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.

[rejesterd]

dougrainer wrote:
Your original tstat wall pic shows a large 14 gauge RED wire connected to a small 20 gauge red wire with a wire nut inside the wall hole. THAT wire should have 12 volts pos. Doug

So I removed the wire nut that joins the red wires at the tstat. I put the positive probe of my multimeter on the smaller red wire (coming from the furnace) and I connected the black probe to one of the screws that holds the thermostat mounting bracket onto the wall. I get 0V. Am I doing that right? Sorry, I'm very new at troubleshooting electrical stuff.

If my test is correct, then I suppose that means the wiring is bad somewhere between the furnace (where the 2 blues are connected to red & white inside the brown wire) and the tstat. Again, I get 13V at the blue wire connected to the furnace circuit breaker.

[rejesterd]

Ok, I figured out what the brown wire is. Here'a a picture on the other side of the furnace housing..



So the 12V dc power is a black wire which goes from the main fuse panel, and connects to that red wire shown in the above pic. That red wire goes to the furnace circuit breaker (it's the top red wire on the breaker, shown in my earlier pic). I can also see that both blue wires (coming from the furnace circuit breaker and the furnace relay) connect to the smaller red and white wires inside that brown wire. So basically, the small red & white wires at the thermostat are the blues coming from the furnace.

BFL13 wrote:
Did you put your voltmeter on the AC control box shown in that photo for any 12v there? With and without the tstat control to "on" for that? Those terminals look easy to get the meter probes on, unlike at that tstat circuit board without doing any harm touching the wrong thing.

Yes, and I'm getting 0V at those connections, regardless of what the tstat is set to (on/off/fan/auto/manual).

Edited: The black wire is 12V dc power, not shore power.

[BFL13]

Did you put your voltmeter on the AC control box shown in that photo for any 12v there? With and without the tstat control to "on" for that? Those terminals look easy to get the meter probes on, unlike at that tstat circuit board without doing any harm touching the wrong thing.

[rejesterd]

Ok Doug, I did like you said and pulled the wires out further. What I thought was a black wire is actually brown. Inside are 2 smaller gauge wires, one red and one white. These are connected to the tstat red and white wires respectively.

I don't know where this goes, but I assume this is shore power.

The lighter blue wire from the tstat connects to the darker blue wire shown in the picture. That darker blue wire appears to go to the AC control box. I assume the green, yellow and gray wires also go to the AC control box.

Still not totally sure what the next step would be. Should I try connecting red to green or gray just to see if that operates the fan?

[dougrainer]

Your original tstat wall pic shows a large 14 gauge RED wire connected to a small 20 gauge red wire with a wire nut inside the wall hole. THAT wire should have 12 volts pos. Doug

[dougrainer]

From your pics, the Roof AC IS controlled from the Wall Tstat. The HOT blue (12 volt positive wire) exiting the furnace goes to the wall tstat and supplies 12 volt power to the wall tstat. The wall tstat then sends the appropriate 12 volt signal to the roof AC to turn on the Fan and or the Compressor. The wall tstat then also sends a 12 volt signal BACK to the furnace to that second blue wire to turn the furnace ON. So, Check all the wires at the tstat for 12 volts. THAT one wire should be from the furnace. IF NONE of those wires have 12 volts you have a disconnect(open) between the furnace and the wall tstat. Be advised, that some OEM's will install an inline fuse inside the wall behind the tstat, so make sure there is NO in line fuse stuck down in the wall. There will be enough wire slack to pull the wiring out if there is a fuse. Doug

[Old-Biscuit]

rejesterd wrote:

Old-Biscuit wrote:
Connect the 2 blue wires togther.furnace should come on
Connect the blue wire (one from furnace on/off switch) to Yellow wire and A/C Unit should come on

BFL13 wrote:
you can confirm the furnace works down at the furnace by jumpering the blues that would go to the tstat (but somehow get to the red and white on yours I think)

Ok, I'm slowly getting this. I've actually looked at the manual Old-Biscuit linked before, but I don't have 2 blue wires. Here's the thermostat wiring.



So there's one of each: blue, red, white, yellow, gray, and green. It's hard to see from the picture, but there is also another red wire inside the wall that is connected to another very small gauge red wire with a wire nut. Neither of these appear to be connected to the thermostat. Also, there are 2 purple wires coming from the thermostat that have been cut and taped over. I haven't done anything to the wiring here, so this is the way it's been for years (and the system has worked generally well for the most part).

Based on what I'm reading in the manual, it seems that if I connect red to green or red to gray, that should operate the fan only. And connecting red to yellow & green, that would operate the AC at low speed. And connecting red to white would run the furnace.

Is that correct? Thanks so much.

YES!

[BFL13]

As a side note, I would be scared of how the wires are on that tstat circuit board too easy to break something can't be fixed, so I would snip the wires half way up to leave length and wire nut them back together after.

That way no chance of breaking anything right at the circuit board and no chance of mixing up the colours, and--it is easy to put a meter on any two and for putting the ends together of the ones coming out of the wall for trying to see if anything makes the furnace work--and without shorting something on the circuit board can't be fixed.

[rejesterd]

dougrainer wrote:
1. You have NOT removed all the wires from the hole for the Tstat.
2. You will NOT have "blue" wires at the tstat. The Blue wires are the 2 just for the furnace AT the furnace that go to the wall tstat and return
3. Most OEM's use standard home type Tstat multistrand wire to connect the Roof AC and to the furnace.
4. Usually they use the small 20 gauge wires---red and white to go from the furnace to the wall tstat.
5. When you have your type tstat, the one blue wire at the furnace that is 12 volt, it goes to the Roof AC control board in the AC. THAT wire is what supplies 12 volt power to the Roof AC control board and then DOWN to the wall tstsat. On other models, the blue 12 v wire goes to the wall tstat and then another wire then feeds the Roof AC for 12 volt power to the Roof AC. You need to get all the wires out of that small hole in the wall and odds are you will see a wire not connected. Doug

Thanks Doug. The thermostat wiring shown in the above pic is the way it's always been. I've looked at it before and made notes about the wires and colors. If I look further in the wall hole, I can also see a black wire, and that one additional red wire that has a wire nut on it (with a very small gauge red wire connected to it).

Here's my AC control box..



I cleaned up these connections when I first noticed the problem a couple days ago. From left-to-right, the colors are yellow (Y), gray (GL), green (GH), and blue/white connected together (B).

Based on what you're seeing, what would you check next?

[dougrainer]

1. You have NOT removed all the wires from the hole for the Tstat.
2. You will NOT have "blue" wires at the tstat. The Blue wires are the 2 just for the furnace AT the furnace that go to the wall tstat and return
3. Most OEM's use standard home type Tstat multistrand wire to connect the Roof AC and to the furnace.
4. Usually they use the small 20 gauge wires---red and white to go from the furnace to the wall tstat.
5. When you have your type tstat, the one blue wire at the furnace that is 12 volt, it goes to the Roof AC control board in the AC. THAT wire is what supplies 12 volt power to the Roof AC control board and then DOWN to the wall tstsat. On other models, the blue 12 v wire goes to the wall tstat and then another wire then feeds the Roof AC for 12 volt power to the Roof AC. You need to get all the wires out of that small hole in the wall and odds are you will see a wire not connected. Doug

[BFL13]

I don't know which wires do what in that photo, but before removing any, make very sure you know where each goes for when putting it back!!

I think you need to find the place up where the relay is for the air conditioner to see if there is 12v at that location. I got the idea the 12v for the tstat comes from up there where a click can be heard, but not sure. The pros here will be able to say what to do now.

[rejesterd]

Old-Biscuit wrote:
Connect the 2 blue wires togther.furnace should come on
Connect the blue wire (one from furnace on/off switch) to Yellow wire and A/C Unit should come on

BFL13 wrote:
you can confirm the furnace works down at the furnace by jumpering the blues that would go to the tstat (but somehow get to the red and white on yours I think)

Ok, I'm slowly getting this. I've actually looked at the manual Old-Biscuit linked before, but I don't have 2 blue wires. Here's the thermostat wiring.



So there's one of each: blue, red, white, yellow, gray, and green. It's hard to see from the picture, but there is also another red wire inside the wall that is connected to another very small gauge red wire with a wire nut. Neither of these appear to be connected to the thermostat. Also, there are 2 purple wires coming from the thermostat that have been cut and taped over. I haven't done anything to the wiring here, so this is the way it's been for years (and the system has worked generally well for the most part).

Based on what I'm reading in the manual, it seems that if I connect red to green or red to gray, that should operate the fan only. And connecting red to yellow & green, that would operate the AC at low speed. And connecting red to white would run the furnace.

Is that correct? Thanks so much.

[Old-Biscuit]

13V on Blue wire..that goes to the thermostat
Remove cover on thermostat
Connect the 2 blue wires togther.furnace should come on
Connect the blue wire (one from furnace on/off switch) to Yellow wire and A/C Unit should come on

Thermostat is bad if in Cool or Heat modes A/C or Furnace do not work


See pg 14 and 15....Flow chart for Heat Mode and Cool Mode
Coleman Thermostat Info

[rejesterd]

Old-Biscuit wrote:
That On/Off CB Switch has 3 wires
RED and BLUE on one terminal and then RED on other terminal

RED/Blue....red is 12VDC + from DC Dist Panel (FUSED)
Blue sends 12VDC+ to the thermostat

RED on other side goes to Timed Delay Relay.
When Thermostat closes then 12VDC+ goes to coil side of Timed Delay Relay which allows the 12VDC+ on RED to go to Fan Motor

So you should have 12VDC + on RED/Blue at the On/Off Switch and on the RED from Switch to the Time Delay Relay (Green Box)

Yellow is NEG/Ground

I have 13V at the 2 red wires and the blue wire shown in the above picture. Not sure what to check next.

I forgot to mention that the AC nor the fan for the AC work either.

[Old-Biscuit]

That On/Off CB Switch has 3 wires
RED and BLUE on one terminal and then RED on other terminal

RED/Blue....red is 12VDC + from DC Dist Panel (FUSED)
Blue sends 12VDC+ to the thermostat

RED on other side goes to Timed Delay Relay.
When Thermostat closes then 12VDC+ goes to coil side of Timed Delay Relay which allows the 12VDC+ on RED to go to Fan Motor

So you should have 12VDC + on RED/Blue at the On/Off Switch and on the RED from Switch to the Time Delay Relay (Green Box)

Yellow is NEG/Ground