires and 1 blue wire going into the breaker. And further to the left (out of the picture) would be the furnace control board. I'll test all the connections at the thermal breaker to see what I get.

[drsteve]

rejestered wrote:
However, I noticed that the furnace circuit itself is getting only 0.2V. I verified this with a multimeter by putting both red & black connectors into the slot where the 15A fuse goes.

That's not how you do it... check the voltage by touching the red probe to the hot side of the fuse and the black one to ground. You are simply placing the meter in series with whatever load is present on the circuit. This doesn't measure the voltage.

[Chris_Bryant]

OK- the blue wire is the thermostat wire, it will not have power unless the thermostat is getting power and calling for heat. You need to measure the red wire, and the power on both sides of the breaker in the furnace.

The black wire likely hooks to the furnace outside and the left (looking from the outside), and from there goes to the breaker.

[rejesterd]

midnightsadie wrote:
pull that fuse don,t just look some time oxidation sets in.clean it .

Thanks. I should clarify that I checked for continuity in the fuse itself with the multimeter. I also changed it twice, and changed the position of the wire on the fuse panel. So I'm pretty certain it's not the 15A furnace fuse. Could certainly be another fuse/breaker, but I don't think it's that one.

[midnightsadie]

pull that fuse don,t just look some time oxidation sets in.clean it .

[rejesterd]

Chris Bryant wrote:
It probably does go right to the furnace, though it may power something else, as well. There is a circuit breaker on the furnace itself, and they do fail.
Where are you measuring the voltage to the furnace?

When I look at the furnace wiring, I don't see a black wire (which is the wire that's connected to the main fuse panel). However, the only voltage reading I took was from a blue wire that was connected to the relay, and it was 0.2V (so the same as what I read from the fuse panel). I'll have to take another look to see where that blue wire comes from. I did try pushing the reset button on the thermal circuit breaker on the furnace, but I didn't take a voltage reading there. So that could be bad. Thanks.

[Chris_Bryant]

It probably does go right to the furnace, though it may power something else, as well. There is a circuit breaker on the furnace itself, and they do fail.
Where are you measuring the voltage to the furnace?

[rejesterd]

Chris Bryant wrote:
You have a 12 volt problem- nothing to do with 120 vac. The thermostat most likely gets power from the furnace, so if you sort the furnace problem out, all will be fine.
The 2 amp fuse is in the thermostat, fwiw.

Oh that's right, I saw the fuse inside the thermostat unit a couple years ago when I was just inspecting it. Thanks. I guess I'll just make sure it's still good, then focus on the furnace circuit. Would you think the wire that starts at the fuse panel goes directly to the furnace control board? I'll have to review the manual again tonight (I don't have it with me).