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Power to thermostat


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I have the older comfort thermostats that is not getting power to even turn it on. I need to know what and where the thermostat is getting it power from? The thermostat have a RJ45 connection in the back.

I took the coach out for my annual fishing trip and both the front and rear air didn't want to get cold. Took the AC cover off and something decided to build a home in the AC unit and ate the compressor wire. Fixed the wire and it ran fine for 20 minutes or so and it shut down. This is how discovered that there is not power to both of thermostat control unit

 

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You have a 12V fuse panel often near the 120V circuit breaker panel, something in the bathroom or wherever. Power comes from that 12v source.

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3 hours ago, planodp said:

I have the older comfort thermostats that is not getting power to even turn it on. I need to know what and where the thermostat is getting it power from? The thermostat have a RJ45 connection in the back.

I took the coach out for my annual fishing trip and both the front and rear air didn't want to get cold. Took the AC cover off and something decided to build a home in the AC unit and ate the compressor wire. Fixed the wire and it ran fine for 20 minutes or so and it shut down. This is how discovered that there is not power to both of thermostat control unit

 

Pull your owners manual.  Look for the “electrical” section. Should be section 8….page 370.  Find that fuse panel.  There will be an index of the fuses.  My bet is one of the 10A fuses is labeled for Thermostat….but the thermostat power (12 VDC) comes out of that panel.  Find the fuse and replace.  NOW…you will have to do a reset of the system….

Verify you have power.  Then turn OFF the Thermostat.  Hold both the MODE and ZONE buttons down.  Turn ON a the Thermostat.  You will see an FF in the display.  

You also need to understand that there is NO direct 12 VDC power that goes to the Thermostat(s).  12 VDC goes to each of the control modules or more if you have a 4th zone,  there is only a phone line to the Thermostat.  The thermostat is on the “end” of the line.  The phone line runs from the Thermostat to the front AC.  There are two phone lines that dangle from the ceiling or inside the AC unit.  To to get to those, you pull the inside cover.  They are in the cavity where the junction box and the AC power cables are.  There is a dual inline (female to female) phone cord connector on the incoming phone line from the Thermostat.  That connects one of the cables (phone) from the controller located outside, under the cover.  Then, there is a return cable that comes down,  the two cables are interchangeable.  So the circuit starts at the thermostat via a phone cable to the front,  then goes up and back down.  The down cable is then connected to the phone cable going to the middle AC…same deal up and back down and then to the rear AC.  There MAY a separate controller, if you see Zone 4 on the Thermostat, then  there will be a phone line to that controller.

These connectors are prone to intermittent connections.  Much like you described.  You MAY find the 10A fuse is good.  That means that there is a bad connection in, typically, the front unit.  Based on the age, you can replace all these connectors.  I carry spares.  BUT, if you disconnect the two male plugs, then you need to clean with  Q-Tip and alcohol or electronic contact cleaner.  Then you need to plug and thoroughly seat each side.  I had to plug mine in and out 4 times after cleaning.  This burnished the contacts and it has not been an issue.  You need to do that inside each unit.  
After you have cleaned all the connections, then there should be DC power (via the phone line) from the front AC to the Thermostat. NOW, do the reset.  Every time you disconnect even ONE of the phone lines, the Thermostat loses “connection” with the rest of the system. Resetting it is the only way to fix that.  
 

Typically, if you lose Zone 2, then there can be a problem with the outgoing line from Zone 1 or the connection from that line in Zone 2 to,the controller…..same for Zone 3…

SO a check the fuse…if power  reset and go.  If still no juice, clean ALL the phone cable connections and then you should have power….reset and that will fix it.

Read the other recent posts….the vibration and dirt and corrosion play havoc with the phone connections,

Let us know what makes it work again…

Go from there…..

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Calling the cable a "phone cable" can be misleading.  People purchase a actual phone cable replacement finding that it does not work.  If you stretch out the cable laying it flat, you will find that the plug clip on the other end faces the opposite direction.  You can test the thermostat by plugging it into the cable going directly to the air unit.

Perform a reset.  You can check the thermostat on all the other air units.

The flat cable can cause other problems depending on how it is run from unit to unit.  There is no twist in the flat cable so might get cross talk from other devices such as a florescent light fixture.  I replaced all mine with a 2 pair cable with twists in it.

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11 hours ago, Chuck B 2004 Windsor said:

I might add that the cable I use has a red, green, yellow, and black wire in it.

Chuck,  OMG.  You ARE still alive.  Glad to see you chiming in on this.  As usual, you are well informed and right.  
 

I agree, but since it worked before, I have a “hunch”, after sleeping on it, as to what happened.  I should have stated this before I got too technical in my response.

Odds are that when the AC repairs were made in the front and middle units, that the “data cable” connections (the female to female connectors) was disturbed.  Later on, maybe due to vibration of running for a few minutes,, the front AC’s connection to the data cable running back to the thermostat “died” or opened up.  Since the data cable provides the power to the thermostat, then the thermostat went “dead”.  I really suspect that the 10A house fuse is OK.

So, without stating that clearly, my advice was to not only fix or repair the front connection, but also to do the preventative maintenance and clean all the data cable connections on all three units.  If you really wanted to complete the job, then pulling off the thermostat and cleaning the male and female connectors is advised.

I also will post some good advice from some members at the “Gathering” a few years back.  The ON/OFF switch at the bottom is a weak point.  I now leave that switch ON and use the Zone and Mode buttons to turn off a Zone.  I know that eventually one of my units will fail and I plan to pull both units and the Thermostat and upgrade.  But until that fateful day, I am “babying” the thermostat.

From a moderator’s perspective here as well as on the original site, 90 - 95% of the “My AC don’t work” or I “lost” an AC unit and it ain’t showing up on the thermostat are the female to female connectors.  We usually get these posts after the winter storage or lack of use.  I have had this happen three times and I on,y have 2 AC’s.  The next time, I will install the new spare female to female connectors and just clean the male contacts.

To ALL…  I should also commented that in talking to many savvy RV techs, they say the same thing,  many of them will not even do the cleaning.  They will snip off the ends of the two male data cables and use a “phone” crimping tool and put on new male ends and install a new female to female inline connector.

This is the third thread on AC in the past week or so….

Thanks for keeping us “educated”.

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Thanks Chuck and Tom for the great information.

Tom it seem that you had this problem multiple time. It been about 5 - 6 months since I use the coach so you hit the nail on the head. I'll let your know what I find out.  

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Yes Tom, I am still on the green side of the grass.  At 80 years of age, I am finding it more difficult to keep you experts on their toes.  

As new members join the group, this topic will be discussed again.  That is why we are still here to help.  Enjoy your CR tonight,  Chuck B

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So the problem was a blown 2 amp fuse# 14 located behind the washing machine. Did a reset on the thermostat and it seems to be working correctly.  Now I need to figure out what going on with the rear AC unit. For now I'm ok till I get back home. Thank you for your help! 

 

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1 hour ago, planodp said:

So the problem was a blown 2 amp fuse# 14 located behind the washing machine. Did a reset on the thermostat and it seems to be working correctly. 

 

Amazing the different places Monaco put things, even in the same year. 04 Dynasty, it’s in the bathroom. 

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