Forum Discussion

Explorer

Nov 10, 2013

How to fix Dometic Comfort Control thermostat problems

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.

Class A

25 Replies

sailor_lou
Explorer
Nov 11, 2013
drusher, good info thanks.

dougrainer, I appreciate all the assistance you provide on this forum, but agree that any manufacturer that will only assist an "Authorized Service Tech" and not the retail customer does stink. This is not to say their people are not knowledgeable because obviously they have been to you, but their corporate policy to not assist the customer is flawed. The bottom line is they would rather have you replace an overprice board that was built with low quality components and poor solder joints than have it repaired. After all, they know a shop or service tech such as yourself will not spent the time to replace a component when a total board replacement is faster and more efficient. Granted, some of this is beyond what the average RV owner will attempt, but there are also many of us out here that do repair our own equipment and the lack of technical information (esp. schematics) makes that extremely difficult. I am a retired Instrument & Controls Design Engineer with 30+ years of experience and like you, I am pretty confident in my abilities to fix anything installed on a MOHO.

My recent experiences with Equalizer Systems (RV levelers) and CAT have shown there are successful companies out there willing to help their customers with technical assistance and for the prices we pay for some of this stuff, that assistance is expected.

Lou
05 Travel Supreme Envoy
dougrainer
Nomad
Nov 11, 2013
The reason the NEW tstat and boards will not work is a simple fact that Dometic decided to make a HP reversing valve failure to default to COOL instead of Heat. They also allowed the New 12 button to have 3 fan speeds on Heat Pump when they only had 2 speeds before. And the tstat is programmable. The reason to update to a 5 button, when they did that 13 years ago was to fix a failure of HP units freezing. The original 4 button did not have a Freeze Sensor For heat Pumps, Cool only units AND to have a ZONE button(hence the 5 versus 4 button). They had a LOT of RV'ers that had no clue how to change Zones on the 4 button. Now, I AM a highly qualified factory trained Dometic Service Technician, with 34 years of working on RV's AND Dometic products. I am also a Master Certified Technician.
I never insulted you. I pointed out your mis-info on what a reset does. I will tell you that, the remote sensors CAN have a fault IF the sensor bulb has the plastic wire loom pinching it behind the wall cover. That pressure can cause intermittent sensing. The wall tstat CCC can also have a problem if the area it is mounted at is on a wall area that the backside is NOT insulated properly--especially mounts on the side walls with no insulationor by a refer cabinet---that causes erroneous readings.As you stated, the BEST way for the CCC tstats is for EACH Zone to have a remote sensor located in an area that only senses the correct inside Zone temp. A LOT of OEM's fail to do that and then Dometic gets the blame. For instance, I had 1 sensor mounted over a LCD TV. LCD TV's generate a LOT of heat. We had to relocate it. I have fixed HUNDREDS of Dometic CCC systems over the years(Correctly). There is probably no one, including Dometic, that knows as much about the CCC system than me. There is no way I would charge anyone $125 to rebuild a 4 button. I would rather have them pay the online Dinosaur guys that $75 for them to rebuild it. They can warranty it, I will not. BTW, the 5 button tstats do not seem to have the problems with internal contacs that the 4 button models have. Doug

Dometic support does NOT stink. They ONLY support authorized trained Technicians, They do Not support retail customers. If your "trained Dometic man" has a problem with Dometic support, I would think it is because he has a lack of knowledge and expects Dometic to figure out his problems instead of him having the knowledge and experience to do it himself.

You just joined. I suggest you look at some of my posts over the years before you try to argue with me.
diveman52
Explorer
Nov 11, 2013
Great information Drusher
Thanks.
drusher
Explorer
Nov 11, 2013
My thermostat was 16 degrees off before I reset it, and only 10 degrees off after I reset it. I made very careful measurements so I know that it did make things better. I still ended up putting a remote sensor on my front unit. Now it is within 2 degrees of spec. Nobody knows what memory settings beyond temp settings are modifiable in the EEPROM. The thermistor resistance is converted to pulses by an oscillator chip, which are read by the microprocessor. As a programmer in professional life, it is reasonable to believe there is at least one value that can be set in the EEPROM to determine how pulses are interpreted by the microprocessor. a 6-degree improvement in performance due to resetting indicates that something related to temperature interpretation was reset.

Please refrain from being insulting to me. A lot of folks can do what I put in this posting. Paying a tech to do an hour of work to do the above sure beats the heck out of paying $600-$800 for a new thermostat and boards, and then paying somebody $125/hr to put it in. That is assuming that it even works. The new 12-button thermostats don't work with CC1/CC2-era heat pumps -- another bad design move by Dometic.

I know a trained dometic service person, who agrees that Dometic's designs have problems, and that their support stinks. That being the case, it is up to us to fix our own equipment and not end up spending a ton of money being stuck in their pricey upgrade racket. Power to the people!
dougrainer
Nomad
Nov 10, 2013
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.

This has NOTHING to do with temp sensing issues. ALL this does is reset to the TEMP preset on the tstat (usually 72 degrees and just verifies the DIP switch settings on each control board. NOTHING else. A LOT of people think the reset is like resetting a PC. It is NOT. It does NOTHING for operational issues. As to your assertion that people are spending money when the system can be fixed. You are correct, but at $125 per hour average labor, It would cost HUNDREDS of dollars to do all things you have suggested with no guarantee that the system will function after it is done. Now, if you have the tech smarts you have, then it is worth the time and effort. MOST rv'ers do not have the knowledge and expertise to do what you did. Besides, 4 button systems were discontinued almost 13 years ago, so paying someone to do that work is not cost efficient. Doug