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2000 Diplomat - HVAC issues - FOUR (4) Button Duo-Therm T'Stat. Can't find REAR DC power. Help appreciated.

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Forgive me if this has been discussed before, which it probably has, i did a search for it and nothing came up.

In this 2000 Diplomat, the front AC unit works as intended. It had a blown fuse in the control box but works after replacing that. The rear AC hasn't started up in any thermostat setting, not even just fan. It has 120v in the control box, but there doesn't seem to be 12v to that small fuse. Breaker panel shows it has its own breaker, which is functioning, and the fuse panel next to it has nothing listed as anything to do with the AC's, only a fuse for "Furnace", which is good to go. 

What i would like to find is where the rear AC gets its DC voltage from, i guess for the the front AC as well. They are Dometic units, the front has heat pump or strip i do not know but it blows out heat when on thermostat heat pump setting. The thermostat is a 4 button Duo-Therm and is functioning i believe as it should be. There is never a selection for individual zones that i have found and the rear AC dip switch setting is i believe set to like a slave status (end pin up rest down).

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Here's your print.  The wiring is simple...

BTW...if you don't have prints....use the SEARCH   Put in Wiring....then click on Everywhere....and select FILES.  Download this one.  The prints are actually broken out into individual PDF's as well as the entire set.  I had theses in my "Monaco Wiring Files" and pulled the Comfort Control one.  Really simple....

There IS no 12 VDC wiring to the REAR.  The "PHONE" cord carries two signals.  One pair is 12 VDC and the other is the Multiplexed or control signal.

The STANDARD FIX...  Open up or find the "phone" wires.  ODDS are (I know how the later ones work....) there are Dual INLINE (phone connectors) that connect the ceiling wiring to the control modules.  There are (is) a control Module in the rear....just like the front.  The FRONT unit actually GETS its 12 VDC from the Furnace....imagine that...

OK...the DRILL.  Remove the inline connectors (one at a time, but they are NOT polarized and the Control Module's leads work either way... Use Alcohol and Q-Tips and CLEAN the Male and Female (inside the connectors) ends or the copper strips.  Then Plug, unplug and reseat FIRMLY a few times.  I actually carry a few spare connectors and they "wear" from vibration.

CLEAN every Phone Male and Female connection that you see.

OK....there is a video that I found....  KEEP IT as you might need to do some "refurbishment".  The guy sounds like he KNOWs what he is doing, but at the end...he admits that he has NO IDEA as to whether all the cleaning or such did any good.  Probably DID...but IF he was really smart...he would have done this.  This reset is HARD TO FIND....took me several minutes...


NOW....follow the above and reset the Thermostat.  THAT would fix it.  I have also included a manual for your files....  This should answer some of your questions.  Also if you look at the prints....  the Furnace ONLY runs from the FRONT thermostat....the rear has NO control....this is TOTALLY different from how the 5 button's work....or maybe I should say...it is DIFFERENT.  


I SUSPECT (hope) that the connection cleaning and resetting will fix....if not...then you can start from there.  





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  • Tom Cherry changed the title to 2000 Diplomat - HVAC issues - FOUR (4) Button Duo-Therm T'Stat. Can't find REAR DC power. Help appreciated.

Thanks Tom - for future use, here is a link to a new file in our Downloads.  The Operating Instructions and Reset Procedure are now in one document.


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Iam guessing the rear AC control box or the unit itself or both is dead. After reviewing the schematics in the files you guys recommended downloading, i switch the dip pins to make the front AC Zone 2. It still worked as Zone 2 on the thermostat, which is the first time i have seen anything "Zone" on my thermostat. The back on still no DC and nothing. The RJ11 cables are fine, i checked continuity with the one going to the back AC unit with a continuity tester and a digital line tester. The line tester actually has the cable plug into it and the remote end also plugs in to the other end of the cable. Oh that doesn't check the plugs on the boxes though, i will clean those with alcohol and swabs and see what happens. 

In the event i do need to look at replacing stuff, how do i check the unit itself besides the continuity motor check, which i know how to do? Is there anyway to directly bypass the thermostat just to see if the unit even works?

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Ok, i determined the comm cable from front unit to rear unit was defective somewhere in the ceiling. I ran a new phone line with the RJ11 plugs installed as per Dometic wiring and it worked, well kind of. It immediately showed up the zones in the thermostat and when i switched that zone to cool, he made a nasty noise while it was pumping freon out into the coach and no fan. So i shut it off and switched it to just fan and nothing. So apparently it is dead.

Now looking at replacement units, i realize i will have to buy the unit itself and the control box for it and adapt it into the Duo-Therm 4 button thermostat. I see a few different price points for these 15k BTU units ranging from $600 to $1700 each. One brand i have never heard of, Advent. Who is this company made or owned by and has anyone ever installed one that can comment on its quality?


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OK….now here is the background.  Your MH was wired for MPX and does NOT have any easy (to me) fixes for the HVAC system.  The Dometic (Duo Therm) thermostat is a Digital Multiplexed system. It was designed to be a central thermostat.  From the wiring diagram, you see now it works.

The conventional fix is to stay with Dometic.  Since you have a “failure”, most folks opt for replacement of both.  That also includes the new CCC2 thermostat.  NO OTHER UNIT will work with the system.  OK, there is ONE.  But then when you install it, the thermostat will NOT control your furnace.  There is no known “workaround….and folks here and all over the internet from Air Streams to god knows how many other brands have tried.  We have some success stories….as well as MORE frustration and never got it to work.

@vanwill52 has a Dynasty the same age.  He is a great resource.  He put in two new “Colemans”….I think.  He had to abandon the MPX system and has, I think, an individual thermostat for each unit….but he had to do some “mods”.  I would PM him and talk.

In a perfect world….you need 12  VDC to each rooftop.  You CAN use exterior grade wire mold and run that across the roof.  You also need a 4 or 5 wire run DOWN to the wall where you mount the thermostat.

You have a bad phone line,  folks have used a wire snake and replaced the cable by snaking a new wire through the duct work.  Same deal…you can run a new phone line on the roof from one unit to the other.

SO…. It is COMPLICATED.  If you replace or run an exterior phone cable….then, you can purchase two new DOMETICS….and the CCC2 thermostat and also hook up your furnace….and it will be the same as an OEM.

If you don’t go that route….put in a replacement (as in it will fit without a lot of cutting of the roof hole)L You can run 12 VDC from the front to the  rear on the roof.  You then use the “interior” thermostat in the unit and it “stands alone”.  So you have no central thermostat…

Then you COULD put in a new Dometic up front and upgrade the thermostat.  The original CCC MPX doesn’t work with the new unit,  but you DO have the central thermostat now…. And the control unit in the new system will work exactly like the old system and you have one thermostat and multiple modes.

Other option….put in a standalone like the rear. You control the cooling from the new unit…some have a BT link so you can use your phone.

You WILL have to install a simple Honeywell ON/OFF wall thermostat....and run a two wire (#16) cable to the furnace. Then, you let that thermostat work.   BUT…you need a special  MECHANICAL one with an OFF Switch.  No power is needed….as the furnace only needs a “on or off”, or like a light switch to control it. I posted the thermostat a while back.

Do some research.  You can use the search function…put in dometic (or maybe duo them) and thermostat in the search.  Then click on everywhere and choose TOPICS….all the past topics and each individual post will be there….

FULLY understand HOW your system works.  Then, if you abandon the MPX….look at the print..and then the instructions for the new unit and draw up a circuit….THEN go inside….figure out how you are going to run the wires….

Good luck







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