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.