Dometic HVAC: Sudden Changes

FlyboyTR · 01-06-2019, 02:07 PM

We have a 2013 Surveyor SC189, Dometic ducted roof air with Dometic Single Zone LCD Thermostat (Fan, A/C, Heat, OFF). This system has been working flawlessly.

In preparing for a camping trip next week I was working in the camper and decided to turn on the heat. This was when I realized something was wrong.

Previously I would run the fan on Low (typically about 20 seconds before the fan would engage) to help circulate the air, then switch to the furnace setting and within seconds the gas furnace would fire up and cycle on-off as needed to maintain temp. The Dometic HVAC fan would continue to run on Low without stopping...just as it should.

Today, when turning on the fan, nothing happened for several minutes. Then suddenly a click and the fan motor came on for about 2-3 seconds, then shut off. Anywhere from a couple of minutes upwards to 10 minutes or so the familiar click and the fan would turn on for 2-3 seconds then off.

I lowered the A/C temp to it's lowest setting, about 10 degrees below ambient temperature. The A/C nor fan would come on. I then tried for heat only, raising the setting about 10 degrees above ambient temp. Nothing.

I have attempted the reset procedure several times with no success. Holding down + key & power/selection key until the dashes appear and it beeps. That has changed nothing.

OK...not sure where to go. All the thermostat wiring is good as well as the connections in the roof air unit. 12 volts continuous at the thermostat. I have unplugged, repluged and tugged on every wire to ensure none of them are loose.

I am presently trying to find a wiring diagram for the Dometic thermostat control box. Any suggestions would be appreciated. Thanks!

FlyboyTR · 01-07-2019, 10:30 AM

Yesterday afternoon I found a wiring diagram and then confirmed the wiring. Also rechecked all connections. Nothing found. I eventually gave up, unplugged the trailer from shore power and disconnected the house battery. It sat like this overnight.

After leaving the trailer unplugged from shore power and the house battery disconnected overnight, I reconnected everything this morning. To my surprise, everything now works just as it did prior to this strange event starting. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

I heated up the inside to 75 and then switched to cooling, set at 65. Within a few seconds the blower came on (set to LO) and a few seconds later the compressor kicked in and it started cooling. OH...I also tried cooling with the fan in Auto mode...does not work (just like the furnace). Oh well...I had rather the fan run continuous anyway but it would be nice to have everything working as Dometic originally intended.

**Bama Rambler** · 01-07-2019, 10:38 AM

There have been reports like this and IIRC it turned out to be the control board in the A/C. There have also been a few instances of strange behavior fixed by replacing the t-stat.

**Flybob** · 01-07-2019, 11:05 AM

Not sure what fan you are referring to here. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

If it is the AC fan then it is operating correctly. In the heat mode the AC fan only operates in LO and HI setting.

5picker · 01-07-2019, 11:08 AM

Quote:

Originally Posted by FlyboyTR

Yesterday afternoon I found a wiring diagram and then confirmed the wiring. Also rechecked all connections. Nothing found. I eventually gave up, unplugged the trailer from shore power and disconnected the house battery. It sat like this overnight.

After leaving the trailer unplugged from shore power and the house battery disconnected overnight, I reconnected everything this morning. To my surprise, everything now works just as it did prior to this strange event starting. However, the fan still does NOT come on with the gas furnace if the fan is set to Auto.

I heated up the inside to 75 and then switched to cooling, set at 65. Within a few seconds the blower came on (set to LO) and a few seconds later the compressor kicked in and it started cooling. OH...I also tried cooling with the fan in Auto mode...does not work (just like the furnace). Oh well...I had rather the fan run continuous anyway but it would be nice to have everything working as Dometic originally intended.

The A/C fan is designed to NOT come on in the AUTO (AU) setting when using the furnace. Only if set to Hi or Low.

Did you mis-type or are you saying it worked on AUTO before?

alsdas88 · 01-07-2019, 11:29 AM

I have a different RV than you, but had a similar problem shortly after I bought it used. After talking with a good RV repair place, they recommended disconnecting the heater from the Dometic controller and installing an independent heater controller. Very easy to do and it solved ALL my control issues. Now the wall controller controls just the AC unit and the simple heater controller controls just the heater. It used to be the signals from the Dometic controller had to be routed thru the AC control board before the heater could/would engage and the AC control board would not function correctly. I still have the ability to turn on the fans to circulate air if I desire, but usually I don't need to.

FlyboyTR · 01-07-2019, 11:36 AM

Thanks for the replies and info. I have had numerous people tell me the a/c fan works on Auto with the furnace...mine never has. So it is very possible mine is functioning correctly. There sure seems to be a lot of confusion about the operation of this system.

To clarify, before this event yesterday when everything stopped working except for the a/c fan coming on for 2-3 seconds... the a/c fan never worked in Auto mode with the gas furnace. ...but that is now sounding like this is normal!

Any ideas why disconnecting everything overnight seemed to fix the problem I was having as described in my original post?

Thanks again for the replies and helpful support!

**Bama Rambler** · 01-07-2019, 11:52 AM

Disconnecting the power allowed the computer to reset and that will probably work for a while. Hopefully there was just a power-on glitch and it won't come back.

5picker · 01-07-2019, 12:10 PM

Quote:

Originally Posted by Bama Rambler

Disconnecting the power allowed the computer to reset and that will probably work for a while. Hopefully there was just a power-on glitch and it won't come back.

X2

**Flybob** · 01-07-2019, 12:25 PM

During a discussion I had with Dometic support some time ago, The tech said that occasionally if the Rv has not been used for a while it may be necessary to reset the thermostat/controller. Not sure why, but it has worked for me.

FlyboyTR · 01-07-2019, 12:46 PM

Man.... a simple hardwired conventional thermostat system would sure be a lot simpler...in my opinion.

Glad to know about doing the reset. I will keep that in my notes! Thanks!

jcbrock · 01-07-2019, 01:28 PM

We have a 2019 Surveyor and dealer told us to always run the furnace on auto for the fan. We have been doing this since October without any issues. (Other than me accidentally changing it to C instead of F)

FlyboyTR · 01-07-2019, 04:58 PM

AND...it's back!

Stepped outside to the camper a few minutes ago and I am back to nothing except the random a/c fan turning on for 1-2 seconds. That's it.

Tonight I am going to try to reseat the electronic components on the control board and the thermostat by using a heat gun. This is my last shot before replacing the thermostat and control board.

I have done this on other Chinese junk electronic boards where they had cold solder joints and such. If I melt it...I don't care. I will update later!

**Bama Rambler** · 01-08-2019, 01:10 PM

Might want to try "initializing" the thermostat before trying too much else. It may not work, but it doesn't cost you anything.

Nevermind, I see you've already tried that.

FlyboyTR · 01-08-2019, 04:59 PM

I resoldered every connection on the thermostat PCB (printed circuit board). After a cool down I reconnected it. No joy...

Next I removed the PCB from the control box. I resoldered all the main joints but due to the many tiny chip joints I superheated it with my heat gun. It is easy to get parts hot enough to melt the solder...at the same time running the risk of damaging a component...but most can withstand a bit of heat. Anyway...the board was starting to smoke a bit when I laid it down. After cooling I reconnected all the connections. When I fired it up everything worked as expected! YEA!!!! I cycled things on and off about 20 times.

Due to the potential for another failure, I went ahead and purchased a new kit with the updated thermostat and control box. Camping World near me had one for $90. It took about 30 minutes to install the box and thermostat. I have packaged the original parts and have them stored in my on-board parts box. While I hate carrying stuff I don't need, I feel this is good insurance. In our Allegro bus, if the thermostat failed I can replace it with one from WalllyWorld...but not this Dometic system.

So, my assumption is there was a cold or poor solder joint on the control board. Based on this, repair is possible. As far as longevity of the parts after being heated (with a heat gun)....well...that will remain unknown. I think this is good to know because if one of these parts fail you have no heating or cooling.

The new thermostat is touch sensitive and when I snapped on the faceplate the settings went all over the place, A/C to 80, furnace to 40! I have read about people having this problem with the new style. I was able to make some adjustment to the little foam pieces and then everything worked without incident!

Thanks again to everyone for your suggestions, support and encouragement! ...greatly appreciated!

5picker · 01-08-2019, 05:49 PM

Thank you for coming back and reporting the outcome.

**Bama Rambler** · 01-09-2019, 09:55 AM

So heating the control board with a heat gun, at least temporarily fixed the problem. that's actually good news. At least we know that it's possibly a defective solder joint.

Now I'd like to get hold of one that's not working and look at it under magnification to see if I could find the bad joint. Because I bet that the same general location is the problem in many of the failures.

Thanks much for coming back and letting us know what you did.

FlyboyTR · 01-09-2019, 10:03 AM

Bama,
You're welcome! I was wearing my magnifying goggles and only saw one joint that looked suspect. But so often you really can't tell just by looking. I wish I knew which joint was the actual culprit. Yep, knowledge is power. I am sure whatever is failing is the same on all the units. At least now we know it is possible to make it work again!

Quote:

Originally Posted by Bama Rambler

So heating the control board with a heat gun, at least temporarily fixed the problem. that's actually good news. At least we know that it's possibly a defective solder joint.

Now I'd like to get hold of one that's not working and look at it under magnification to see if I could find the bad joint. Because I bet that the same general location is the problem in many of the failures.

Thanks much for coming back and letting us know what you did.

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