Thermostat on A/C went dark

[Georgia_Biker]

Today out of the clear blue my bedroom A/C went off and the panel is dark. I looked at the breaker panel and nothing has flipped. Are the A/C units breakers somewhere else?

I have a Big Horn 3875FB with twin Dometics and both have been replaced since new in 2016 (under warranty) - have been great ever since.

Any ideas on where to proceed next?

 

[danemayer]

Larry, if by "panel" you mean the thermostat, first thing to check is the fuse box. The thermostat is dependent on DC power.

 

[Georgia_Biker]

Larry, if by "panel" you mean the thermostat, first thing to check is the fuse box. The thermostat is dependent on DC power.

Hey Dan-

Yes the Thermostat panel is dark like no power.

Thanks for the quick reply as I mentioned I did go to the breaker panel and nothing was tripped - my question is there a different panel for the A/C units if not what is the next step?

 

[danemayer]

You have a circuit breaker panel with 120V AC circuit breakers. One of those powers the rooftop A/C unit.

You also have a fuse box that has 12V DC fuses. One of those probably powers the thermostat.

The fuse fox is usually next to the circuit breaker panel.

 

[Dahillbilly]

Larry
Thinking Dan is referring to the 12 VDC spade fuses not the 120 VAC circuit breakers that trip & can be reset. The spade fuses are the automotive type & they blow; must replace these.

 

[Flick]

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Larry, if the info offered brings no results, it’s likely your thermostat itself has gone bad. They’re relatively inexpensive, readily available on line and simple to install. They do go bad every now and then.

 

[Georgia_Biker]

Larry
Thinking Dan is referring to the 12 VDC spade fuses not the 120 VAC circuit breakers that trip & can be reset. The spade fuses are the automotive type & they blow; must replace these.

Many thanks - I forgot about those - I will check.

We are in the middle of a "remodel" repainting the inside of our RV and changing up light fixtures and plumbing fixtures so thankfully we are not having to stay in it overnight without A/C

 

[Georgia_Biker]

Many thanks - I forgot about those - I will check.

We are in the middle of a "remodel" repainting the inside of our RV and changing up light fixtures and plumbing fixtures so thankfully we are not having to stay in it overnight without A/C

Alright - I checked out the fuse on the A/C and it is fine, after thinking the other A/C is running and they are both on the same fuse.

Based on another suggestion I am going to replace the thermostat and see if that is it.

 

[danemayer]

Alright - I checked out the fuse on the A/C and it is fine, after thinking the other A/C is running and they are both on the same fuse.

Based on another suggestion I am going to replace the thermostat and see if that is it.

You might want to check the thermostat wiring both at the thermostat and on the rooftop unit.

 

[jmarnell]

You might want to check the thermostat wiring both at the thermostat and on the rooftop unit.

The rear A/C thermostat in our Cyclone went dead a while back. Turned out to be exactly what Dan suggested, a loose wire connection up in the A/C unit. Redid the connection and the thermostat came back on and no problems since.

 

[Georgia_Biker]

You might want to check the thermostat wiring both at the thermostat and on the rooftop unit.

Good Idea! will do

 

[Georgia_Biker]

Good Idea! will do

OK - I ordered a new thermostat and it came in this morning. I went and plugged it in and nothing - so that rules out the thermostat and I can send it back. I then got my back pack full of tools and went up top and checked everything out. I think I took apart everything on the unit where there was wiring and connections and I could not find anything out of place. I even got shocked checking one connector which tells me there is juice going up there.

I still think there is a short or something somewhere blocking power but, I am at a loss where else to look. I guess I may have to take it to the dreaded repair shop.

One question - the wire coming out of the wall is just a common phone wire - any idea where this wire goes and where I can find the other end without removing or cutting into walls?

Thanks

 

[cashmore]

OK -

One question - the wire coming out of the wall is just a common phone wire - any idea where this wire goes and where I can find the other end without removing or cutting into walls?

Thanks

That is the control wire that runs up to the control box for the AC. That box is on the inside of the AC upper unit on the inside of the RV. Remove the filter grate and there should be a grey box that controls the AC where that cord plugs into.

 

[Flick]

OK - I ordered a new thermostat and it came in this morning. I went and plugged it in and nothing - so that rules out the thermostat and I can send it back. I then got my back pack full of tools and went up top and checked everything out. I think I took apart everything on the unit where there was wiring and connections and I could not find anything out of place. I even got shocked checking one connector which tells me there is juice going up there.

I still think there is a short or something somewhere blocking power but, I am at a loss where else to look. I guess I may have to take it to the dreaded repair shop.

One question - the wire coming out of the wall is just a common phone wire - any idea where this wire goes and where I can find the other end without removing or cutting into walls?

Thanks

Just another comment. You mentioned a shock. That tells you there is 120v at the ac but you more than likely have lost your 12v. Once again, you’re back to a fuse problem or a disruption in the 12v (+ or -) wire to the control box which I think could originate at the fuse panel, battery compartment or converter. Just a guess.

 

[Georgia_Biker]

That is the control wire that runs up to the control box for the AC. That box is on the inside of the AC upper unit on the inside of the RV. Remove the filter grate and there should be a grey box that controls the AC where that cord plugs into.

Thanks for the suggestion - back up the ladder again - will report back

 

[Georgia_Biker]

Thanks for the suggestion - back up the ladder again - will report back

After another trip up the ladder I did find the phone connectors and they both look solid - I think it's time for a repairman as bad as I hate it.

Thanks again!

 

[TomSt]

When I had issues with the A/C I ran a new 12 volt line to the thermostat and have no problems since. As the whole system is on the same 12 volts it does not seem to hurt anything

 

[bobw]

The connector one the phone wire at the AC side is bad. You can get new connectors and crimping tool at Lowe's or Home Depot cheap. I've had to do this on a couple of RVs, it's a simple job.

 

[Georgia_Biker]

You might want to check the thermostat wiring both at the thermostat and on the rooftop unit.

I wanted to share my $200. lesson with everyone! I was finally stumped so I called out a local Mobile RV guy and to my surprise he came right away working me between calls. Of course I had told him I had checked fuses and removed the up top cowl and could not find anything. He went through the system and confirmed there was no 12v coming to the A/C. He checked the convertor and all wiring behind the fuse panel. Then he asked if the furnace was working - I did not know because it is in the 90's in Georgia. The furnace would not come on - 12v. He went out to the furnace and checked everything there and there was no 12v. He took a gismo he had with 12 volt power and hooked to the a/c and it came on so we now know the A/C is fine it's just the 12v.

My wife happened to mention when Larry (that's me) was installing some new wall lights that he shorted a couple wires but we could never get the lights to work. HELLO!!!

My guy then went back to the fuse panel and started pulling out each blade fuse and low there was a blown fuse on a different circuit than the a/c or furnace. He installed the fuse and we are back in business.

His service call price was $175 - service call and labor - I gladly paid him $200. and wrote it off as a lesson well learned.

Hope this helps other folks in the group.

 

[cashmore]

I wanted to share my $200. lesson with everyone! I was finally stumped so I called out a local Mobile RV guy and to my surprise he came right away working me between calls. Of course I had told him I had checked fuses and removed the up top cowl and could not find anything. He went through the system and confirmed there was no 12v coming to the A/C. He checked the convertor and all wiring behind the fuse panel. Then he asked if the furnace was working - I did not know because it is in the 90's in Georgia. The furnace would not come on - 12v. He went out to the furnace and checked everything there and there was no 12v. He took a gismo he had with 12 volt power and hooked to the a/c and it came on so we now know the A/C is fine it's just the 12v.

My wife happened to mention when Larry (that's me) was installing some new wall lights that he shorted a couple wires but we could never get the lights to work. HELLO!!!

My guy then went back to the fuse panel and started pulling out each blade fuse and low there was a blown fuse on a different circuit than the a/c or furnace. He installed the fuse and we are back in business.

His service call price was $175 - service call and labor - I gladly paid him $200. and wrote it off as a lesson well learned.

Hope this helps other folks in the group.

Good lesson, always check the fuses even ones you don't think are on that circuit. Ive had fuses that "look" good, but when you put an ohm meter in it, boom, not good....