SOLVED: No power getting to the thermostat

[rejesterd]

SOLVED: It was a short somewhere between the blue wires at the furnace and the tstat. Refer to this post

Hi All,

1997 Terry 30g RV with an Atwood 8531-III DCLP furnace and a Coleman 7300 air conditioner.

I just opened up my camper for this season, and found that power is not getting to the thermostat. The heat, AC nor the fan work. The last time this occurred, it was due to the small 2A fuse on the thermostat itself. However, i removed it today and it has continuity and the connection points look clean. This is an older Coleman thermostat..



I can confirm that I get 13.something volts at the circuit breaker on the furnace.



I checked the voltage at all 3 connection points to that breaker (the 2 red wires and the blue wire).

What would be the next step to troubleshoot this further?

Thanks as always.

[wa8yxm]

If you can get into the utility tunnels (A riser that runs along one or both side walls from there the world is your oyster. follow the tunnels to where the water lines pop through to the basement then you can cross over in the wet bays if you need to (or just compartment to comprtment along) and then up to the Furnace... NOT through the same hole as the propane line (drill a new one if you must) or even come up and enter from the side. Lots of options with not much drilling and no holes where anyone can see 'em.

[wnjj]

Your best bet may be to head down, through the floor and back up into the furnace. Consider cutting a hole in the wall just above the floor and then cover it with a mud ring and blank cover plate. By putting this near the floor, you can angle a drill down through or drill up from the bottom and use hole to allow you to feed the wire. This assumes you can drop a loose wire down all the way from the t-stat hole without framing in the way.

[rejesterd]

Ok boys.. based on my previous tests, I decided to just snip the tstat connections and re-wire them temporarily (without going through the walls). The tstat has power now. The fan runs. I didn't test the heat because I actually need to install a new regulator for the gas lines (when it rains, it pours eh?). But I assume it will work when I actually have gas flowing again.

So now I just have to run the new wires behind the walls. The problem is with the old wire going to the thermostat (the brown wire which contains the smaller gauge red & white wires). It is totally stuck. I tried pulling on it from both ends and it just won't budge. I was hoping I could just tape the new wires to one end and pull them through, but nope.

So it's stuck and I don't know where exactly the wire travels behind the walls/ceiling. I only know that from the tstat, it goes up toward the ceiling. From there, I have no idea where it's going. I did notice that it only moves on the tstat end if I also push up on all the tstat wires.. it moves with them. So I'm guessing it goes to the ceiling plenum, but I don't think it's worth taking that apart. So hopefully I'll figure out a way to run it behind my cabinets then through the walls. Any advice on doing that would be greatly appreciated. But I'm happy I at least understand the wiring now.

[rejesterd]

Well, wnjj's suspicion was correct. After I shoved the probe in a little further, I get 12V at the both sides of this connector. I also get 12V where the blue wire connects to the thermostat's red wire (in the same pic above) with that crimp cap. It feels like a pretty solid connection, but still no voltage at the thermostat end.

So I think that means I need to run a new wire. Anything else I should check before concluding that this is a short somewhere behind the wall? That makes the most sense, since the fuse on the thermostat blew a couple years back.

On the plus side, at least I've found where the mice are getting in..

[wnjj]

That style of connector is just a housing with individual pins that snap into place. You should be able to look closely and see why there isn’t a connection. Check both sides of the connector. Worst case, splice a wire onto each side and connect them with a bullet style connector in parallel with the bad one.

It does seem unlikely that a connector like that just went open circuit but I suppose anything is possible.

[BFL13]

Not clear to me, but the "two blues" that meet at the tstat on a heat only, are actually one wire that is connected by the tstat to get 12v back down to the other end.

So whatever the wires are on yours, only one would have 12v on it until it is connected to the other I think.

I took a photo of mine to muddify things for you 🙂 The connector has a blue looped at the top and a blue at the bottom and there are just two blues at the tsat

[rejesterd]

Ok.. I decided to come back up to camp today, so I can hopefully fix this before the weekend.

I confirmed there is zero volts at the red wire at the thermostat. I hooked my negative probe up to 2 known good grounds this time (the breaker panel frame and a nearby outlet). So I went back to the furnace and confirmed that I still have 12V at the blue wire connected to the furnace circuit breaker. Then I just started working back from there. In the access panel where the plastic 6-pin connector is, I confirmed that I get 12V at the blue wire on one side of the plastic connector, but not on the other side. This is the blue wire that supplies power to the thermostat (i.e. it's the one that's connected to the thermostat's red wire).

Are these connectors something I could find at the hardware store? Again, here's the pic that shows it..



I jiggled the connector around a bit, but no change in the voltage.

Thanks again, guys.

[rejesterd]

dougrainer wrote:
You screwed up big time:B You attempted to use the Tstat mount screw for a ground????? How would that be a ground screwed into WOOD?:B Find another ground source. Now that SMALL 2 wires(Red and White), those come from the Furnace. See your pic. The RED should be 12 volt positive at the furnace and at the wall tstat. Once you get a good ground verify that at the tstat. Doug

PS a Ground should be that Large BLUE wire at the wall tstat. BUT, if that wire has lost its ground, that would be your power problem, so it is best to use a good ground from another source. I usually use the GROUND pin on the 120 wall receptacle.

hahaha I was thinking you would come back and rip me a new one for that. Well deserved. I've been lucky with this RV in that every problem I've had thus far was just a replacement part... either a fuse or a relay. But now I have to get my hands dirty.

[dougrainer]

You screwed up big time:B You attempted to use the Tstat mount screw for a ground????? How would that be a ground screwed into WOOD?:B Find another ground source. Now that SMALL 2 wires(Red and White), those come from the Furnace. See your pic. The RED should be 12 volt positive at the furnace and at the wall tstat. Once you get a good ground verify that at the tstat. Doug

PS a Ground should be that Large BLUE wire at the wall tstat. BUT, if that wire has lost its ground, that would be your power problem, so it is best to use a good ground from another source. I usually use the GROUND pin on the 120 wall receptacle.

[rejesterd]

Thanks again, everyone. All your comments helped me clearly understand the wiring involved, and now I have a good plan for when I go back to camp this Friday. The troubleshooting chart in the Coleman manual (provided earlier by Old-Biscuit) is now very clear. And it seems my earlier voltage test at the thermostat might not have been valid. So I'll start by checking for 12 VDC between the white and blue wires at the thermostat, and go from there, just like the chart says. I'll update again this weekend.

[rejesterd]

Thanks again, everyone. All your comments helped me clearly understand the wiring involved, and now I have a good plan for when I go back to camp this Friday. The troubleshooting chart in the Coleman manual (provided earlier by Old-Biscuit) is now very clear. And it seems my earlier voltage test at the thermostat might not have been valid. So I'll start by checking for 12 VDC between the white and blue wires at the thermostat, and go from there, just like the chart says. I'll update again this weekend.

[BFL13]

Before snipping the wires to the tstat so you can join the red and white from the wall end to end (which is the "two blues" AFAIK), I would try the whole thing with that 2a fuse jumpered. You cleaned it up, but just to confirm. You said that was the culprit last time.

Also I would undo that wire nut on the reds in case it is all corroded in there.

Then if that didn't help, I would snip the wires as mentioned before, leaving lots of wire out from the wall to connect your new (as might be needed) thermostat. (but also long wire from the tstat to rejoin with) If the furnace and AC run by the wires from the wall, then the tstat is not acting as a switch anymore.

[rejesterd]

BFL13 wrote:
What do you mean by "shore power"? That is 120v. Furnace is all 12v. A/C has 120v but controls for it are 12v.

Sorry, I mean the 12V dc power supply. On my rv, I don't need to have the battery hooked up to operate the slideout and other 12V components. I'm plugged into shore power, and I see 118V ac at the main breaker and the breaker for the air conditioner.

[BFL13]

What do you mean by "shore power"? That is 120v. Furnace is all 12v. A/C has 120v but controls for it are 12v.