How to fix Dometic Comfort Control thermostat problems

[drusher]

Here is how to fix the most common dometic thermostat problems for the 4 and 5 button Comfort Control thermostats. Dometic support is awful and there are a lot of folks out there spending large sums of money on new thermostats and upper board conversion kits. With this posting you can save yourself big bucks.

I am a geek and figured it all out. There are procedures here you will not find anyplace else.

Below are the 3 most common problems and the way to fix them.

AC UNITS TURN OFF WHEN GOING OVER BUMPS, THERMOSTAT WILL NOT TURN ON, OR WILL NOT STAY ON:

-- The on/off switch on the thermostat probably has cracked solder joints. Re-solder the switch to the board and make sure the switch is snug on the circuit board so the solder does not crack again. Use a small 25-watt soldering iron with a finer tip on it. Of course, use electronic solder, not the solder used from plumbing! (Note: Dometic boards are famous for cold solder joints -- you might need to touch up the other solder points -- but don't try to do the microprocessor unless you know what you are doing.)

-- Also, soak the power switch with spray contact cleaner and operate the switch a few times.

-- If the above does not work, you might need to take the switch off the board, take it apart and fix it (you won't find a replacement for this switch on the internet due to the long paddle on it).

-- These switches were not properly mounted to the circuit board in the first place. You might have a cracked trace on the circuit board, which can be fixed by carefully scraping the green backing off the board with a small razor knife and soldering a small piece of solid wire (such as telephone wire) across the crack.

PUSHBUTTONS DO NOT WORK PROPERLY

(Note: the logic on the board triggers functions when a button is released, not when it is pressed. You have to let go of the button before a function is processed.)

Clean the switches. Remove the PC board from the housing. Do this in a clean work area so you don't lose any parts. Using a small flat jewelers screwdriver, carefully prise the locking tabs holding the top of the switch on the base JUST ENOUGH to pull the top of the switch off. Be sure not to lose any parts. Clean the contacts with spray contact cleaner and perhaps Q-tips. Snap the top back on the switch.

TEMPERATURE SENSING PROBLEMS:

First you have to understand how the system works.

For zone 1, the thermistor (little yellow thing near the thermostat on/off switch) inside the thermostat is usually used. If a remote sensor is plugged into the AC unit upper board (the white 2-pin plug), then the remote sensor is automatically activated by the thermostat and the temperature sensing electronics inside the thermostat is disabled. Do not bother trying to fix the electronics on the thermostat. It is complicated nonlinear circuitry and can't be fixed since Dometic provides no documentation.

Zone 2 usually has a remote sensor wired to the rear upper AC unit board. The temperature is processed by circuitry on the rear AC upper board. Look for a white nylon 2-pin connector on the upper board to see if a sensor is connected to it.

NOTE: In both cases, if a remote sensor is used, the electronics on the upper board are used to read and process signals for that zone from the thermistor. If communications between the boards and thermostat are hung up, it won't work right and there is no telling what it will do.

NOTE: If you are going to unplug the thermostat from the phone connector, always turn the thermostat off first, and also be sure the thermostat off before plugging the connector back in. If the thermostat is on when you unplug it or plug it back in, communications will not establish.

For testing, you need a thermometer from around the house so you can see what the real room temp is. Use a fan blowing on the home thermometer and the thermostat so you don't have to wait all day for thermostat and thermometer to sync up.

Dometic says these thermostats are accurate to +/- 5 degrees, which is abysmal. My experience is that they are usually within +/- 3 degrees (lousy). You will probably have to live with it if your thermostat is in this range.

If your thermostat isn't turning on/off at the right temperature (+/- 3 degrees), here is what to do (in this order):

1. Clearing communications issues: your unit can get into a situation where the thermostat is not communicating with one or both upper boards. Turn the thermostat off, and turn power to the AC units off and on. Then turn the thermostat on and see if it works better.

2. Reset the thermostat to factory specs. This can correct some problems with invalid temperature measurement when the thermistor inside the thermostat is being used. This sequence definitely works with 4-button thermostats. I do not have a 5-button unit to test so I cannot say if it works on 5-button thermostats. Do the following in the order below:
-- Turn the thermostat on
-- press any button once
-- press and hold the bottom 2 buttons in
-- press and release the mode button one time
-- release the bottom 2 buttons
-- press and release the mode button
-- if you did the above sequence correctly, the thermostat backlight will turn off. If it does not turn off, do the above sequence again.

3. Rear unit: If the temperature setting still does not work right, the problem is in the upper board (not reading the remote sensor thermistor correctly). Get a new upper board (available on the internet)

4. Front unit without remote sensor: Since the thermostats are no longer available and the circuitry not repairable, install a remote sensor 3106486.008 in your front AC unit. Just plug it into the upper board and the thermostat will use it. You will have to figure out how to route the thermistor down into the RV. If this does not work, the upper board is bad and needs to be replaced.

2. Front AC with remote sensor: Either the thermistor sensor is bad, or the circuitry on the upper board is bad. The sensors rarely go bad, but you can check the sensor with an ohmmeter. Unplug the sensor from the white plug on the upper board, and measure the resistance. The resistance should be about 7.2k at 70 degrees, 9.8k at 60 degrees, or 12.4k at 50 degrees. If you are seeing no resistance, then the thermistor is definitely bad If the thermistor is good, replace the upper board.

[dougrainer]

dicksop wrote:
I was reading the posts of how to fix Dometic issues and wanted to chime in with my issue and if anyone had seen it or knew how to fix it.
I have a 4 button dometic with two AC units.
When it worked, the digital display showed all the info including the zone 1 or zone 2 that I was configuring and looking at.

All of a sudden when I turned on the comfort control the zones did not show up on the display, nor did either of the AC units work.
After fumbling through a few wires on the AC 1 unit, I disconnected one of the phone connector cables and the front AC unit began to work. The rear still would not work. Also after having the front AC run for a while, I noticed it was not turning off when the temp on the display (by the way the zones 1 and 2 still was not displayed) was reached. It just runs continuously.
Any thoughts or ideas? I really do not want ot go buy the 10 button unit with control boards if I dont have to..
Peter

Y

Your 4 button is over 20 years old and they dropped the 4 button due to the same issues you are having. You can either buy the 2-- 10 control boards and the 10 button or you can buy 2 -- 5 Button control boards and the 5 button. You also need to replace the 4 wire phone end plugs as a maintenance issue. Doug

[dicksop]

I was reading the posts of how to fix Dometic issues and wanted to chime in with my issue and if anyone had seen it or knew how to fix it.
I have a 4 button dometic with two AC units.
When it worked, the digital display showed all the info including the zone 1 or zone 2 that I was configuring and looking at.

All of a sudden when I turned on the comfort control the zones did not show up on the display, nor did either of the AC units work.
After fumbling through a few wires on the AC 1 unit, I disconnected one of the phone connector cables and the front AC unit began to work. The rear still would not work. Also after having the front AC run for a while, I noticed it was not turning off when the temp on the display (by the way the zones 1 and 2 still was not displayed) was reached. It just runs continuously.
Any thoughts or ideas? I really do not want ot go buy the 10 button unit with control boards if I dont have to..
Peter

[jjcook]

We have 2017 cedar creek silverback with 2 a/c units controlled by a Dometic comfort control center 2 thermostat I cannot get it to display anything about zone 2 I have reset it the way the manual says but still nothing. Any help will be greatly appreciated .

[dougrainer]

Cwest17 wrote:
I have a 2016 Grand Design Imagine. When I turn the furnace on, the air conditioner comes on also so both hot and cold air is blowing. Any suggestions?

I doubt cold air is blowing. In the Tstat settings on a Dometic 12 button or Zone Control tstat. You MUST select AUTO for Fan speed selection to prevent the AC blower from coming ON. If the Fan is set to Lo-Med-Hi the AC blower will run when Furnace is ON. Doug

[Cwest17]

I have a 2016 Grand Design Imagine. When I turn the furnace on, the air conditioner comes on also so both hot and cold air is blowing. Any suggestions?

[dougrainer]

Megmcc08 wrote:
The a/c wont come on. It's a domestic dual thermostat. The 10 button. We tried switching with new thermostat. We flipped breakers in/outside. I looked at the circuit boards and don't see any burned wires. We just had the whole unit replaced in both a/c. I am not sure if it's a voltage problem. I get nothing on thermostat so it's incredibly hard for me to diagnose issue. Any help greatly appreciated. 5th wheel crusader 2014.

You need to start a NEW Thread, not hijack a old thread. This will get you the info you need faster. WHO replaced the unit/s. Your post does not make sense. ----"We just had the whole unit replaced in both a/c". BOTH roof top units replaced? Once we determine who replaced them will determine what to look for. Doug

[hostage]

Megmcc08 wrote:
The a/c wont come on. It's a domestic dual thermostat. The 10 button. We tried switching with new thermostat. We flipped breakers in/outside. I looked at the circuit boards and don't see any burned wires. We just had the whole unit replaced in both a/c. I am not sure if it's a voltage problem. I get nothing on thermostat so it's incredibly hard for me to diagnose issue. Any help greatly appreciated. 5th wheel crusader 2014.

whats weird to me is if you have two roof AC systems and one thermostat
which is pretty much the norm these days there still has to be two communication wires going to each Unit; the chances of both failing at the same time is confusing for sure;; how long after replacing both units did the problem start? Did both stop operating at the same time?
looking for a common denominator here may help
Will the thermostat run the furnace systems?

[Megmcc08]

The a/c wont come on. It's a domestic dual thermostat. The 10 button. We tried switching with new thermostat. We flipped breakers in/outside. I looked at the circuit boards and don't see any burned wires. We just had the whole unit replaced in both a/c. I am not sure if it's a voltage problem. I get nothing on thermostat so it's incredibly hard for me to diagnose issue. Any help greatly appreciated. 5th wheel crusader 2014.

[MagicMikeD]

I have the 5 button and it all of a sudden just flashes Zone 3.

I can turn it off and on about 6 or 7 times, and every mode shows up except furnace. The A/C or fan doesn't kick on. Then after a minute or so, it will go back to blank except Zone 3.

Any ideas? Tstat? Board?

[rgatijnet1]

dougrainer wrote:

rgatijnet1 wrote:
On my Monaco coach, they installed the Dometic CC thermostat on the exterior wall. This caused all kinds of problems with maintaining a comfortable temperature inside the coach, depending on if the exterior wall was in the sun or in the shade. I removed the thermostat, placed a 1" piece of styrofoam behind it and installed it spaced out from the wall that 1". It now does a much better job of maintaining a comfortable temp in the coach.

You can buy the remote Thermister for the front Zone 1 and when that is connected to the Zone 1 AC board will remove the Wall CCC from being the temp selector. Just route the temp bulb out the ceiling of the unit about 1 foot from the intake filter of the Zone 1 AC unit. Doug

Thanks, that makes sense on a location for a remote sensor.

[dougrainer]

rgatijnet1 wrote:
On my Monaco coach, they installed the Dometic CC thermostat on the exterior wall. This caused all kinds of problems with maintaining a comfortable temperature inside the coach, depending on if the exterior wall was in the sun or in the shade. I removed the thermostat, placed a 1" piece of styrofoam behind it and installed it spaced out from the wall that 1". It now does a much better job of maintaining a comfortable temp in the coach.

You can buy the remote Thermister for the front Zone 1 and when that is connected to the Zone 1 AC board will remove the Wall CCC from being the temp selector. Just route the temp bulb out the ceiling of the unit about 1 foot from the intake filter of the Zone 1 AC unit. Doug

[rgatijnet1]

On my Monaco coach, they installed the Dometic CC thermostat on the exterior wall. This caused all kinds of problems with maintaining a comfortable temperature inside the coach, depending on if the exterior wall was in the sun or in the shade. I removed the thermostat, placed a 1" piece of styrofoam behind it and installed it spaced out from the wall that 1". It now does a much better job of maintaining a comfortable temp in the coach.

[Pete_k]

Found the issue with ours. Still not sure what to do for the fix. But found the issue. Went to unplug the phone plug from the back of the controller. AS I started to unplug it. The Controller lights came on. Pushed the plug back in all the way. Lights go's out, back it out some. Works again, so now to figure out if its the plug male or female end?
Pete

[drusher]

The power switch on bottom of the older dometic thermostats had no physical support. The solder joint can easily crack with normal use of the power switch. In the early stages, your AC will turn on and off while you are going down the road and might not turn on at all depending on what you had for lunch (hee hee). It gets worse with time as the cracked solder gets junk and pollutants in the crack and then it wont work at all.

Take the board out of the case and resolder the power switch. Be sure to use electronic solder, not plumbing solder.

Check the circuit traces where the switch is mounted to make sure they are not cracked. If the trace is broken, you can scrape the green insulation off the trace carefully with a small razor knife. Use a short piece of wire cut off a resistor to bridge the break and make it stronger too. Cut the piece of wire too long and bend the end up so you can hold the wire in place with small needlenose pliers while soldering, then cut the end off with a little wire wire cutter.

On mine I resoldered all the joints because there were a lot of cold solder joints (which crack easily). You can tell a cold joint because it is not shiny. Do this carefully with a fine tipped iron, and inspect it to make sure you did not make any solder bridges.

After soldering, clean rosin off the back of the board with a clean toothbrush dipped in rubbing alcohol. You do not want the impurities and chemicals in the rosin to attack the solder joint later.